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Emerging infectious diseases (EIDs) of free-living wild animals can be classified into three major groups on the basis of key epizootiological criteria: (i) EIDs associated with “spill-over” from domestic animals to wildlife populations living in proximity; (ii) EIDs related directly to human intervention, via host or parasite translocations; and (iii) EIDs with no overt human or domestic animal involvement. These phenomena have two major biological implications: first, many wildlife species are reservoirs of pathogens that threaten domestic animal and human health; second, wildlife EIDs pose a substantial threat to the conservation of global biodiversity.
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DOI: 10.1126/science.287.5452.443
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Emerging Infectious Diseases of Wildlife-- Threats
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REVIEW: WILDLIFE ECOLOGY
Emerging Infectious Diseases of Wildlife
Threats to Biodiversity and Human Health
Peter Daszak,
1, 2
* Andrew A. Cunningham,
3
Alex D. Hyatt
4
Emerging infectious diseases (EIDs) of free-living wild animals can be classified into
three major groups on the basis of key epizootiological criteria: (i) EIDs associated
with “spill-over” from domestic animals to wildlife populations living in proximity; (ii)
EIDs related directly to human intervention, via host or parasite translocations; and
(iii) EIDs with no overt human or domestic animal involvement. These phenomena
have two major biological implications: first, many wildlife species are reservoirs of
pathogens that threaten domestic animal and human health; second, wildlife EIDs
pose a substantial threat to the conservation of global biodiversity.
T
he past two decades have seen the
emergence of pathogenic infectious
diseases, such as acquired immunode-
ficiency syndrome, multidrug-resistant tuber-
culosis, and tick-borne diseases, which rep-
resent a substantial global threat to human
health (1). Emergence is associated with a
range of underlying causal factors (1, 2).
These include interactions with zoonotic
pathogens within a host-parasite continuum
between wildlife, domestic animal, and hu-
man populations (Fig. 1). In this review, we
identify a number of EIDs that predominantly
involve wildlife [(3, 4 ), Table 1, and Web table
1(5)]. We define wildlife EIDs by applying
criteria similar to those used to define human
EIDs (1, 2) and categorize them according to
their specific characteristics that are “emerging”
or novel (Table 2) and to their epizootiology.
Wildlife EID, Past and Present
Parallels between human and wildlife EIDs
extend to early human colonization of the
globe and the dissemination of exotic patho-
gens. In the same way that Spanish conquis-
tadors introduced smallpox and measles to
the Americas, the movement of domestic and
other animals during colonization introduced
their own suite of pathogens. The African
rinderpest panzootic of the late 1880s and
1890s is a paradigm for the introduction,
spread, and impact of virulent exotic patho-
gens on wildlife populations (4, 6). This
highly pathogenic morbillivirus disease, en-
zootic to Asia, was introduced into Africa in
1889. The panzootic front traveled 5000 km
in 10 years, reaching the Cape of Good Hope
by 1897, extirpating more than 90% of Ken-
ya’s buffalo population and causing second-
ary effects on predator populations and local
extinctions of the tsetse fly. Populations of
some species remain depleted and the persis-
tence of rinderpest in eastern Africa contin-
ues to threaten bovid populations.
Pandemics of cholera, influenza, and oth-
er diseases seriously impact human popula-
tions. Such clear-cut panzootic outbreaks of
diseases in wildlife are probably rare events,
but a lack of awareness and reporting, partic-
ularly during the earlier decades of European
expansion, almost certainly belies their true
extent. Historically, wildlife diseases have been
considered important only when agriculture or
human health have been threatened. However,
because of outbreaks of disease in endangered
species (7), increasing veterinary involvement
(8, 9), and advances in host-parasite population
biology (4, 10), the threat of wildlife diseases is
now taken more seriously (11–13).
Common Causal Themes
The increasing number of wildlife EIDs may
reflect increasing vigilance, but parallels be-
tween causal factors driving the emergence of
human and wildlife EIDs suggest that this trend
is valid (14) (Fig. 1). Disease emergence most
frequently results from a change in ecology of
host, pathogen, or both (15). Human population
expansion has driven the emergence of EIDs
via increasing population density, especially in
urban areas (dengue, cholera), and encroach-
ment into wildlife habitat (Ross River virus
disease) (2, 16). This encroachment may have
been a key factor in Africa for the global emer-
gence of Marburg and Ebola viruses and human
immunodeficiency virus (HIV) (2, 17). Pres-
sures of human encroachment on shrinking
wildlife habitat also cause increased wildlife
population densities and the emergence of wild-
life EIDs (11–13, 18). The international move-
ment of livestock and modern agricultural prac-
tices have led to EIDs such as rinderpest in
Africa and bovine spongiform encephalitis
(BSE) in Europe. Similar situations occur in
wildlife populations managed either in situ or in
captivity. The extent of in situ management
may be substantially underestimated. Recent
analysis (19) suggests that 15,000 tons of pea-
1
Institute of Ecology, University of Georgia, Athens,
GA 30602, USA.
2
Infectious Disease and Pathology
Activity, Division of Viral and Rickettsial Diseases,
National Center for Infectious Diseases, Centers for
Disease Control and Prevention, Atlanta, GA 30333,
USA.
3
Institute of Zoology, Zoological Society of Lon-
don, Regent’s Park, London NW1 4RY, UK.
4
Australian
Animal Health Laboratory, CSIRO, Private Bag 24,
Geelong, Victoria 3220, Australia.
*To whom correspondence should be addressed. E-
mail: daszak@uga.edu
Fig. 1. The host-parasite ecological
continuum (here parasites include
viruses and parasitic prokaryotes).
Most emerging diseases exist
within a host and parasite contin-
uum between wildlife, domestic
animal, and human populations.
Few diseases affect exclusively any
one group, and the complex rela-
tions between host populations
set the scene for disease emer-
gence. Examples of EIDs that over-
lap these categories are canine dis-
temper (domestic animals to wild-
life), Lyme disease (wildlife to hu-
mans), cat scratch fever (domestic
animals to humans) and rabies (all
three categories). Arrows denote
some of the key factors driving
disease emergence.
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nuts are fed annually to United Kingdom gar-
den birds. This form of provisioning has led to
the emergence of infection by Salmonella ty-
phimurium DT40 and Escherichia coli 086:
K61 in Britain and Mycoplasma gallisepticum
in the United States. because of a high density
and diversity of birds at feeding stations (19).
The maintenance of brucellosis in bison in the
Grand Teton National Park (United States) is
related to the presence of disease in managed
sympatric elk (20). Even changes in arable
farming may lead to disease emergence, such as
the shift in agriculture from the eastern United
States to the Midwest, which allowed refores-
tation of New England, providing the condi-
tions for Lyme disease emergence (21).
Anthropogenic global climate change is
likely to cause major changes to the geographic
range and incidence of arthropod-borne infec-
tious diseases. Expansion of mosquito vector
geographical ranges has been proposed to ex-
plain the reemergence of malaria and dengue in
South America, central Africa, and Asia during
the 1980s and 1990s (22). Similarly, the biting
midge vector for African horse sickness (AHS)
and bluetongue has recently invaded Europe
and North Africa (23).
Spill-Over and “Spill-Back”
The transmission of infectious agents from
reservoir animal populations (often domesti-
cated species) to sympatric wildlife, termed
spill-over, underpins the emergence of a
range of wildlife EIDs. Spill-over is a partic-
ular threat to endangered species, because the
presence of infected reservoir hosts can lower
the pathogen’s threshold density and lead to
local (population) extinction (8, 9, 11). Pop-
ulations of the African wild dog (Lycaon
pictus) have been declining since the 1960s.
This species is now endangered and, with a
fragmented population of less than 5000, is
susceptible to stochastic events such as dis-
ease outbreaks. Wild dogs became extinct in
the Serengeti in 1991, concurrent with
epizootic canine distemper in sympatric do-
mestic dogs (18, 24). Rabies has also caused
mortality of wild dogs, and a viral variant
Table 1. Selected emerging* infectious diseases (EIDs) of humans and
terrestrial wildlife, classified to demonstrate degrees of involvement of
humans, domesticated animals, and wildlife. Taken together with those
mentioned in text, this list is representative, and examples are chosen
purely to demonstrate the range of pathogens, hosts, and factors under-
lying emergence. The expanded table (Web table 1) is available as
supplementary material (5). EIDs that involve only humans, both humans
and domesticated animals, or domesticated animals only are not in-
cluded. EIDs of marine environments are covered in a separate, related
paper (3).
Disease and
class of EID
Pathogen Hosts
Geography of
emergence
Impact on wildlife
populations
Factors associated
with emergence
Refs.
Humans– domestic animals–wildlife
Hendra virus disease 1 Hendra virus
(paramyxovirus)
Humans, horses, fruit bat
reservoir
Australia,
Papua New
Guinea
Unknown Unknown (16)
Nipah virus disease 1 Nipah virus
(paramyxovirus)
Humans, domestic pigs
and dogs, fruit bats
Malaysia and
Singapore
Unknown Unknown (45)
Cryptosporidiosis 4 Cryptosporidium
parvum (protozoan
parasite)
Humans, cattle, wild
rodents and other
mammals
Europe, USA Unknown Farming practices,
emergence of
HIV, cross-
species transfer
(36)
Humans–wildlife
Hantavirus pulmonary
syndrome 1
Sin Nombre and other
strains of hantavirus
(bunyaviruses)
Humans, Peromyscus spp.,
and other rodents
Americas, esp.
SW USA
Probably little
impact
ENSO event and
human
encroachment
(37)
Marburg virus and
Ebola virus
hemorrhagic fever 1
Marburg and Ebola
virus (filoviruses)
Humans and nonhuman
primates, insectivorous or
fruit bat reservoir
suspected
Sub-Saharan
Africa,
Indonesia,
Philippines
High mortality in
captive and wild
nonhuman
primates
Marburg:
translocation of
infected
monkeys for lab
research; Ebola:
contact with
infected human
or nonhuman
carcasses or
patients
(17)
Human monocytotropic
granulocytotropic
ehrlichioses 1,4
Ehrlichia chaffeensis, E.
phagocytophila and
E. equi (tick-borne
rickettsia)
Humans, cervids, horses,
dogs and others
USA, Europe,
Africa
Apparently little
impact, but
underresearched
Uncertain (64)
Plague 4 Yersinia pestis
(bacterium)
Humans, wide range of
mammalian (especially
rodent) hosts
Panglobal,
notably
India, SW
USA
High mortality in
prairie dog
towns during
epizootics
leading to
declines in
endangered
black-footed
ferret
Enzootic foci are
remnants of last
panzootic
outbreak in
early 1900s
(65)
Domestic animals–wildlife
Canine distemper 3 Canine distemper virus
(morbillivirus)
Wide range of carnivores USA, Africa Extinction of
African wild dog
and black-footed
ferret
populations;
threat to
Ethiopian wolf
Spill-over from
domesticated
dogs
(7, 24)
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common in sympatric domestic dogs has
been identified from one such incident (25).
The geographic expansion of human popula-
tions and the consequent encroachment of
domestic dog carriers may explain the emer-
gence and impact of rabies in wild dogs in the
Serengeti (25).
Spill-over epizootic outbreaks represent
a serious threat both to wildlife and, via
reverse spill-over (“spill-back”), to sym-
patric populations of susceptible domesti-
cated animals. Brucellosis was probably
introduced into America with cattle. In Yel-
lowstone National Park (United States), the
presence of this disease in elk and bison is
considered a potential threat to domesti-
cated cattle grazing at the park boundaries
(20). Other examples of spill-over infec-
tions include sarcoptic mange in foxes (Eu-
rope) and wombats (Australia) and bovine
tuberculosis (global). The latter threatens to
spill back to domestic livestock (8, 9) and,
ultimately, to humans.
Emergence Owing to Host or Parasite
Translocations
The translocation of wildlife for conserva-
tion, agriculture, and hunting occurs on a
global scale, with an inherent risk of exposure
of wildlife species to exotic infectious agents
Table 1. (continued)
Disease and
class of EID
Pathogen Hosts
Geography of
emergence
Impact on wildlife
populations
Factors associated
with emergence
Refs.
Humans– domestic animals–wildlife (continued)
Canine parvovirus disease 1 Canine parvovirus Canids Europe, USA Suspected cause
of gray wolf
population
declines; threat
to Ethiopian
wolf
Evolution of novel
strain, contact
with domestic
dogs
(66)
Varroasis 2 Varroa jacobsoni
(mite)
Wild and domesticated
honeybees
Panglobal
except
Australasia
and C. Africa
Catastrophic mass
mortality, e.g.,
75% loss of
feral colonies in
California
Introduction of
hosts into
enzootic region
(28)
Neurotropic velogenic
Newcastle disease 2
Newcastle disease
virus
(paramyxovirus)
Double-crested cormorants,
pelicans, gulls, poultry
Canada, USA High mortality
rates (up to 80
to 90%)
Unknown (67)
Sarcoptic mange 2 Sarcoptes scabiei
(mite)
Mammals Australia, UK,
Sweden
Recent threat to
wildlife in
Sweden;
emerging threat
to wombats in
Australia
Dispersal of
infected
wildlife;
domestic
dog–wildlife
interactions
(68)
Wild animals only
Amphibian
chytridiomycosis 1
Batrachochytrium
dendrobatidis
(fungus)
Range of amphibian species,
including anurans and
salamanders
Australia,
Central
and North
America
Mass mortalities,
population
declines, local
and possibly
global
extinctions
Unknown;
evidence
indicates
introduced
pathogen and
possibly
associated with
climate change
in C. America
(40, 41)
Viral chorioretinitis
“Kangaroo blindness” 1
Wallal virus and
possibly Warrego
virus; vector-borne
orbivirus
Kangaroo spp. Australia Substantial
mortalities
Unknown;
possibly
weather related
(69)
Crayfish plague 2 Aphanomyces astaci
(fungus)
Crayfish Europe High mortality
rates with
population
declines,
threatening
native species
with extinction
Introduction of
infected North
American
crayfish (in
which the
infection is
enzootic and
nonlethal)
(70)
Captive wild animals
Steinhausiosis Steinhausia sp.
(protozoan parasite)
Partula snails Global extinction
of P. turgida
Unknown (54, 55)
Avian malaria Plasmodium spp.
(protozoan
parasites)
Birds High mortality in
susceptible
species, e.g.,
penguins
Translocation of
naı¨ve animals
to enzootic
regions
(71)
Pneumonia Ophidian
paramyxovirus
Snakes Epizootics with
high mortality
rates
Unknown (72)
*Before this review, few wildlife diseases had been labeled “emerging” (19, 73). The criteria used to distinguish emerging from established infectious diseases are described in the
introduction and in Table 2. EID are classified on the basis of their “emerging” characteristics, according to criteria listed in Table 2. EID of captive wild animals are not classified
since geographic range is not relevant in these cases. Not all hosts are listed. The identity of reservoir hosts for some EID remains uncertain.
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(4, 8, 9). Translocation and introduction of
animals to new geographic regions corre-
spond to increased human global travel and
commerce as underlying factors for infec-
tious disease emergence (2, 14). The translo-
cation of fish, and possibly amphibians, may
have driven the emergence of ranavirus
epizootics as threats to freshwater fish and
wild herpetofauna (26 ). Similarly, a rabies
epizootic in the mid-Atlantic region of the
United States resulted from translocation of
infected raccoons from a southeastern U.S.
enzootic focus (27). The introduction of po-
tential hosts into new geographic areas with-
out co-introduction of pathogens can also
result in disease emergence. For example,
varroasis, a disease of honeybees caused by
the mite Varroa jacobsoni, spread globally
(except Australia) after the European honey-
bee (Apis mellifera) was introduced into Asia
(28).
This form of emergence is a particular
concern to conservation programs that bring
allopatric endangered species into close prox-
imity or that alter basic host-parasite vari-
ables such as population density and structure
(8, 9, 11, 13). Molecular analyses of a newly
discovered herpesvirus associated with dis-
ease in captive elephants indicate that a nor-
mally benign herpesvirus of the African ele-
phant can be lethal to its Asian cousin (29).
Another notable example is the exposure of
zoo animals in the United Kingdom to food
contaminated by the BSE agent (30). Scrapie-
like spongiform encephalopathies thought to
result from exposure to the BSE agent have
been confirmed in 58 zoo animals of 17
species (31). Recommendations have been
published to preempt the potentially disas-
trous consequences to wildlife, agriculture,
and public health should BSE be introduced
into free-living wildlife (31).
Risk factors for disease emergence in con-
servation programs are complex. For exam-
ple, epizootic toxoplasmosis, with high mor-
tality rates, has occurred in captive lemurs,
New World primates, and Australian marsu-
pials. These animals evolved in the absence
of Toxoplasma gondii, and only recently, af-
ter human intervention (translocation), they
have been exposed to the parasite (32). The
feeding of contaminated neonate mice to cap-
tive callitrichid primates (marmosets and
tamarins) led to the emergence of callitrichid
hepatitis (32), caused by a variant of the
zoonotic pathogen, lymphocytic choriomen-
ingitis virus (LCMV). The zoonotic risk of
LCMV is mirrored by the transfer of patho-
gens from humans to wildlife species. For
example, measles contracted from humans
threatens wild mountain gorillas habituated to
tourists, and poliovirus has killed chimpan-
zees in the Gombe National Park in Tanzania
(33).
Captive breeding programs aim to main-
tain genetically viable, healthy populations
for subsequent release into the wild. The
potential transfer of pathogens into previous-
ly unexposed wild populations in often sen-
sitive, protected areas represents a serious
challenge to conservation efforts (8, 9, 13).
This can impinge on release programs even
when no apparent disease is observed. The
release of captive-reared field crickets (Gryl-
lus campestris) was suspended in England
after the discovery of unidentified, potential-
ly exotic parasites that were not associated
with ill-health, but that posed a disease threat
to sympatric wild species at release sites (34 ).
The loss of host-specific parasites from en-
dangered species in captive breeding pro-
grams is also a substantial threat to biodiver-
sity conservation. In addition to ethical obli-
gations to conserve parasite assemblages
along with their more favored hosts (35), the
maintenance of established host-parasite re-
lations may be important for the overall well-
being of the host species both at an individual
level (maintenance of immunity) and at a
population level (maintenance of genetic di-
versity) (8, 9, 11–13).
Emergence Without Overt Human
Involvement
Correlations between emergence of human
diseases (such as cryptosporidiosis, hemor-
rhagic fevers, cholera, and malaria) and
weather patterns [flooding, the El Nin˜o
Southern Oscillation (ENSO)] are common
(36, 37 ). These patterns may also cause
changes in parasite prevalence and intensity
and host mortality rates in wild animals such
as the 3- to 4-year cycles of population crash-
es in feral sheep on the St. Kilda archipelago,
Scotland (38), and major epizootics of AHS
every 10 to 15 years in South Africa (39).
There is increasing evidence that the frequen-
cy and severity of these events are influenced
by anthropogenic effects on the climate.
A newly discovered fungal disease, cuta-
neous chytridiomycosis, has recently been
identified as the cause of amphibian mortality
linked to declines in Central American and
Australian rain forests (40). The emergence
of chytridiomycosis in amphibians radically
changes our view of wildlife EIDs, because it
is the first such disease to emerge in “pris-
tine” sites, to infect a wide range of hosts, and
to cause declines and possibly extinctions in
disparate regions. Hypotheses for the rela-
tively synchronous emergence of amphibian
chytridiomycosis globally include human-as-
sisted introduction to previously unexposed
amphibian populations (41), or an alteration
of preexisting host-parasite relations owing to
climate change (42).
The Zoonotic Threat
Most human EIDs result from exposure to
zoonotic pathogens, that is, those transmitted
naturally between animals and humans, with
or without establishment of a new life-cycle
in humans. Wildlife play a key role in their
emergence by providing a “zoonotic pool”
from which previously unknown pathogens
may emerge (2). This occurs classically for
influenza virus, which causes pandemics in
humans after periodic exchange of genes be-
tween the viruses of wild and domestic birds,
pigs, and humans. Recent nucleic acid se-
quence analyses have demonstrated direct
transmission of avian influenza to humans
(43) and have identified potential nonhuman
primate reservoirs from which HIV types 1
and 2 originated (44). Natural reservoir hosts
for Ebola and Marburg viruses have proved
more elusive (17 ), although fruit or insectiv-
orous bats, insectivores, and rodents have
been tentatively implicated. The link to bats
is strengthened because (i) they can support
replication of experimentally inoculated vi-
rus, (ii) human infection has occurred near
bat-roosting sites, and (iii) Ebola virus sub-
types have been identified in geographically
dispersed regions (including Madagascar and
the Philippines). Sequence analysis suggests
that separate Ebola outbreaks are associated
with distinct emergence events, occurring ei-
ther directly from the primary reservoir, or
via secondary or tertiary intermediate hosts.
Similar chain events are thought to have oc-
curred in Australia for Hendra virus (fruit bat
reservoir, horses, and humans) and Menangle
virus (fruit bat reservoir, domesticated pigs,
and humans) (16 ), and in Malaysia and Sin-
gapore for Nipah virus (fruit bat reservoir),
which causes a fatal disease of humans, dogs,
and pigs (45). The involvement of fruit bats
in this high-profile group of EIDs has impli-
cations for further zoonotic disease emer-
gence. A number of species are endemic to
remote oceanic islands, and these may harbor
enzootic, potentially zoonotic, pathogens.
Searches for new zoonotic pathogens have
become part of the strategy to counter emerg-
ing disease threats to humans, and knowledge
from studies of known pathogens can assist in
Table 2. Definition and classification of EIDs of
wildlife based on fundamental epizootiological pa-
rameters derived from (1, 2). EIDs of humans are
defined as diseases that are newly recognized, newly
appeared in the population, or are rapidly increasing
in incidence or geographic range (1, 2). Here, and in
Table 1, we classify EIDs according to their specific
characteristics that are emerging or novel. E, emerg-
ing, new or increasing; R, recognized.
EID
type
Infectious
agent
Host
species
Incidence or
geographic
range
1E E E
2R E E
3R E R
4R R E
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this surveillance. Telford et al. (46) com-
pared guilds of deer tick-transmitted zoonotic
pathogens in Eurasian Ixodes spp. ticks with
those described from America and discovered
a novel flavivirus, “deer tick virus,” related to
the virulent Powassan virus. This work
showed similar host-parasite guilds in wild-
life host-vector assemblages separated since
the Pleistocene, and has important impli-
cations for future targeting of surveillance
efforts.
“Pathogen Pollution”: Implications for
Global Biodiversity
One of the costs of human domination of the
Earth’s ecosystem is increasing global bio-
geographical homogeneity caused by the
widespread introduction of nonnative flora
and fauna into new areas (14, 47). This an-
thropogenic form of invasion, sometimes
termed “biological pollution” (14, 47, 48) has
caused loss of biodiversity globally, particu-
larly on oceanic islands (49).
Similar loss of biodiversity occurs when
disease is introduced into naı¨ve populations.
The introduction of smallpox, typhus, and
measles by the conquistadors in the 15th and
16th centuries resulted in catastrophic depop-
ulation and 50 million deaths among native
South Americans (4 ). A number of epizootio-
logical equivalents of these “first-contact”
depopulations have occurred, but considering
the global scale of anthropogenic domestic
and feral animal introduction, their true ex-
tent has probably been grossly underestimat-
ed. MacPhee and Marx (50) implicate the
introduction of infectious diseases in the
striking loss of biodiversity after human col-
onization of continental landmasses and large
islands over the past 40,000 years, including
many of the Pleistocene megafaunal extinc-
tions. If pathogens have been introduced on a
global scale within recent human history,
how many wildlife diseases currently consid-
ered native actually originated from these
introduction events? Anthropogenic introduc-
tion of exotic pathogens, which we term here
pathogen pollution (human-mediated patho-
gen invasion), is implicated in many wildlife
EIDs listed in Table 1, often acting in consort
with spill-over events to drive emergence.
Pathogen pollution poses a substantial
threat to global biodiversity. First, it has the
potential to cause catastrophic depopulation
of the new and naı¨ve host population. Sec-
ond, when introduced diseases become enzo-
otic, initial declines may be followed by
chronic population depression, and if the
threshold host density for disease transmis-
sion is lowered, local extinction may occur.
In some cases, the success of invading host
species may be enhanced by parasite-mediat-
ed competition (“apparent competition”) due
to the impact of co-introduced diseases on
resident species (10). Disease co-introduction
may also impact humans, either directly
(Marburg virus importation into Germany) or
via effects on domesticated animals (the in-
troduction of AHS into Spain with zebra).
Although there are numerous examples
of disease emergence after pathogen intro-
duction (Table 1), there undoubtedly are
many more that have not been identified as
such. For example, the decline of red squir-
rels in Britain, recorded since 1900, may
have been caused by a parapoxvirus trans-
mitted from introduced grey squirrels in
which it is benign (51). Whether the patho-
gen was co-introduced to Britain with the
grey squirrel, or whether the establishment
of this reservoir host in Britain led to an
increased exposure of red squirrels to a
preexisting pathogen, is unknown.
The mechanics of pathogen pollution in-
volve international traffic in agricultural ma-
terials, domesticated animals, food crops, and
timber, and in biologically contaminated
wastes such as landfill and ballast water (47,
48). Global hotspots of biodiversity and wil-
derness sites such as the Gala´pagos and Ant-
arctica are not exempt (52). Evidence of in-
troduced disease in Antarctic wildlife (anti-
bodies to the domestic chicken pathogen, in-
fectious bursal disease virus, in Antarctic
penguins) has prompted legislation to main-
tain stricter controls against pathogen pollu-
tion (52).
The impact of pathogen pollution may be
augmented by secondary or “knock-on” ef-
fects that are difficult to predict. High mor-
tality of rabbits after the introduction of myx-
omatosis in the United Kingdom caused pop-
ulation declines in stoats, buzzards, and owls
(4). Myxomatosis also led to local extinction
of the endangered large blue butterfly, by
reducing grazing pressure on heathlands
which, in turn, removed the habitat for an ant
species that assists developing butterfly lar-
vae (12). The effect on rain forest ecology
after disease-mediated local extinction of
multispecies amphibian assemblages is yet to
be assessed, but is likely to be substantial
(41).
Vitousek et al. (47) suggest that introduc-
tion of alien species is the next most impor-
tant cause of extinction to habitat loss. The
introduction of pathogens might achieve a
similar status. Introduced diseases have been
implicated in the local extinction of a number
of species (7–11, 18, 24, 25) and the global
(species) extinction of Hawaiian birds (53),
the thylacine (11), Mascarene reptiles (49),
Pleistocene megafauna (50), and others. In
the first definitively proven example of ex-
tinction by infection, a microsporidian para-
site extirpated the captive remnant population
of the Polynesian tree snail, Partula turgida
(54). Thus, the 20 or so other species of
Partula occurring solely in captivity may be
at greater risk of extinction than previously
thought. This case highlights the inherent
problems parasites present to the conserva-
tion community, in which there is reliance on
captive propagation and reintroduction as a
safeguard against extinction. Global extinc-
tion as a secondary effect of disease occurred
after mass mortality of the eel grass (Zostera
marina) on the U.S. Atlantic seaboard caused
by the slime mold Labyrinthula zosterae.
Here, a Z. marina eelgrass-specific limpet,
Lottia alveus, was driven to extinction after
more than 90% loss of its habitat (55). These
two cases also highlight the consequences of
ignoring diseases of invertebrates, which are
the most speciose form of life (47 ) and are
crucial components of most ecosystems.
Perspectives
There is a clear economic cost of wildlife
EIDs. For example, in 1994, postexposure
prophylaxis for 665 people who had potential
contact with a single rabid kitten in a pet store
in New Hampshire cost $1.1 million, and it
has been estimated that the economic burden
of Lyme disease treatment in the United
States may be around $500 million per an-
num (56). The cost of importing AHS into
Spain was estimated at $20 million (23). In
Australia, a recent epizootic of pilchards re-
duced fisheries production by around A$12
million over 3 years (57 ). The economic
impacts of zoonotic EIDs may be difficult to
predict and may have complex consequences.
For example, the recent proposal to ban blood
donation in the United States by persons who
have spent longer than 6 months cumulative-
ly in the United Kingdom during 1980 –96
and are considered as potential carriers of the
BSE agent, will reduce the U.S. blood supply
by 2.2% (58). The cost of introduced disease
to human, livestock, and crop plant health is
over $41 billion per year in the United States
(48). Although the value of biodiversity and
significance of disease threats can be calcu-
lated (59), the cost of global biodiversity loss
due to disease is yet to be assessed.
There are few regulations concerning ex-
otic disease threats to wild animals, and few
systems for surveillance are in place. Current
measures for the detection and control of
human and livestock EIDs are inadequate for
the identification of similar threats to wild-
life. The conservation community has drawn
up guidelines to prevent the release of ani-
mals carrying exotic pathogens to novel areas
(8, 9). These recommendations are currently
underused: of almost 700 terrestrial verte-
brate translocations (within conservation pro-
grams) per year between 1973 and 1986 in
the United States, Australia, Canada, and
New Zealand, 24% occurred without any dis-
ease screening, and fewer than 25% involved
investigations into causes of death of the
translocated animals (60).
Future strategies for wildlife EID surveil-
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lance and control may adapt techniques now
used to study EIDs of humans and domestic
animals such as satellite imaging, used in
analyzing ENSO-related cholera outbreaks
and forecasting ENSO-related Rift Valley fe-
ver epidemics (37). An increasing use of
moderated Internet newsgroups in rapidly
disseminating quality information on out-
breaks is evident, and some (ProMED, 61)
regularly include data on plant and wildlife
EIDs. Control measures for wildlife EIDs
have largely been attempted as part of a
strategy to prevent spread to humans (rabies
control) or domesticated animals (culling of
wildlife reservoir hosts). Recent attempts to
control wild dog rabies by vaccination of
domesticated dogs adjacent to the Serengeti
National Park, and the vaccination of moun-
tain gorillas against measles and of chimpan-
zees against poliovirus suggest a growing
trend (25, 34). Woodroffe (9) predicted an
increasing role of population management,
building on modeling studies (13, 20), as an
alternative, or complement, to direct veteri-
nary intervention.
Important ethical differences exist be-
tween domesticated animal and human EIDs,
where many diseases are notifiable and con-
trol measures easily conducted, and wildlife
EIDs, for which few notifiable diseases exist
and control is often politicized and under-
funded. New initiatives are required. Mc-
Sweegan (62) proposed that infectious dis-
ease impact plans be submitted for large-scale
developmental projects. Similarly, wildlife dis-
ease impact plans could be incorporated into
environmental impact statements. In addition,
ecological studies, which have demonstrated
the extent of parasite influence on community
structure and biodiversity via host population
regulation and apparent competition (10), may
also allow prediction of the combination of
parasite, host, and environmental parameters
most likely to lead to disease emergence.
Future research on wildlife EIDs will need
to adopt a multidisciplinary approach to iden-
tify underlying causes and to control their
spread. Efforts to increase surveillance for
known pathogens and to identify previously
unknown infectious agents will be increased.
Investigations into the ecology, pathology,
and population biology of host-parasite sys-
tems will be approached from individual,
population, and environmental perspectives.
This integrative approach has been success-
fully applied to human EIDs (16, 63) and
wildlife EIDs that threaten public or domestic
animal health (27, 20). For wildlife EIDs this
integration will involve a synthesis of both
classical and cutting edge technologies from
diverse disciplines.
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On the evening of Sunday, 18 January 1778, Captain James Cook first sighted the island of Kauai, thus discovering for the western world the Hawaiian Islands. Since then, that beau-tiful archipelago has experienced a series of extraordinary biotic changes. In general, the pattern has been one of destruction and re-placement of native forms-human, plant, vertebrate and invertebrate animals. The en-demic land avifauna, a unique assemblage of forest birds, was not exempt from these effects of "discovery." This avifauna has suffered extinction of nearly half its total species, and restriction of the remainder as remnant populations to a few high-mountain forest sanctuaries. This report examines first the historical evi-dence of changes in the indigenous avifauna, especially as related to the effects of intro-duced diseases. Then it presents experimental evidence clearly demonstrating the high sus-ceptibility of existing drepaniid species to avian malaria and birdpox. Other ecological data are presented that substantiate the theory of extinction and restriction of range by exotic pathogens, which in turn are spread by intro-duced mosquitoes. Some predictions, based on the evidence derived from field studies and controlled experiments, are offered regarding the future of the Drepaniidae and the endemic Hawaiian avifauna in general. PRIMEVAL AND EARLY HISTORICAL CONDITIONS 1 DISTRIBUTION Thk early record of Hawaii' s avifauna. is at best fragmentary. At the time of discovery the native forests, together with their bird popula-tions, extended from the high mo.untain slopes to the ocean. Captain Cook (178.5). reported seeing several species of birds in the l&land forests and among the coconut palms .lining the ocean shores (see also Stresemann 1950). Other early visitors, such as the English bot-anist David Douglas who during a visit to Oahu in 1830 recorded large numbers of na-tive birds in the flowered canopies of trees at the edge of Honolulu, substantiated the seashore-to-high-forest distribution of the en-demic avifauna. Sporadic and desultory collecting continued throughout the 19th century. Finally, in 1887 the English ornithologist Scott Wilson began an extended period of field activities that re-sulted in publication of the definitive work Aves Hawaiienses (Wilson and Evans ISgO-1899). This publication was followed shortly by a magnificent two-volume work by Roths-child (1893-1900) containing more data on the distribution of the drepaniids. From these re-ports, together with the later excellent publi-cations by Perkins (1893, 1903), Henshaw (1902), and Munro (1944), it is possible to estimate the distribution patterns and general abundance of the drepaniids and other en-demic bird species before their populations were disrupted. Some species were very limited in their dis-tribution. For example, the Grosbeak Finch, Psittirostra konu, was restricted to the Kona district of Hawaii where it inhabited an area of a few square miles. In contrast, the Iiwi (Vestiaria coccinea) and the Apapane (Hima-tione sanguinea) were ubiquitous, ranging from the coastal lowlands to the upper limits of the high forests on all the high islands.