Clin. Microbiol. Rev.
Duane J. Gubler
Dengue and Dengue Hemorrhagic Fever
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CLINICAL MICROBIOLOGY REVIEWS,
July 1998, p. 480–496 Vol. 11, No. 3
Dengue and Dengue Hemorrhagic Fever
DUANE J. GUBLER*
Division of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, Centers for Disease Control
and Prevention, Public Health Service, U.S. Department of Health and Human Services,
P.O. Box 2087, Fort Collins, Colorado 80522
EMERGENCE OF DENGUE AS A GLOBAL PUBLIC HEALTH PROBLEM................................................480
Factors Responsible for the Increased Incidence...............................................................................................481
Dengue in the Continental United States ...........................................................................................................482
Dengue Hemorrhagic Fever...................................................................................................................................486
Host Immune Factors.............................................................................................................................................488
Mammalian cell culture.....................................................................................................................................490
Mosquito cell culture..........................................................................................................................................490
New Diagnostic Technology...................................................................................................................................491
PREVENTION AND CONTROL..............................................................................................................................491
Disease Prevention Programs ...............................................................................................................................492
Active surveillance ..............................................................................................................................................492
Prevention of Dengue in Travelers.......................................................................................................................493
Although first reports of major epidemics of an illness
thought to possibly be dengue occurred on three continents
(Asia, Africa, and North America) in 1779 and 1780 (73, 75,
109, 128), reports of illnesses clinically compatible with dengue
fever occurred even earlier. The earliest record found to date
is in a Chinese encyclopedia of disease symptoms and reme-
dies, first published during the Chin Dynasty (265 to 420 A.D.)
and formally edited in 610 A.D. (Tang Dynasty) and again in
992 A.D. (Northern Sung Dynasty) (108). The disease was
called water poison by the Chinese and was thought to be
somehow connected with flying insects associated with water.
Outbreaks of illness in the French West Indies in 1635 and in
Panama in 1699 could also have been dengue (75, 103). Thus,
dengue or a very similar illness had a wide geographic distri-
bution before the 18th century, when the first known pandemic
of dengue-like illness began. It is uncertain whether the epi-
demics in Batavia (Jakarta), Indonesia, and Cairo, Egypt, in
1779 were dengue, but it is quite likely that the Philadelphia
epidemic of 1780 was dengue (19). A more detailed discussion
of the history of dengue viruses has recently been published
EMERGENCE OF DENGUE AS A GLOBAL
PUBLIC HEALTH PROBLEM
The disease pattern associated with dengue-like illness from
1780 to 1940 was characterized by relatively infrequent but
often large epidemics. However, it is likely that dengue viruses
became endemic in many tropical urban centers during this
time because during interepidemic periods, when there was no
apparent disease transmission, nonimmune visitors invariably
contracted a dengue-like illness within months of their arrival.
The ecologic disruption in the Southeast Asia and Pacific
theaters during and following World War II created ideal con-
* Mailing address: Division of Vector-Borne Infectious Diseases,
National Center for Infectious Diseases, Centers for Disease Control
and Prevention, Public Health Service, U.S. Department of Health and
Human Services, P.O. Box 2087, Fort Collins, CO 80522. Phone: (970)
221-6428. Fax: (970) 221-6476. E-mail: firstname.lastname@example.org.
on June 5, 2013 by guest
ditions for increased transmission of mosquito-borne diseases,
and it was in this setting that a global pandemic of dengue
began. With increased epidemic transmission, hyperendemicity
(the cocirculation of multiple dengue virus serotypes) devel-
oped in Southeast Asian cities and epidemic dengue hemor-
rhagic fever (DHF), a newly described disease, emerged (37,
48, 61, 63). The first known epidemic of DHF occurred in
Manila, Philippines, in 1953 to 1954, but within 20 years the
disease in epidemic form had spread throughout Southeast
Asia; by the mid-1970s, DHF had become a leading cause of
hospitalization and death among children in the region (1). In
the 1970s, dengue was reintroduced to the Pacific Islands and
epidemic activity increased there and in the Americas. During
the 1980s and 1990s, epidemic dengue transmission intensified,
and there is now a global resurgence of dengue fever, with
expanding geographic distribution of both the mosquito vec-
tors and the viruses, increased incidence of disease caused by
an increased frequency of epidemic transmission, and the
emergence of DHF in many new countries (36, 39, 41, 45, 48,
61, 63, 110, 111, 124).
In Asia, epidemic DHF has expanded geographically from
Southeast Asian countries west to India, Sri Lanka, the Mal-
dives, and Pakistan and east to China (42). Several island
countries of the South and Central Pacific (Niue, Palau, Yap,
Cook Islands, Tahiti, New Caledonia, and Vanuatu) have ex-
perienced major or minor DHF epidemics (41). Epidemiologic
changes in the Americas, however, have been the most dra-
matic. In the 1950s, 1960s, and most of the 1970s, epidemic
dengue was rare in the American region because the principal
mosquito vector, Aedes aegypti, had been eradicated from most
of Central and South America (36–38, 110). The eradication
program was discontinued in the early 1970s, and this species
then began to reinvade the countries from which it had been
eradicated (38, 110). By the 1990s, A. aegypti had nearly re-
gained the geographic distribution it held before eradication
was initiated (Fig. 1). Epidemic dengue invariably followed re-
infestation of a country by A. aegypti. By the 1980s, the Amer-
ican region was experiencing major epidemics of dengue in
countries that had been free of the disease for 35 to 130 years
(36–38, 111). New dengue virus strains and serotypes were
introduced (DEN-1 in 1977, a new strain of DEN-2 in 1981,
DEN-4 in 1981, and a new strain of DEN-3 in 1994). More-
over, many countries of the region evolved from nonendemic-
ity (no endemic disease) or hypoendemicity (one serotype
present) to hyperendemicity (multiple serotypes present), and
epidemic DHF emerged, much as it had in Southeast Asia 25
years earlier (36–38). From 1981 to 1997, 24 American coun-
tries reported laboratory-confirmed DHF (Fig. 2) (42, 43, 111).
While Africa has not yet had a major epidemic of DHF,
sporadic cases have occurred, with increased epidemic dengue
fever, in the past 15 years. Before the 1980s, little was known
of the distribution of dengue viruses in Africa. Since then, how-
ever, major epidemics caused by all four serotypes have oc-
curred in both East and West Africa (41, 48). Outbreaks have
been more common in East Africa and the Middle East in the
1990s, with major epidemics in Djibouti in 1991 and in Jeddah,
Saudi Arabia, in 1994; both were the first outbreaks in those
countries in over 50 years (41, 120).
In 1997, dengue viruses and A. aegypti mosquitoes have a
worldwide distribution in the tropics (Fig. 3); over 2.5 billion
people now live in areas where dengue is endemic (42, 45, 48,
61, 63). Currently, dengue fever causes more illness and death
than any other arbovirus disease of humans (124). Each year,
an estimated 100 million cases of dengue fever and several hun-
dred thousand cases of DHF occur, depending on epidemic ac-
tivity (42, 45, 104). DHF is a leading cause of hospitalization and
death among children in many Southeast Asian countries (1).
Factors Responsible for the Increased Incidence
The factors responsible for the dramatic resurgence and
emergence of epidemic dengue and DHF, respectively, as a
FIG. 1. A. aegypti distribution in the Americas during the 1930s and in 1970 and 1998.
VOL. 11, 1998DENGUE AND DENGUE HEMORRHAGIC FEVER481
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global public health problem in the past 17 years are complex
and not fully understood. However, the resurgence appears to
be closely associated with demographic and societal changes
over the past 50 years (36, 41, 42, 48). Two major factors
have been the unprecedented global population growth
and the associated unplanned and uncontrolled urbanization,
especially in tropical developing countries. The substandard
housing, crowding, and deterioration in water, sewer, and waste
management systems associated with unplanned urbanization
have created ideal conditions for increased transmission of
mosquito-borne diseases in tropical urban centers.
A third major factor has been the lack of effective mosquito
control in areas where dengue is endemic (36, 38, 42, 48). The
emphasis during the past 25 years has been on space spraying
with insecticides to kill adult mosquitoes; this has not been
effective (38, 107, 115) and, in fact, has been detrimental to
prevention and control efforts by giving citizens of the com-
munity and government officials a “false sense of security”
(38). Additionally, the geographic distribution and population
densities of A. aegypti have increased, especially in urban areas
of the tropics, because of increased numbers of mosquito larval
habitats in the domestic environment. The latter include non-
biodegradable plastics and used automobile tires, both of
which have increased dramatically in prevalence during this
A fourth factor responsible for the global emergence of
dengue and DHF is increased air travel, which provides the
ideal mechanism for the transport of dengue and other urban
pathogens between population centers of the world (36, 40, 42,
48). For instance, in 1994, an estimated 40 million persons
departed the United States by air, over 50% of whom traveled
for business or holiday to tropical countries where dengue is
endemic. Many travelers become infected while visiting tropi-
cal areas but become ill only after returning home, resulting in
a constant movement of dengue viruses in infected humans to
all areas of the world and ensuring repeated introductions of
new dengue virus strains and serotypes into areas where the
mosquito vectors occur (40, 119).
A fifth factor that has contributed to the resurgence of
epidemic dengue has been the decay in public health infra-
structures in most countries in the past 30 years. Lack of
resources has led to a critical shortage of trained specialists
who understand and can develop effective prevention and con-
trol programs for vector-borne diseases. Coincident with this
has been a change in public health policy that placed emphasis
on emergency response to epidemics by using high-technology
mosquito control methods rather than on preventing those
epidemics by using larval source reduction through environ-
mental hygiene, the only method that has been shown to be
In summary, demographic and societal changes, decreasing
resources for vector-borne infectious disease prevention and
control, and changes in public health policy have all contrib-
uted to increased epidemic dengue activity, the development of
hyperendemicity, and the emergence of epidemic DHF.
Dengue in the Continental United States
Each year, dengue cases imported to the Continental United
States are documented by the Centers for Disease Control and
Prevention (CDC) (40, 119). These cases represent introduc-
tions of all four virus serotypes from all tropical regions of the
world. Most cases of dengue introduced into the United States
come from the American and Asian tropics, reflecting the
increased number of persons traveling to and from those areas.
Overall, from 1977 to 1995, a total of 2,706 suspected cases
FIG. 2. DHF in the Americas before 1981 and from 1981 to the present.
482GUBLERCLIN. MICROBIOL. REV.
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of imported dengue were reported to CDC (21, 40, 119). Al-
though adequate blood samples were received from only some
of these patients, 584 (22%) were confirmed in the laboratory
These cases represent only the tip of the iceberg, because
most physicians in the United States have a low index of sus-
picion for dengue, which is often not included in the differen-
tial diagnosis of acute febrile illness, even if the patient re-
cently returned from a tropical country. As a result, the
majority of imported dengue cases are never reported (21). It
is important to increase awareness of dengue and DHF among
physicians in temperate areas, however, because the disease
can be life-threatening. For example, two cases of dengue
shock syndrome (DSS) were recently described in Swedish
tourists returning from holiday in Asia (152). In the United
States, imported cases appear to be increasingly severe (21).
From 1986 to 1993, for example, only 13 of 166 patients (8%)
with laboratory-confirmed dengue were hospitalized. In 1994
and 1995, however, 6 of 46 patients (13%) and 11 of 86 patients
(13%) with confirmed imported disease required hospitaliza-
tion, respectively. Moreover, 3 (7%) of the patients in 1994 had
severe, hemorrhagic disease (21). Therefore, it is important
that physicians in the United States consider dengue in the
differential diagnosis of a viral syndrome in all patients with a
travel history to any tropical area.
The potential for epidemic dengue transmission in the
United States still exists. After an absence of 35 years, auto-
chthonous transmission, secondary to importation of the virus
in humans, occurred on four occasions in the past 17 years
(1980, 1986, 1995, and 1997) (21, 22). Although all of these
outbreaks were small, they underscore the potential for dengue
transmission in the United States, where two competent mos-
quito vectors are found (48) (Fig. 4). A. aegypti, the most im-
portant and efficient epidemic vector of dengue viruses, has
been in the United States for over 200 years and was respon-
sible for transmitting major epidemics in the southern states in
the 19th and early 20th centuries (34). Currently, this species is
found only in the Gulf Coast states from Texas to Florida,
although small foci have recently been reported in Arizona
(Fig. 4). Aedes albopictus, a secondary vector of dengue virus,
was introduced into the continental United States from Asia in
the early 1980s and has since become widespread in the eastern
half of the country. This species currently is found in 866
counties in 26 of the continental states (22, 105); it has also
been found in Hawaii for over 90 years, as well as in Guam and
Saipan. Both A. aegypti and A. albopictus can transmit dengue
viruses to humans, and their presence in the United States
increases the risk of autochthonous dengue transmission, sec-
ondary to imported cases (37, 40).
There are four dengue virus serotypes, called DEN-1, DEN-
2, DEN-3, and DEN-4. They belong to the genus Flavivirus,
family Flaviviridae (of which yellow fever virus is the type
species), which contains approximately 70 viruses (150). The
flaviviruses are relatively small (40–50 mm) and spherical with
a lipid envelope. The flavivirus genome is approximately 11,000
bases long and is made up of three structural and seven non-
structural proteins. There are three major complexes within
this family—tick-borne encephalitis virus, Japanese encephali-
tis virus, and dengue virus. All flaviviruses have common group
epitopes on the envelope protein that result in extensive cross-
reactions in serologic tests. These make unequivocal serologic
FIG. 3. World distribution map of dengue and A. aegypti in 1998.
VOL. 11, 1998DENGUE AND DENGUE HEMORRHAGIC FEVER 483
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diagnosis of flaviviruses difficult. This is especially true among
the four dengue viruses. Infection with one dengue serotype
provides lifelong immunity to that virus, but there is no cross-
protective immunity to the other serotypes. Thus, persons liv-
ing in an area of endemic dengue can be infected with three,
and probably four, dengue serotypes during their lifetime (37).
The primitive enzootic transmission cycle of dengue viruses
involves canopy-dwelling Aedes mosquitoes and lower primates
in the rain forests of Asia and Africa (Fig. 5) (37). Current
evidence suggests that these viruses do not regularly move out
of the forest to urban areas (116). An epidemic transmission
cycle may occur in rural villages or islands, where the human
population is small. Introduced viruses quickly infect the ma-
jority of susceptible individuals in these areas, and increasing
herd immunity causes the virus to disappear from the popula-
tion. A number of Aedes (Stegomyia) spp. may act as a vector
in these situations, depending on the geographic area, includ-
ing A. aegypti, A. albopictus, A. polynesiensis and other mem-
bers of the A. scutellaris group (37). The most important trans-
mission cycle from a public health standpoint is the urban
endemic/epidemic cycle in large urban centers of the tropics
(Fig. 5). The viruses are maintained in an A. aegypti-human-
A. aegypti cycle with periodic epidemics. Often, multiple virus
serotypes cocirculate in the same city (hyperendemicity).
Humans are infected with dengue viruses by the bite of an
infective mosquito (37). A. aegypti, the principal vector, is a
small, black-and-white, highly domesticated tropical mosquito
that prefers to lay its eggs in artificial containers commonly
found in and around homes, for example, flower vases, old
automobile tires, buckets that collect rainwater, and trash in
general. Containers used for water storage, such as 55-gallon
drums, cement cisterns, and even septic tanks, are important in
producing large numbers of adult mosquitoes in close proxim-
ity to human dwellings. The adult mosquitoes prefer to rest
indoors, are unobtrusive, and prefer to feed on humans during
daylight hours. There are two peaks of biting activity, early
morning for 2 to 3 h after daybreak and in the afternoon for
several hours before dark. However, these mosquitoes will feed
all day indoors and on overcast days. The female mosquitoes
are very nervous feeders, disrupting the feeding process at the
slightest movement, only to return to the same or a different
person to continue feeding moments later. Because of this
behavior, A. aegypti females will often feed on several persons
during a single blood meal and, if infective, may transmit
dengue virus to multiple persons in a short time, even if they
only probe without taking blood (46, 112, 114, 135). It is not
uncommon to see several members of the same household
become ill with dengue fever within a 24- to 36-h time frame,
suggesting that all of them were infected by a single infective
mosquito (43). It is this behavior that makes A. aegypti such an
efficient epidemic vector. Inhabitants of dwellings in the tropics
are rarely aware of the presence of this mosquito, making its
After a person is bitten by an infective mosquito, the virus
undergoes an incubation period of 3 to 14 days (average, 4 to
7 days), after which the person may experience acute onset of
fever accompanied by a variety of nonspecific signs and symp-
toms (136). During this acute febrile period, which may be as
short as 2 days and as long as 10 days, dengue viruses may
circulate in the peripheral blood (51). If other A. aegypti mos-
quitoes bite the ill person during this febrile viremic stage,
FIG. 4. A. aegypti and A. albopictus distribution in the United States in 1998.
484GUBLERCLIN. MICROBIOL. REV.
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those mosquitoes may become infected and subsequently
transmit the virus to other uninfected persons, after an extrin-
sic incubation period of 8 to 12 days (37, 46).
Dengue virus infection in humans causes a spectrum of ill-
ness ranging from inapparent or mild febrile illness to severe
and fatal hemorrhagic disease (1). Infection with any of the
four serotypes causes a similar clinical presentation that may
vary in severity, depending on a number of risk factors (see
below). The incubation period varies from 3 to 14 days (aver-
age, 4 to 7 days) (131, 136). In areas where dengue is endemic,
the illness is often clinically nonspecific, especially in children,
with symptoms of a viral syndrome that has a variety of local
names. Important risk factors influencing the proportion of
patients who have severe disease during epidemic transmission
include the strain and serotype of the infecting virus and the
immune status, age, and genetic background of the human host
(1, 4, 37, 57, 62, 123).
Classic dengue fever is primarily a disease of older children
and adults. It is characterized by the sudden onset of fever and
a variety of nonspecific signs and symptoms, including frontal
headache, retro-orbital pain, body aches, nausea and vomiting,
joint pains, weakness, and rash (1, 71, 131, 136, 149). Patients
may be anorexic, have altered taste sensation, and have a mild
sore throat. Constipation is occasionally reported; diarrhea
and respiratory symptoms are infrequently reported and may
be due to concurrent infections.
The initial temperature may rise to 102 to 105°F, and fever
may last for 2 to 7 days. The fever may drop after a few days,
only to rebound 12 to 24 h later (saddleback). A relative
bradycardia may be noted despite the fever. The conjunctivae
may be injected, and the pharynx may be inflamed. Lymph-
adenopathy is common. Rash is variable but occurs in up to
50% of patients as either early or late eruptions. Facial flushing
or erythematous mottling may occur coincident with or slightly
before onset of fever and disappears 1 to 2 days after onset of
symptoms. A second rash, varying in form from scarlatiniform
to maculopapular, may appear between days 2 and 6 of illness.
The rash usually begins on the trunk and spreads to the face
and extremities. In some cases, an intense erythematous pat-
tern with islands of normal skin is observed. The average du-
ration of the second rash is 2 to 3 days. Toward the end of the
febrile phase of illness or after the temperature falls to or
below normal, petechiae may appear; these may be scattered
or confluent. Intense pruritus followed by desquamation on the
palms of the hands and soles of the feet may occur.
Hemorrhagic manifestations in dengue fever patients are
not uncommon and range from mild to severe. Skin hemor-
rhages, including petechiae and purpura, are the most com-
mon, along with gum bleeding, epistaxis, menorrhagia, and gas-
trointestinal (GI) hemorrhage. Hematuria occurs infrequently,
and jaundice is rare.
Clinical laboratory findings associated with dengue fever in-
clude a neutropenia followed by a lymphocytosis, often marked
by atypical lymphocytes. Liver enzyme levels in the serum may
be elevated; the elevation is usually mild, but in some patients,
alanine aminotransferase and aspartate aminotransferase lev-
els reach 500 to 1,000 U/liter. In one epidemic of DEN-4, 54%
of confirmed patients with data reported on liver enzymes had
elevated levels (32). Thrombocytopenia is also common in
FIG. 5. Transmission cycles of dengue viruses.
VOL. 11, 1998 DENGUE AND DENGUE HEMORRHAGIC FEVER485
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dengue fever; in the above epidemic, 34% of patients with
confirmed dengue fever who were tested had platelet counts of
less than 100,000/mm3(32).
Dengue fever is generally self-limiting and is rarely fatal.
The acute phase of illness lasts for 3 to 7 days, but the conva-
lescent phase may be prolonged for weeks and may be associ-
ated with weakness and depression, especially in adults. No
permanent sequelae are known to be associated with this in-
Dengue Hemorrhagic Fever
DHF is primarily a disease of children under the age of 15
years, although it may also occur in adults (1, 32). It is char-
acterized by sudden onset of fever, which usually lasts for 2 to
7 days, and a variety of nonspecific signs and symptoms. During
the acute phase of illness, it is difficult to distinguish DHF from
dengue fever and other illnesses found in tropical areas. The
differential diagnoses during the acute phase of illness should
include measles, rubella, influenza, typhoid, leptospirosis, ma-
laria, other viral hemorrhagic fevers, and any other disease that
may present in the acute phase as a nonspecific viral syndrome.
Children frequently have concurrent infections with other vi-
ruses and bacteria causing upper respiratory symptoms. There
is no pathognomonic sign or symptom for DHF during the
acute stage; on the other hand, as fever remits, characteristic
manifestations of plasma leakage appear, making accurate
clinical diagnosis possible in many cases (1).
The critical stage in DHF is at the time of defervescence, but
signs of circulatory failure or hemorrhagic manifestations may
occur from about 24 h before to 24 h after the temperature
falls to normal or below (1). Blood tests usually show that the
patient has thrombocytopenia (platelet count, ?100,000/mm3)
and hemoconcentration relative to baseline as evidence of a
vascular leak syndrome. Common hemorrhagic manifestations
include skin hemorrhages such as petechiae, purpuric lesions,
and ecchymoses. Epistaxis, bleeding gums, GI hemorrhage,
and hematuria occur less frequently. The tourniquet test,
which indicates that the patient has increased capillary fragil-
ity, may be diagnostically helpful to the physician.
Scattered petechiae are the most common hemorrhagic
manifestation observed; they appear most often on the extrem-
ities but are also found on the trunk, other parts of the body,
and on the face in patients with severe dengue shock syndrome
(DSS). Purpuric lesions may appear on various parts of the
body but are most common at the site of venipuncture. In some
patients, large ecchymotic lesions develop on the trunk and
extremities; other patients bleed actively at the site of veni-
puncture, some profusely. More severely ill patients have GI
hemorrhage. Classic hematemesis with coffee-ground vomitus
and melena usually occur after prolonged shock, but patients
may develop massive, frank upper GI hemorrhage as well,
often before the onset of shock. Without early diagnosis and
proper management, some patients experience shock from
blood loss, which may be mild or severe (35, 138, 139). More
commonly, shock is caused by plasma leakage; it may be mild
and transient or progress to profound shock with undetectable
pulse and blood pressure (1). Children with profound shock
are often somnolent, exhibit petechiae on the face, and have
In patients with severe DHF or DSS, fever and nonspecific
constitutional signs and symptoms of a few days duration are
followed by the sudden deterioration of the patient’s condition
(1). During or shortly before or after the fall in temperature,
the patient’s skin may become cool, blotchy, and congested;
circumoral cyanosis is frequently observed, and the pulse be-
comes rapid and weak. Although some patients appear lethar-
gic at first, they become restless and then rapidly pass into a
critical stage of shock. They frequently experience acute ab-
dominal pain shortly before the onset of shock (1, 138, 139).
In patients with mild DHF, all signs and symptoms abate
shortly after the fever subsides. Subsidence of fever, however,
may be accompanied by profuse sweating and mild changes in
pulse rate and blood pressure, together with coolness of the
extremities and skin congestion. These changes reflect mild
and transient circulatory disturbances as a result of plasma
leakage. Patients usually recover spontaneously or after fluid
and electrolyte therapy (1). Patients in shock are in danger of
dying unless appropriately managed. The duration of shock is
usually short; the patient may die within 8 to 24 h, but recovery
is usually rapid following antishock therapy. Convalescence for
patients with DHF, with or without shock, is usually short and
uneventful. Once the shock is overcome, even patients with
undetectable pulse and blood pressure will usually recover
within 2 to 3 days (1).
As with dengue fever, leukopenia is common; thrombocyto-
penia and hemoconcentration are constant findings in DHF
and DSS. A platelet count of ?100,000/mm3is usually found
between the days 3 and 8 of illness. Hemoconcentration, indi-
cating plasma leakage, is almost always present in classic DHF
but is more severe in patients with shock. Hepatomegaly is a
common but not constant finding (35, 138, 139). In some coun-
tries, most patients with confirmed DHF and DSS have en-
larged livers. In other countries, however, hepatomegaly varies
from one epidemic to another, suggesting that the strain and/or
serotype of virus may influence liver involvement (35). Ele-
vated liver enzyme levels are common.
The primary pathophysiologic abnormality seen in DHF and
DSS is an acute increase in vascular permeability that leads to
leakage of plasma into the extravascular compartment, result-
ing in hemoconcentration and decreased blood pressure (1,
77). Plasma volume studies have shown a reduction of more
than 20% in severe cases. Supporting evidence of plasma leak-
age includes serous effusion found postmortem, pleural effu-
sion on X-ray, hemoconcentration, and hypoproteinemia. Early
diagnosis and aggressive fluid replacement therapy with good
nursing care can decrease fatality rates to 1% or less. Normal
saline or lactated Ringer’s solution can be used in patients with
mild DHF and DSS, but plasma or plasma expanders may be
necessary in those with severe cases. Details of effective man-
agement of DHF and DSS have been published previously (1).
There are no apparent destructive vascular lesions, suggest-
ing that the transient functional vascular changes are due to a
short-acting mediator (1). Once the patient is stabilized and
begins recovery, the extravasated fluid is rapidly reabsorbed,
causing a drop in the hematocrit.
Hemostatic changes in DHF and DSS involve three factors:
vascular changes, thrombocytopenia, and coagulation disor-
ders (1). Almost all DHF patients have increased vascular
fragility and thrombocytopenia, and many have abnormal
coagulograms, suggesting disseminated intravascular coagu-
lation, which is also evidenced by concomitant thrombocyto-
penia, a prolonged partial thromboplastin time, a decreased
fibrinogen level, and increased levels of fibrinogen degradation
products. GI hemorrhage is found at autopsy in the majority of
patients who die.
The pathogenesis of DHF and DSS is still controversial. Two
theories, which are not mutually exclusive, are frequently cited
to explain the pathogenetic changes that occur in DHF and
486 GUBLERCLIN. MICROBIOL. REV.
on June 5, 2013 by guest
DSS. The most commonly accepted is known as the secondary-
infection or immune enhancement hypothesis (57, 61, 62). This
hypothesis implies that patients experiencing a second infec-
tion with a heterologous dengue virus serotype have a signifi-
cantly higher risk for developing DHF and DSS (62). Preex-
isting heterologous dengue antibody recognizes the infecting
virus and forms an antigen-antibody complex, which is then
bound to and internalized by immunoglobulin Fc receptors on
the cell membrane of leukocytes, especially macrophages. Be-
cause the antibody is heterologous, however, the virus is not
neutralized and is free to replicate once inside the macro-
phage. Thus, it is hypothesized that prior infection, through a
process known as antibody-dependent enhancement (ADE),
enhances the infection and replication of dengue virus in cells
of the mononuclear cell lineage (15, 62, 66, 67, 106). It is
thought that these cells produce and secrete vasoactive medi-
ators in response to dengue infection, which causes increased
vascular permeability leading to hypovolemia and shock (see
The other hypothesis assumes that dengue viruses, like all
animal viruses, vary and change genetically as a result of se-
lection pressures as they replicate in humans and/or mosqui-
toes and that there are some virus strains that have greater
epidemic potential (37, 49, 123). Phenotypic expression of ge-
netic changes in the virus genome may include increased virus
replication and viremia, severity of disease (virulence), and
There is epidemiologic and laboratory evidence to support
both of these hypotheses; however, a detailed discussion is
beyond the scope of this review. They are not mutually exclu-
sive, and both are most probably valid (37). Excellent reviews
have recently been published on both viral pathogenesis and
immunopathogenesis (92, 127), which have summarized the
evidence concluding that both viral and host immunologic fac-
tors are involved in the pathogenesis of severe dengue disease.
This evidence is briefly presented below.
The pathology of DHF and DSS has been well studied (6, 7,
9), but that of dengue infections has not. Gross and micro-
scopic pathologic studies of tissues taken at autopsy in Thai-
land have shown diffuse petechial hemorrhages of most organs,
as well as serous effusions in the pericardial, pleural, and peri-
toneal cavities. Microscopically, perivascular edema and loss of
integrity of endothelial junctions are found. Dengue antigen
can be demonstrated in endothelial cells, but there is no ap-
parent damage to the blood vessels or endothelial cells.
In the liver, midzonal necrosis is common and is often in-
distinguishable from the pathologic changes caused by the
closely related yellow fever virus; Councilman bodies are com-
mon. In the brain, edema and hemorrhage have been observed
but pathologic changes associated with encephalitis have not.
However, recent isolations of dengue virus from the brain and
cerebrospinal fluid and intrathecal antibody production in the
latter suggest that on occasion, the dengue virus crosses the
blood-brain barrier. There is increased proliferation of reticu-
loendothelial cells in the bone marrow, spleen, lymph nodes,
Unfortunately, there are no good animal models for DHF
and DSS, making studies on pathogenesis difficult to interpret.
Primates are natural hosts for dengue virus, but those that have
been studied generally show no signs of disease; these animals
become infected and develop viremia, although at a lower titer
than humans (126). However, the results obtained with these
animals are conflicting. One of the few studies cited as evi-
dence that ADE occurs in vivo showed that rhesus monkeys
that experienced a secondary DEN-2 infection or had been
infused with dengue immune serum had higher viremias than
did monkeys with primary infections (60, 64, 65). All monkeys
were infected parenterally by needle inoculation. These results
could not be repeated in macaque monkeys infected naturally
by a mosquito bite or in chimpanzees infected parenterally;
primary and secondary infections of all serotypes and combi-
nations routinely showed that monkeys with primary infection
had viremia of the same or higher titer and longer duration
(126, 134). Clinical and laboratory studies on humans have
shown the same results (35, 47, 49, 51, 87).
In humans, viremias range in titer from barely detectable
(103), measured as 50% mosquito infection doses (MID50)
(125) to over 108.5MID50(51). Viremia usually peaks at the
time of or shortly after the onset of illness and may remain
detectable for various periods ranging from 2 to 12 days, de-
pending on the strain of virus and the immune status of the
individual (35, 43, 47, 49, 51, 87, 147). It has been suggested
that the severity of the disease associated with dengue infec-
tion is determined by the number of cells infected with the
virus and that the number of cells infected is related to ADE
infection of peripheral blood leukocytes in secondary infec-
tions (77). It follows that viremias should be higher in second-
ary infections, but this is not borne out by experimental infec-
tion of lower primates or by clinical studies on humans (35, 49,
51, 87, 126, 134). In fact, the opposite has usually been ob-
served; that is, viremias are usually higher in primary infec-
In secondary infections, the virus may be complexed with
antibody, making it undetectable by most current virus isola-
tion techniques. However, studies in humans during an out-
break of DEN-2 on an island in the Pacific (Tonga) showed
great variation in both the magnitude and duration of viremia
in primary infections (49). Some patients were identified on
the day of onset of mild illness and monitored for as long as 8
days. Blood samples were taken daily for viremia studies, and
uninfected mosquitoes were allowed to feed on some patients.
The majority of patients, confirmed as DEN-2 infection by
seroconversion, had undetectable viremia both by virus isola-
tion and by isodiagnosis (feeding mosquitoes on patients) (49).
When virus was detectable, viremia was at a low titer (?106
MID50) and of short duration (1 to 3 days). The same DEN-2
virus had caused explosive epidemics associated with severe
disease in neighboring islands in the previous 3 years, but in
Tonga it circulated for nearly a year without being detected in
a human population that was fully susceptible to DEN-2 virus
(silent transmission) (49). Two species of vector mosquitoes
(A. aegypti and Aedes tabu) were present in large numbers. The
data suggested that the virus had changed from an epidemic
strain to one that circulated in nature silently, causing mild or
inapparent disease. Similar observations have been made with
DEN-3 and DEN-1 viruses (41).
Molecular studies have demonstrated that dengue viruses
vary genetically in nature; unfortunately, phenotypic changes
that have been observed in the field have not yet been associ-
ated with genetic changes in the virus (26, 99, 100, 116, 117,
143). Collectively, however, the data suggest that viral factors
play a significant role in the pathogenesis of severe dengue
VOL. 11, 1998DENGUE AND DENGUE HEMORRHAGIC FEVER487
on June 5, 2013 by guest
Host Immune Factors
There is a large body of evidence, mostly obtained in vitro,
suggesting that heterotypic, nonneutralizing antibody binds
with dengue virus, facilitating the entry of the virus into cells of
the monocytic line and hence facilitating infection (15, 61, 62,
67, 68, 83). These data, along with epidemiologic observations
that the majority of patients with reported DHF cases are
experiencing a secondary infection, form the basis for the hy-
pothesis that preexisting heterotypic dengue antibody is a risk
factor for DHF (18, 57, 61, 62, 83, 133). The lack of a good
animal model for human disease and limitations of human
clinical studies have made it difficult to confirm this hypothesis.
In recent years, however, detailed, well-designed studies that
support the concept of immunopathogenesis of dengue infec-
tion in humans have been conducted. The results of these
studies have been comprehensively reviewed in a recent article
Briefly, the data show that dengue virus-specific memory
CD4?CD8?and CD4?CD8?lymphocytes are detectable in
humans after natural dengue infections. Infection with a single
dengue serotype induces both serotype-specific and serotype-
cross-reactive CD4?memory T cells, while CD8?T lympho-
cytes have virus-specific cytotoxic activity.
The pathogenetic mechanism responsible for the increased
vascular permeability observed in DHF and DSS is not known,
but it has been suggested that cytokines and chemical media-
tors such as tumor necrosis factor (TNF), interleukin-1 (IL-1),
IL-2, IL-6, platelet-activating factor (PAF), complement acti-
vation products C3a and C5a, and histamine may play a role.
CD4?T lymphocytes produce a number of cytokines, in-
cluding gamma interferon (IFN-?), IL-2, IL-4, IL-5, IL-6,
IL-10, and lymphotoxin. Moreover, monocytes/macrophages
which are infected by dengue viruses produce TNF, IL-1, IL-
1B, IL-6, and PAF. Finally, cytokine and chemical mediator
production is induced by other cytokines. Thus, once cytokines
are produced, a complex network of induction may further
increase the levels of cytokines and chemical mediators, result-
ing in even higher levels with synergistic effects on vascular
Kurane and Ennis have proposed a model of immunopatho-
genesis based on these observations (92). Briefly, it is hypoth-
esized that dengue virus infections of monocytes/macrophages
is enhanced by ADE. This enhancement is facilitated by the
fact that the dengue virus-specific CD4?T lymphocytes pro-
duce IFN-?, which in turn up-regulates the expression of FC-?
receptors. The increased number of dengue virus-infected
monocytes/macrophages results in increased T-cell activation,
which results in the release of increased levels of cytokines and
chemical mediators. Kurane and Ennis (92) hypothesized that
the rapid increase in the levels and the synergistic effects of
mediators such as TNF, IL-2, IL-6, IFN-?, PAF, C3a, C5a, and
histamine induce increased vascular permeability, plasma leak-
age, shock, and malfunction of the coagulation system, which
may lead to hemorrhage.
In summary, available evidence suggests that both viral and
host immune factors are involved in the pathogenesis of severe
dengue disease. Unfortunately, the role of each is not fully
understood and the lack of an animal model makes this a
difficult area to study. It would appear that different clinical
pathologic manifestations of the disease may be caused by
different pathogenetic mechanisms (37). For example, it has
been suggested that hepatic injury may relate more to viral
factors whereas vascular permeability may be mediated pre-
dominantly by the immune response (92, 127). Clearly, the
strain of virus is important since ADE apparently occurs only
with selected virus strains when tested in vitro. Also, the rate of
virus replication and infectivity in various tissues varies with
the strain of virus. Collectively, the data suggest that only
certain strains of dengue virus are associated with major epi-
demics and severe disease, and it is most likely that these are
the viruses that infect cells of the monocytic line via ADE (12,
37, 49, 116, 117).
A definitive diagnosis of dengue infection can be made only
in the laboratory and depends on isolating the virus, detecting
viral antigen or RNA in serum or tissues, or detecting specific
antibodies in the patient’s serum (47, 55, 148). There have
been two recent reviews of this topic (55, 148).
An acute-phase blood sample should always be taken as
soon as possible after the onset of suspected dengue illness,
and a convalescent-phase sample should ideally be taken 2 to
3 weeks later. Because it is frequently difficult to obtain con-
valescent-phase samples, however, a second blood sample
should always be taken from hospitalized patients on the day of
discharge from hospital.
Five basic serologic tests have been routinely used for diag-
nosis of dengue infection; hemagglutination-inhibition (HI),
complement fixation (CF), neutralization test (NT), immuno-
globulin M (IgM) capture enzyme-linked immunosorbent as-
say (MAC-ELISA), and indirect immunoglobulin G ELISA
(47, 55, 148). Regardless of the test used, unequivocal serologic
diagnosis depends upon a significant (fourfold or greater) rise
in the titer of specific antibodies between acute- and convales-
cent-phase serum samples. The antigen battery for most of
these serologic tests should include all four dengue virus sero-
types, another flavivirus (such as yellow fever virus, Japanese
encephalitis virus, or St. Louis encephalitis virus), a nonflavi-
virus (such as Chikungunya virus or eastern equine encepha-
litis virus), and ideally, an uninfected tissue control antigen
Of the above tests, HI has been the most frequently used; it
is sensitive, is easy to perform, requires only minimal equip-
ment, and is very reliable if properly done (28). Because HI
antibodies persist for long periods (up to 48 years and probably
longer) (58), the test is ideal for seroepidemiologic studies. HI
antibody usually begins to appear at detectable levels (titer of
10) by day 5 or 6 of illness, and antibody titers in convalescent-
phase serum specimens are generally at or below 640 in pri-
mary infections, although there are exceptions (4, 47). By con-
trast, there is an immediate anamnestic response in secondary
and tertiary dengue infections, and reciprocal antibody titers
increase rapidly during the first few days of illness, often reach-
ing 5,120 to 10,240 or more. Thus, a titer of ?1,280 in an
acute-phase or early convalescent-phase serum sample is con-
sidered presumptive evidence of a current dengue infection.
Such high levels of HI antibody persist for 2 to 3 months in
some patients, but antibody titers generally begin to wane by 30
to 40 days and fall below 1,280 in most patients (47). The major
disadvantage of the HI test is its lack of specificity, which
generally makes it unreliable for identifying the infecting virus
serotype. However, some patients with primary infections show
a relatively monotypic HI response that generally correlates
with the virus isolated (47).
The CF test is not widely used for routine dengue diagnostic
serologic testing. It is more difficult to perform, requires highly
trained personnel, and therefore is not used in most dengue
488 GUBLERCLIN. MICROBIOL. REV.
on June 5, 2013 by guest
laboratories. It is based on the principle that complement is
consumed during antigen-antibody reactions (20). CF antibod-
ies generally appear later than HI antibodies, are more specific
in primary infections, and usually persist for short periods,
although low levels of antibodies persist in some persons (47).
It is a valuable test to have in a diagnostic laboratory because
of the late appearance of CF antibodies; some patients thus
show a diagnostic rise in antibody titers by CF but have only
stable antibody titers by HI or ELISA (47). The greater spec-
ificity of the CF test in primary infections is demonstrated by
the monotypic CF responses when HI responses are broadly
heterotypic; it is not specific in secondary infections. The CF
test is useful for patients with current infections but is of
limited value for seroepidemiologic studies, where detection of
persistent antibodies is important.
The NT is the most specific and sensitive serologic test for
dengue viruses (33, 129). The most common protocol used in
dengue laboratories is the serum dilution plaque reduction NT.
In general, neutralizing-antibody titers rise at about the same
time or slightly more slowly than HI and ELISA antibody titers
but more quickly than CF antibody titers and persist for at least
48 years (58). Because the NT is more sensitive, neutralizing
antibodies are present in the absence of detectable HI anti-
bodies in some persons with past dengue infection.
Because relatively monotypic neutralizing-antibody re-
sponses are observed in properly timed convalescent-phase
serum, the NT can be used to identify the infecting virus in
primary dengue infections (4, 47, 129, 148). As noted above,
the HI and CF tests may also give monotypic responses to
dengue infection that generally agree with NT results. In cases
when the responses are monotypic, the interpretation of all
these tests is generally reliable. In secondary and tertiary in-
fections, determining the infecting virus serotype by NT or any
other serologic test is not reliable (90). Because of the long
persistence of neutralizing antibodies, the test may also be
used for seroepidemiologic studies. The major disadvantages
are the expense, time required to perform the test, and tech-
nical difficulty. It is therefore not used routinely by most lab-
MAC-ELISA has become the most widely used serologic
test for dengue diagnosis in the past few years. It is a simple,
rapid test that requires very little sophisticated equipment (17,
47, 78, 89, 97). Anti-dengue IgM antibody develops a little
faster than IgG antibody. By day 5 of illness, most patients
(80%) in Puerto Rico whose cases were subsequently con-
firmed by HI on paired serum samples or by virus isolation had
detectable IgM antibody in the acute-phase serum in this assay
(47). Nearly all patients (93%) developed detectable IgM an-
tibody 6 to 10 days after onset, and 99% of patients tested
between 10 and 20 days had detectable IgM antibody. The
rapidity with which IgM develops varies considerably among
patients. Although the dates of onset are not always recorded
accurately, some patients have detectable IgM on days 2 to 4
after the onset of illness whereas others may not develop IgM
for 7 to 8 days after onset (47). This variation is also reflected
in the amount of IgM produced and the length of time detect-
able IgM persists after infection. IgM antibody is produced by
patients with both primary and secondary dengue infections
and probably by persons with tertiary infections, although the
response in some secondary and probably most tertiary infec-
tions is low level and transient (89). IgM antibody titers in
primary infections are significantly higher than in secondary
infections, although it is not uncommon to obtain IgM titers of
320 in the latter cases (47). In some primary infections, detect-
able IgM persists for more than 90 days, but in most patients,
it has waned to an undetectable level by 60 days. A small
percentage of patients with secondary infections have no de-
tectable IgM antibody (89).
MAC-ELISA with a single acute-phase serum sample is
slightly less sensitive than the HI test with paired serum sam-
ples for diagnosing dengue infection (47). However, it has the
advantage of frequently requiring only a single, properly timed
blood sample. In one series of 288 patients during the 1986
epidemic in Puerto Rico, paired blood samples were tested by
HI and the single acute-phase sample from the same pairs were
tested by MAC-ELISA. The HI test on the pairs indicated that
228 (79%) were considered positive, while MAC-ELISA on
the single samples indicated that 203 (70%) were positive. Five
samples (1.7%) showed a false-positive response and 30 sam-
ples (10%) showed a false-negative response by MAC-ELISA
(47). When one considers the difficulty in obtaining second
blood samples and the long delay in obtaining conclusive re-
sults from the HI test, this low error rate would be acceptable
in most surveillance systems. It must be emphasized, however,
that because of the persistence of IgM antibody for 1 to 3
months, MAC-ELISA-positive results obtained with single se-
rum samples are only provisional and do not necessarily mean
that the dengue infection was current (47, 148). These results
do mean that it is reasonably certain that the person had a
dengue infection sometime in the previous 2 to 3 months.
Similarly, a negative result with an acute-phase sample may be
a false-negative result because the sample was taken before
detectable IgM appeared. Unfortunately, many dengue diag-
nostic laboratories have adopted MAC-ELISA as a confirma-
tory test and do not conduct follow-up tests to confirm the
presumptive IgM results. As noted above, this may be accept-
able for surveillance reports, but it is unacceptable in a clinical
setting. If this test is used to make patient management deci-
sions, it could result in a higher case fatality rate among pa-
tients with false-negative results.
The specificity of MAC-ELISA is similar to that of HI. In
both primary and secondary dengue infections, some mono-
typic responses may be observed, but in general, the response
is broadly reactive among both dengue virus and other flavivi-
rus antigens. With serum samples from patients with other
flavivirus infections such as Japanese encephalitis, St. Louis
encephalitis, and yellow fever, however, the response is gener-
ally more specific; while there may be some cross-reaction with
dengue antigens, most specimens show relatively monotypic
IgM responses to the infecting flavivirus (47). In dengue infec-
tions, monotypic IgM responses frequently do not correlate
with the virus serotype isolated from a patient. Therefore,
MAC-ELISA cannot be reliably used to identify the infecting
MAC-ELISA has become an invaluable tool for surveillance
of dengue, DHF, and DSS. In areas where dengue is not
endemic, it can be used in clinical surveillance for viral illness
or for random, population-based serosurveys, with the cer-
tainty that any positive results detected indicate recent infec-
tions (within the last 2 to 3 months). A properly timed sero-
survey by MAC-ELISA during an epidemic can determine very
quickly how widespread transmission has become. In areas
where dengue is endemic, MAC-ELISA can be used as an
inexpensive way to screen large numbers of serum specimens
with relatively little effort. It is especially useful for hospitalized
patients, who are generally admitted late in the illness after
detectable IgM is present in the blood (47), but it must be
emphasized again that this test should not be used to make
patient management decisions.
An indirect IgG-ELISA has been developed that is compa-
rable to the HI test and can also be used to differentiate
primary and secondary dengue infections (27). The test is sim-
VOL. 11, 1998 DENGUE AND DENGUE HEMORRHAGIC FEVER489
on June 5, 2013 by guest
ple and easy to perform and is thus useful for high-volume
testing. The IgG-ELISA is very nonspecific and exhibits the
same broad cross-reactivity among flaviviruses as the HI test
does; therefore, it cannot be used to identify the infecting
dengue virus serotype. However, it has a slightly higher sensi-
tivity than the HI test. As more data are accumulated on the
IgG-ELISA, it is expected to replace the HI test as the most
commonly used IgG test in dengue laboratories.
A number of commercial test kits for anti-dengue IgM and
IgG antibodies have become available in the past few years.
Unfortunately, the accuracy of most of these tests is unknown
because proper validation studies have not been done. Some
evaluations have been published (91, 96, 146, 153), but the
sample sizes have been too small to accurately measure sensi-
tivity and specificity. Moreover, the samples generally used
have represented only strong positives and negatives, with few
samples representing optical densities or positive-negative val-
ues in the equivocal range. One exception to this were kits that
were independently evaluated at CDC; both IgM and IgG test
kits had a high rate of false-positive results compared to stan-
dard tests, especially with samples with optical densities in the
equivocal range (91). Other studies, however, have given re-
sults comparable to those of standard tests (96, 146, 153). It is
anticipated that these test kits can be reformulated to make
them more accurate, making global laboratory-based surveil-
lance for dengue and DHF an attainable goal in the near
Four isolation systems have routinely been used for dengue
viruses; intracerebral inoculation of 1- to 3-day-old baby mice,
the use of mammalian cell cultures (primarily LLC-MK2cells),
intrathoracic inoculation of adult mosquitoes, and the use of
mosquito cell cultures (47, 55, 148).
Baby mice. Although all four dengue serotypes were initially
isolated from human serum by using baby mice (70, 74, 131),
over, because of the low sensitivity of the method, many wild-
type viruses cannot be isolated with baby mice. Those that are
isolated frequently require numerous passages to adapt the
viruses to growth in mice. This method is no longer recom-
mended for isolation of dengue viruses, but some laboratories
continue to use it (47). One advantage of using baby mice,
however, is that other arboviruses that cause dengue-like ill-
ness may be isolated with this system.
Mammalian cell culture. Mammalian cell cultures have
many of the same disadvantages as baby mice for isolation of
dengue viruses—they are expensive, slow, and insensitive (47,
55, 148, 155). As with isolation systems that use baby mice,
viruses that are isolated frequently require many passages be-
fore a consistent cytopathic effect can be observed in the in-
fected cultures. Although the use of this method continues in
some laboratories, it is not recommended (47, 148).
Mosquito inoculation. Mosquito inoculation is the most sen-
sitive method for dengue virus isolation (47, 125). Isolation
rates of up to 100% of serologically confirmed dengue infec-
tions are not uncommon, and this is the only method sensitive
enough for routine successful virologic confirmation of fatal
DHF and DSS cases (47, 50, 139, 147). Moreover, there are
many endemic dengue virus strains that can be recovered only
by this method (47, 49, 54).
Four mosquito species have been used for virus isolation,
A. aegypti, A. albopictus, Toxorhynchities amboinensis, and
T. splendens. Male and female mosquitoes are equally suscep-
tible; dengue viruses generally replicate to high titers (106to
107MID50) in as little as 4 to 5 days, depending on the tem-
perature of incubation. Dengue viruses replicate in most mos-
quito tissues, including the brain. A recent variation on this
method involves intracerebral inoculation of larval and adult
Toxorhynchities mosquitoes (95, 142). However, these modifi-
cations neither increase sensitivity nor provide other advan-
tages over intrathoracic inoculation (125).
Virus detection in the mosquito, regardless of the species, is
generally performed by the direct fluorescent-antibody DFA
test on mosquito tissues, usually brain or salivary glands (47,
50, 86). The direct conjugate is prepared from pooled human
serum and has broadly reactive anti-dengue (or anti-flavivirus)
activity. Alternatively, a polyclonal mouse ascitic fluid or a
flavivirus group-reactive monoclonal antibody can be used in
an indirect fluorescent-antibody (IFA) test with an anti-mouse
immunoglobulin G–fluorescein isothiocyanate conjugate that
is commercially available.
The mosquito inoculation technique has the disadvantages
of being labor-intensive and requiring an insectary to produce
large numbers of mosquitoes for inoculation. Also, unless strict
safety precautions are maintained, the chance of laboratory
infections increases, although this risk can be eliminated by
using male Aedes mosquitoes or nonbiting Toxorhynchites spe-
cies for inoculation (47, 125).
Mosquito cell culture. Mosquito cell cultures are the most
recent addition to dengue virus isolation methodology (47, 52,
76, 88, 141). Three cell lines of comparable sensitivity are most
frequently used (88). The first cell line developed, and still the
most widely used, is the C6/36 clone of A. albopictus cells (76).
The use of these cell lines has provided a rapid, sensitive,
and economical method for dengue virus isolation. Moreover,
many serum specimens can be processed easily, making the
method ideal for routine virologic surveillance (52). However,
this system is less sensitive than mosquito inoculation (47). For
example, on average, 10 to 15% more viruses were isolated
from patients in Puerto Rico by the mosquito inoculation tech-
nique than by mosquito cell cultures (22, 43, 47). However, the
sensitivity of the mosquito cell lines may vary with the strain of
virus. In samples from an epidemic in Mozambique, more than
twice as many DEN-3 viruses were isolated by mosquito inoc-
ulation than by the use of mosquito cells (54).
Dengue antigen can be detected in infected-cell cultures by
DFA or IFA tests with the conjugates used for mosquito tis-
sues (52). Some workers, however, prefer to use cytopathic
effect to detect infection, especially with AP-61 cells. However,
this method alone will miss many dengue viruses that do not
replicate rapidly in mosquito cells (47).
The methods selected for virus isolation depend upon the
laboratory facilities available. Because the mosquito inocula-
tion technique is the most sensitive, it is the method of choice
for fatal cases or patients with severe hemorrhagic disease. Use
of the mosquito cell lines is the method of choice for routine
virologic surveillance. Even though cell cultures are less sen-
sitive than mosquito inoculation, this disadvantage is more
than offset by the ease with which large numbers of samples
can be processed in a relatively short time.
The method of choice for dengue virus identification is IFA
with serotype-specific monoclonal antibodies produced in tis-
sue culture or mouse ascitic fluids and an anti-mouse immu-
noglobulin G-fluorescein isothiocyanate conjugate (47, 52,
55, 72). This test can be easily performed with infected cell
cultures, mosquito brain or tissue squashes, mouse brain
squashes, or even on formalin-fixed tissues embedded in par-
490 GUBLERCLIN. MICROBIOL. REV.
on June 5, 2013 by guest
affin and sectioned for histopathologic testing (56). It is simple
and reliable and is the most rapid method. Moreover, it allows
the detection of multiple viruses in patients with concurrent
infections with more than one serotype (53, 94).
The success of isolating dengue virus from human serum
depends on several factors (47). First, the manner in which the
specimen has been handled and stored is important. Virus
activity can be inhibited by heat, pH, and several chemicals;
therefore, improper handling is often an important cause of
unsuccessful virus isolation. Second, the level of viremia may
vary greatly depending on the time after onset, the antibody
titers, and/or the strain of the infecting virus. Viremia usually
peaks at or shortly before the onset of illness and may be
detectable for an average of 4 to 5 days (43, 47, 51, 147). The
success of virus isolation decreases rapidly with the appearance
of IgM antibody (47, 148). With some virus strains, however,
viremia may remain below the level of detectability throughout
the illness (47, 49). Finally, the virus isolation system used
influences the success of isolation, as discussed above.
New Diagnostic Technology
In recent years, several new methods of diagnosis have been
developed and have proven very useful in dengue diagnosis.
This topic has recently been reviewed extensively (29). The
various methods are discussed briefly below.
PCR. Reverse transcriptase PCR (RT-PCR) has been de-
veloped for a number of RNA viruses in recent years and has
the potential to revolutionize laboratory diagnosis; for dengue,
RT-PCR provides a rapid serotype-specific diagnosis. The
method is rapid, sensitive, simple, and reproducible if properly
controlled and can be used to detect viral RNA in human
clinical samples, autopsy tissues, or mosquitoes (29, 55, 98,
148). Although RT-PCR has similar sensitivity to virus isola-
tion systems that use C6/36 cell cultures, poor handling, poor
storage, and the presence of antibody usually do not influence
the outcome of PCR as they do virus isolation. A number of
methods involving primers from different locations in the ge-
nome and different approaches to detect the RT-PCR prod-
ucts have been developed over the past several years (29, 55,
It must be emphasized, however, that RT-PCR should not
be used as a substitute for virus isolation. The availability of
virus isolates is important for characterizing virus strain differ-
ences, since this information is critical for viral surveillance and
pathogenesis studies. Unfortunately, many laboratories are
now conducting RT-PCR tests without proper quality control,
i.e., virus isolation or serologic testing. Since RT-PCR is highly
sensitive to amplicon contamination, without proper controls
false-positive results may occur. Improvements in this technol-
ogy, however, should make it even more useful in the future
Hybridization probes. The hybridization probe method de-
tects viral nucleic acids with cloned hybridization probes (29,
148). Probes with variable specificity ranging from dengue
complex to serotype specific can be constructed depending on
the genome sequences used. The method is rapid and relatively
simple and can be used on human clinical samples as well as
fixed autopsy tissues. Unfortunately, hybridization probes have
not been widely used or evaluated in the diagnostic laboratory.
Preliminary data suggest that this method is less sensitive than
RT-PCR, but like PCR, the outcome of the test is not influ-
enced by the presence of neutralizing antibodies or other in-
hibitory substances. Even so, the difficulties of working with
RNA and the technical expertise required to obtain reproduc-
ible results make this method more suitable as a research tool
than as a routine diagnostic test (29, 30, 148).
Immunohistochemistry. A major problem in dengue labo-
ratory diagnosis has been confirmation of fatal cases. In most
instances, only a single serum sample is obtained and serologic
testing is therefore of limited value. Also, most patients die at
the time of or slightly after defervescence, when virus isolation
is difficult. With new methods of immunohistochemistry, it is
now possible to detect dengue viral antigen in a variety of
tissues (56, 156). Although immunofluorescence tests were
used in the past, newer methods involving enzyme conjugates
such as peroxidase and phosphatase in conjunction with either
polyclonal or monoclonal antibodies are greatly improved
(156). Because tissues can be fresh or fixed, autopsies should
be performed in all cases of suspected DHF with a fatal out-
come (47, 50).
PREVENTION AND CONTROL
Prevention and control of dengue and DHF has become
more urgent with the expanding geographic distribution and
increased disease incidence in the past 20 years (36, 39, 41, 42,
45, 48, 61, 63, 104). Unfortunately, tools available to prevent
dengue infection are very limited. There is no vaccine currently
available (see below), and options for mosquito control are
limited. Clearly, the emphasis must be on disease prevention if
the trend of emergent disease is to be reversed.
Effective disease prevention programs must have several
integrated components, including active laboratory-based sur-
veillance, emergency response, education of the medical com-
munity to ensure effective case management, community-based
integrated mosquito control, and effective use of vaccines when
they become available (37, 44).
The first candidate dengue vaccines were developed shortly
after the viruses were first isolated by Japanese and American
scientists (81, 132). Despite considerable work over the years,
an effective safe vaccine was never developed (3, 59, 69, 130,
151). The World Health Organization designated the develop-
ment of a tetravalent dengue vaccine a priority for the most
cost-effective approach to dengue prevention (13, 14). Effec-
tive vaccination to prevent DHF will most probably require a
tetravalent vaccine, because epidemiologic studies have shown
that preexisting heterotypic dengue antibody is a risk factor for
DHF (18, 57, 61, 62, 133). With the support of the World
Health Organization, considerable progress in developing a
vaccine for dengue and DHF has been made in recent years (8,
10, 11, 145, 154). Promising candidate attenuated vaccine vi-
ruses have been developed and have been evaluated in phase
I and II trials in Thailand as monovalent, bivalent, trivalent,
and tetravalent formulations (8). A commercialization contract
has been signed, and the tetravalent vaccine formulation is
currently undergoing repeat phase I trials in the United States.
Current progress on the live attenuated dengue vaccine has
been recently reviewed (8).
Promising progress in the development of alternative vac-
cine strategies using new molecular technology has also been
made in recent years. Recent approaches include the use of
inactivated whole-virion vaccines (23), synthetic peptides (5,
121, 122), subunit vaccines (31, 101, 140), vector expression,
recombinant live vector systems (23, 102), infectious cDNA
clone-derived vaccines (16, 25, 79, 80, 82, 93, 113), and naked
DNA (24, 84). The last two approaches appear to be the most
promising. An infectious clone of the DEN-2, PDK-53 vaccine
VOL. 11, 1998 DENGUE AND DENGUE HEMORRHAGIC FEVER 491
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candidate virus from Thailand (11) has been constructed, and
work is in progress to construct chimeric viruses by inserting
the capsid, premembrane, and envelope genes of DEN-1,
DEN-3 and DEN-4, into the DEN-2 PDK-53 backbone (82).
Through genetic manipulation, these recombinants may be
made to grow better and to be more immunogenic and safer
than the original live attenuated virus vaccine candidates. In
addition, chimeras are being constructed by inserting the struc-
tural proteins of dengue viruses into the infections clones of
the 17D yellow fever and the SA14-14-2 Japanese encephalitis
vaccine viruses (103a). The development of naked DNA vac-
cines is in its infancy but shows great promise (24). This area
has been recently reviewed (23, 144).
Despite the promising progress, it is unlikely that an effec-
tive, safe, and economical dengue vaccine will be available in
the near future. A major problem has been and continues to be
lack of financial support for dengue vaccine research. Thus,
other approaches to disease prevention must be developed by
using the program components outlined above.
Disease Prevention Programs
Active surveillance. Active disease surveillance is an impor-
tant component of a dengue prevention program. In addition
to monitoring secular trends, the goal of surveillance should be
to provide an early-warning or predictive capability for epi-
demic transmission, the rationale being that if epidemics can
be predicted, they can be prevented by initiating emergency
mosquito control. For epidemic prediction, health authorities
must be able to accurately monitor dengue virus transmission
in a community and be able to tell at any point in time where
transmission is occurring, which virus serotypes are circulating,
and what kind of illness is associated with dengue infection (44,
118). To accomplish this, the system must be active and labo-
This type of proactive surveillance system must have at least
three components that place the emphasis on the inter- or pre-
epidemic period. These components include a sentinel clinic
and physician network, a fever alert system that uses commu-
nity health workers, and a sentinel hospital system (Table 1).
The sentinel clinic and physician network and fever alert sys-
tem are designed to monitor nonspecific viral syndromes in the
community. This is especially important for dengue viruses
because they are frequently maintained in tropical urban cen-
ters in a silent or unrecognized transmission cycle, often pre-
senting as nonspecific viral syndromes. The sentinel clinic and
physician network and fever alert system are also very useful
for monitoring other common infectious diseases such as in-
fluenza, measles, malaria, typhoid, and leptospirosis.
In contrast to the sentinel clinic and physician component,
which requires sentinel sites to monitor routine viral syn-
dromes, the fever alert system relies on community health and
sanitation workers to be alert to any increase in febrile activ-
ity in their community and to report this to the central epide-
miology unit of the health department. Investigation by the
health department should be immediate but flexible; it may
involve telephone follow-up or active investigation by an epi-
demiologist who visits the area to take samples.
The sentinel hospital component should be designed to
monitor severe disease. Hospitals used as sentinel sites should
include all of those that admit patients for severe infectious
diseases in the community. This network should also include
infectious-disease physicians, who usually consult on such
cases. The system can target any type of severe disease, but for
dengue, it should include all patients with any hemorrhagic
manifestation; an admission diagnosis of viral encephalitis,
aseptic meningitis, or meningococcal shock; and/or a fatal out-
come following a viral prodrome (50).
All three proactive surveillance components require a good
public health laboratory to provide diagnostic support in virol-
ogy, bacteriology, and parasitology. The supporting laboratory
does not have to be able to test for all agents but should know
where to refer specimens for testing, e.g., to the World Health
Organization Collaborating Centers for Reference and Re-
This proactive surveillance system is designed to monitor
disease activity during the interepidemic period, prior to epi-
demic transmission. Individually, the three components are not
sensitive enough to provide effective early warning, but when
used collectively, they can often accurately predict epidemic
activity (44). Table 1 outlines the proactive surveillance system
for dengue and DHF, listing the types of specimens and labo-
ratory tests required. It must be emphasized that once epi-
demic transmission has begun, the surveillance system should
be refocused on severe disease rather than viral syndromes.
The surveillance system should be designed and adapted to the
local conditions where it will be initiated. However, this system
should be closely tied to the mosquito control programs that
will be responsible for reacting to surveillance data to initiate
emergency disease prevention in all areas.
Mosquito control. Prevention and control of dengue and
DHF currently depends on controlling the mosquito vector,
A. aegypti, in and around the home, where most transmission
occurs. Space sprays with insecticides to kill adult mosquitoes
are not usually effective (38, 107, 115) unless they are used
indoors. The most effective way to control the mosquitoes that
transmit dengue is larval source reduction, i.e., elimination or
cleaning of water-holding containers that serve as the larval
TABLE 1. Components of laboratory-based, proactive surveillance for dengue and DHFa
Type of surveillanceSamplesb
Sentinel clinic and physician
Blood from representative cases of viral syndrome,
taken 3–15 days after onset of illness
Representative samples taken year round and processed
weekly for virus isolation and for IgM antibodies
Fever alert system Blood samples from representative cases of febrile
Increased febrile illness in community investigated
immediately; samples tested as above
Sentinel hospital systemc
Blood and tissue samples taken during
hospitalization and/or at death
All hemorrhagic disease and all viral syndromes with fatal
outcome investigated immediately and tested as above
aEmphasis should be placed on the interepidemic period, using a nonspecific case definition. After an epidemic begins and after the virus serotype(s) is known, the
case definition should be made more specific and surveillance should be focused on severe disease.
bAll samples are processed weekly for virus isolation and/or for dengue virus-specific IgM antibodies.
cSentinel sites should be geographically representative.
492 GUBLERCLIN. MICROBIOL. REV.
on June 5, 2013 by guest
habitats for A. aegypti in the domestic environment (38, 115,
There are two approaches to effective A. aegypti control in-
volving larval source reduction. In the past, the most effective
programs have had a vertical, paramilitary organizational
structure with a large staff and budget (137). These successful
programs were also facilitated by the availability of residual
insecticides, such as DDT, that contributed greatly to ridding
the mosquito from the domestic environment. Unfortunately,
in all of these programs, without exception, there has been no
sustainability, because once the mosquito and the disease were
controlled, limited health resources were moved to other com-
peting programs and the A. aegypti population rebounded to
levels where epidemic transmission occurred. The most recent
example of this lack of sustainability is Cuba, where A. aegypti
had been effectively controlled and dengue transmission had
been prevented since 1981. The vertically structured Cuban
program has recently failed, most probably because of lack of
support; the result was a major dengue epidemic in 1997 (2,
In recent years, emphasis has been placed on community-
based approaches to larval source reduction to provide pro-
gram sustainability (38). The rationale is that sustainable
A. aegypti control can be accomplished only by the people who
live in the houses where the problems occur and by the people
who help create the mosquito larval habitats by their lifestyles
(38). Community participation in and ownership of prevention
programs require extensive health education and community
outreach. Unfortunately, this approach is a very slow process.
Therefore, it has been proposed that a combination top-down
and bottom-up approach be used, the former to achieve im-
mediate success and the latter to provide program sustainabil-
ity (38). The effectiveness of this approach remains unknown.
Mosquito control for dengue prevention has recently been
Prevention of Dengue in Travelers
There is no completely effective method of preventing den-
gue infection in travelers visiting tropical areas. The risk of
infection can be significantly decreased, however, by under-
standing the basic behavior and feeding habits of the mosquito
vector and by taking a few simple precautions to decrease
exposure to infective mosquito bites. Female A. aegypti mos-
quitoes prefer to feed indoors, with peak biting activity occur-
ring for 2 to 3 hours after daybreak and for 3 to 4 hours before
nightfall. Although the risk may be higher at these times, it is
important to remember that the mosquito may feed indoors at
anytime during the day as well as outdoors, especially on over-
cast days. Precautions, therefore, include staying in screened
or air-conditioned rooms, spraying these rooms with aerosol
bomb insecticides to kill adult mosquitoes indoors (especially
in bedrooms), using a repellent containing dimethyl-metatolu-
amide (DEET) on exposed skin, and wearing protective cloth-
ing treated with a similar repellant. The risk of exposure may
be lower in modern, air-conditioned hotels with well-kept
grounds and in rural areas.
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