Increasing Legionellosis in the United States • CID 2008:47 (1 September) • 591
M A J O R A R T I C L E
Increasing Incidence of Legionellosis in the United
States, 1990–2005: Changing Epidemiologic Trends
Karen Neil and Ruth Berkelman
Department of Epidemiology, Emory University, Atlanta, Georgia
(See the editorial commentary by Ng et al. on pages 600–2)
An abrupt increase in the incidence of legionellosis in the United States has been noted since
2003. Whether the recent increase is associated with shifting epidemiologic trends has not been well characterized.
We analyzed all cases of legionellosis reported to the Centers for Disease Control and Prevention
through the National Notifiable Disease Surveillance System from 1990 through 2005.
A total of 23,076 cases of legionellosis were reported to the Centers for Disease Control and Prevention
from 1990 through 2005. The number of reported cases increased by 70% from 1310 cases in 2002 to 2223 cases
in 2003, with a sustained increase to 12000 cases per year from 2003 through 2005. The eastern United States
showed most of the increases in age-adjusted incidence rates after 2002, with the mean rate in the Middle Atlantic
states during 2003–2005 exceeding that during 1990–2002 by 96%. During 2000–2005, legionellosis cases were
most commonly reported in persons aged 45–64 years. Persons aged !65 years comprised 63% of total cases in
2000–2005. Age-adjusted incidence rates in males exceeded those in females for all age groups and years. Legion-
ellosis incidence showed marked seasonality in eastern states, with most cases reported in the summer or fall.
Reported legionellosis cases have increased substantially in recent years, particularly in theeastern
United States and among middle-aged adults. Legionella infection should be considered in the differential diagnosis
of any patient with pneumonia. Public health professionals should focus increased attention on detection and
prevention of this important and increasing public health problem.
More than 30 years have passed since the recognition
of Legionella species as the cause of a severe pneumonia
outbreak in Philadelphia in 1976 . Since then, we
have made great progress in understanding this disease
and its environmental sources. Despite this, an abrupt
increase in the incidence of legionellosis has been noted
since 2003 , with recent increases in the Bronx
prompting the New York City Department of Health
to issue a press release in July 2007 . This trend has
also been noted internationally, as evidenced by a press
release issued in August 2007 by the Health Protection
Agency in England .
Received 23 January 2008; accepted 29 March 2008; electronically published
29 July 2008.
Reprints or correspondence: Dr. Karen Neil, Enteric Diseases Epidemiology
Branch, Div. of Foodborne, Bacterial and Mycotic Diseases, US Centers for Disease
Control and Prevention, 1600 Clifton Rd., MS-A38, Atlanta, GA 30333
Clinical Infectious Diseases2008;47:591–9
? 2008 by the Infectious Diseases Society of America. All rights reserved.
Legionella species are weakly gram-negative bacteria
found primarily around fresh waterenvironments,such
as lakes and streams, where the bacteria use free-living
amoeba as hosts for intracellular survival and multi-
plication . More than 45 species of Legionella have
been identified. However, Legionella pneumophila is as-
sociated with ∼90% of reported cases in the United
States, with L. pneumophila serogroup 1 causing ∼80%
of these cases . Disease is usually associated with
man-made environments, such as cooling towers,
whirlpools, and building water systems, where warm
water (25?C–42?C) and biofilms support growth and
survival of Legionella species . Disease caused by Le-
gionella longbeachae has been associated with use of
potting soil and gardening .
Legionella species are implicated in 2 clinical syn-
dromes: legionnaires disease and Pontiac fever, collec-
tively known as legionellosis. Pontiac fever is generally
a self-limited, influenza-like illness, whereas legion-
naires disease is a common cause of serious bacterial
pneumonia. Risk factors for legionnaires disease in-
clude older age, smoking, male sex, and underlying
592 • CID 2008:47 (1 September) • Neil and Berkelman
diseases (immunosuppression, diabetes, chronic lung disease,
and renal failure) . Cases have been reported in otherwise
healthy individuals [7–9] and in all age groups, including in-
fants . Although !20% of legionnaires disease cases are out-
break related [5, 6], outbreaks have been associated with whirl-
pool spas, cooling towers, decorative fountains, hotels,
hospitals, nursing homes, and cruise ships [10–12]. To inves-
tigate whether the recent increase in legionellosis in the United
States is associated with shifting epidemiologic trends, we an-
alyzed data on cases reported to the Centers for DiseaseControl
and Prevention (CDC) from 1990 through 2005.
The CDC collects data on voluntarily nationally notifiable dis-
eases through the National Notifiable DiseasesSurveillanceSys-
tem. Legionellosis has been a nationally notifiable disease since
1980 . Because only summary data are available before
1990, we analyzed legionellosis cases reported from 1990
through 2005, which is the last year for which finalized data
DATA SET AND CASE DEFINITION
Subsequent to a data-use agreement, the CDC provided data
on cases reported in states where legionellosis was designated
as notifiable from 1990 through 2005 . The 1990–2003 data
included all reported legionellosis cases, whereasthe2004–2005
data were limited to “confirmed” cases of legionellosis with the
exception of data from California . Data set variables were
year, event month (based on the report month), state, sex, race,
ethnicity, and age, categorized as !1 year, 1–4 years, 5-year
groups from 5 to 74 years, and ?75 years.
Three case definitions were used by the CDC from 1990
through 2005 [16–18]. For “confirmed” legionellosis, all 3 re-
quire a clinically compatible case plus either culture isolation
of any Legionella organism from respiratory secretions, lung
tissue, pleural fluid, or other normally sterile fluid; detection
of L. pneumophila serogroup 1 antigen in urine; or at least a
4-fold increase in serum antibody titer for L. pneumophila se-
rogroup 1 [17, 18]. Before 2005, criteria also included detection
of L. pneumophila serogroup 1 by direct fluorescent antibody
staining. Before 1996, a “probable” status based on a single
convalescent-phase serum antibody titer of ?256 was included
Data were analyzed using SAS statistical software, version 9.1
(SAS Institute). Analysis was limited to the 50 states and the
District of Columbia. Broader age categories and US Census
Bureau regions and divisions were coded. Event months were
combined into seasons: spring was defined as March, April,
and May; summer as June, July, and August; fall as September,
October, and November; and winter as December, January, and
February. Pediatric cases were defined as cases that occurred
in individuals aged ?19 years.
Sex distribution was compared with the 2000 US Census
population . Crude and age-specific incidence rates were
calculated using the case count and the corresponding yearly
population estimate . Populations of states where legion-
ellosis was not notifiable in a given year were excluded in the
denominator for affected rate calculations. Rates for periods
11 year were obtained by averaging annual rates. Age-adjusted
rates were calculated using the 2000 US standard population
A total of 23,076 cases of legionellosis were reported to the
CDC from 1990 through 2005. The annual number ranged
from 1094 to 2291 cases (figure 1). The number of reported
cases increased by 70%, from 1310 cases in 2002 to 2223 cases
in 2003, with a sustained increase to 12000 cases per year from
2003 through 2005. During 1990–2002, the mean (?SD) an-
nual legionellosis case count was 1268 ? 139.40 cases (range,
1094–1610 cases), whereas from 2003 through 2005, the yearly
mean was 2198 ? 107.15 cases (range, 2081–2291 cases). The
age-adjusted incidence rate for legionellosis in the UnitedStates
paralleled this rise, increasing 65%, from 0.45 cases per 100,000
residents in 2002 to 0.75 cases per 100,000 in 2003.
gionellosis cases. Mean age-specific incidence rates for the
1990–2005 period generally increased with increasingagegroup
(figure 2). Legionellosis cases are now mostcommonlyreported
in persons aged 45–64 years (figure 3). From 1990 through
1999, the 65–74-year-old age group had the highest mean
(?SD) number of reported cases annually (275 ? 38.48 cases
per year). In contrast, from 2000 through 2005, the 55–64-
year-old age group had the highest mean annual case count
(388 ? 154.22 cases per year), followed by the 45–54-year-old
age group. Persons aged !65 years comprised 63% of total cases
From 1990 through 2005, 375 cases (1.7%) were reported
in pediatric age groups; 209 cases (0.93%) were reported in
children aged ?14 years. Most pediatric cases were reported
in children 15–19 years old (44.3%), followed by infants aged
!1 year (18.1%).
Males comprised 61% of the 22,763 case patients for
whom sex was known, compared with 49% of the 2000 US
Census population. Rates in males exceeded those in females
for all age groups and years. The gap between male and female
incidence rates steadily widened in adults as the age group
Age was known in 22,604 (98%) of the reported le-
Increasing Legionellosis in the United States • CID 2008:47 (1 September) • 593
Surveillance System and the corresponding annual age-adjusted incidence rate per 100,000 for 1990–2005.
Annual number of legionellosis cases reported through the Centers for Disease Control and Prevention National Notifiable Disease
increased. The male rate exceeded the female rate by 11% in
the 15–24-year-old age group (0.073 vs. 0.066 cases per 100,000
residents). This sex difference increased to 116% in those aged
?75 years (2.62 vs. 1.21 cases per 100,000). The sex difference
in annual incidence rates for legionellosis was highest in recent
years. Yearly age-adjusted rates in males were 12 times higher
than those in females from 2003 through 2005.
During 1990 through 2005, cases
were reported from the District of Columbia and every state
except Alaska. The Northeast region reported the largest per-
centage of cases (31.5%), followed by the Midwest (30.6%),
the South (26.7%), and the West (11.2%). Most reported cases
(69%) were concentrated in 3 contiguous eastern divisions:
Middle Atlantic (26%), East North Central (25%), and South
Atlantic (19%). The states with the highest reported casecounts
were Pennsylvania (11.5% of total cases), New York (11.0%),
and Ohio (10.3%). Age-adjusted incidence rates were highest
in Delaware (1.8 cases per 100,000 residents). Lowest age-ad-
justed rates were in North Dakota (0.04 cases per 100,000) and
Oregon (0.07 cases per 100,000).
The increase in reported legionellosis cases after 2002 is
mainly reflective of increased incidence in the states east of the
Mississippi River (figure 4). The Northeast and South regions
showed the greatest change in the mean annual number of
cases from 1990–2002 to 2003–2005, increasing by 104% in the
Northeast and 113% in the South. Regional mean age-adjusted
incidence rates reveal similar findings. The mean rate (per
100,000) for 2003–2005 exceeded that for 1990–2002 by 82%
in the Northeast (1.30 vs. 0.72), 76% in the South (0.60 vs.
0.34), 22% in the Midwest (0.81 vs. 0.66), and 4% in the West
(0.30 vs. 0.29). By US Census Bureau division, thehighestmean
annual case counts for both the 1990–2002 and 2003–2005
periods were seen in the Middle Atlantic, East North Central,
and South Atlantic divisions.The MiddleAtlanticstatesshowed
the greatest increases in mean age-adjustedincidencerates,with
the 2003–2005 rate exceeding the 1990–2002 rate by 96% (1.47
vs. 0.75), followed by the South Atlantic division (85%; 0.80
vs. 0.43). Divisional changes in age-adjusted incidence rates
between these periods are shown in figure 5.
From 1990 through 2005, legionellosis cases were most fre-
quently reported to the CDC in the fall and summer: 30% of
the cases were reported in the fall, 29% in the summer, 23%
in the winter, and 18% in the spring. Cases were reported most
frequently in August (11.2%) and least frequently in February
(5.6%). The West region had the least monthly variation in
reported cases during this time (figure 6).
The number of reported legionellosis cases in the United States
has increased substantially in recent years, particularly in the
eastern United States. The number of reported legionellosis
cases increased abruptly, from a mean of 1268 yearly cases
before 2003 to 12000 cases per year from 2003 through 2005,
with a brief spike in 1994, which appears to primarily reflect
a few outbreaks in the South that year . Final data from
2006 show a sustained increase: 2834 legionellosis cases were
reported , which is the greatest number reported since
legionellosis surveillance began.
The passive nature of the notifiable diseasesystemlikelyleads
to underreporting of cases: 1 population-based study estimated
that Legionella species cause 8000–18,000 pneumonia cases an-
nually , suggesting that more than three-quarters of cases
are currently undiagnosed or unreported. Whether the recent
increase in reported legionellosis cases and the predilection for
cases in the eastern states reflect true changes in the incidence
594 • CID 2008:47 (1 September) • Neil and Berkelman
by all age groups (A) and pediatric age groups (B).
Total number of legionellosis cases reported for 1990–2005 and mean age group–specific incidence rates (per 100,000) for this period
of legionellosis, rather than artifact due to changes in legion-
ellosis testing or reporting practices over time is unclear. We
found no evidence that changes in diagnostic testing were re-
sponsible for the increase after 2000. Increased use of urine
antigen testing had already occurred in the 1990s, when di-
agnosis by this test increased from 0% to 69% . Currently,
there is no commercially available PCR approved for clinical
diagnostic use in the United States, making widespread routine
use of PCR for diagnosis of legionellosis less likely. An increase
due to introduction of other new diagnostic methods or
changes in reimbursement in ∼2003 is also unlikely(V.Baselski,
Although completeness of notifiable disease reporting is dif-
ficult to assess, we found no evidence that variations in case-
reporting procedures or completeness contributed to the in-
creased incidence over time. Although physicians in states with
Increasing Legionellosis in the United States • CID 2008:47 (1 September) • 595
64 years of age, and ?65 years of age during 1990–2005. B, Comparison of the mean number of legionellosis cases per year for the 1990–1999
versus 2000–2005 periods among different age groups. Note that cases in the 45–64-year-old age group surpassed those in the ?65-year-old age
group in ∼2000.
Trends in age distribution of reported legionellosis cases. A, Annual number of reported legionellosis cases by 0–45 years of age, 45–
historically higher legionellosis rates or recent outbreaks may
have increased awareness and may be more likely to test for
and report Legionella species, evaluation of the Middle Atlantic,
East North Central, and South Atlantic divisions on a state-
by-state basis reveals that the number of case reports increased
across almost all these states after 2002, rather than being lim-
ited to a few states. This makes the geographic variation and
post-2002 increase less suggestive of state-specific reporting ar-
tifacts. We also found no changes in national water-quality
standards that would promote increased of risk of proliferation
of Legionella species in water sources.
Past research has suggested a link between weather and le-
gionellosis. A 1990–2003 study by Hicks et al.  analyzing
the 2003 increase in the incidence of legionellosis in several
Middle Atlantic states correlated the 2003 increase in legion-
ellosis with increased total monthly rainfall. Because legionel-
losis occurrence has continued to increase after 2003 despite
decreased rainfall in some areas—for example, case reports
increased in South Atlantic states through 2006 despite a
drought in that area [23, 26]—the correlation to total rainfall
is less certain. A separate study by Fisman et al.  that
evaluated the association of weather patterns and legionellosis
596 • CID 2008:47 (1 September) • Neil and Berkelman
Annual number of reported legionellosis cases by US Census Bureau region, 1990–2005
in Philadelphia from 1995 through 2003 alludes to a more
complex weather pattern than just increased monthly rainfall.
Although this study did not find an association between
monthly incidence of legionellosis and total monthly precipi-
tation after controlling for other meteorologic variables,itiden-
tified a short-term association between legionellosis and the
presence of precipitation and increased humidity at 6–10 days
before disease . Given that climate trends predict continued
precipitation increases in northeastern states , more de-
tailed analyses are needed to clarify the association among cli-
mate, weather, and temporal and geographic variations in le-
In our analysis, as in previous studies[6,13],annualreported
incidence rates for legionellosis increased with age across all
age groups older than 1 year. However, we noted a trend toward
younger ages in recent years. Despite the common perception
that legionnaires disease is a disease primarily of elderly people,
since the year 2000 the highest number of legionellosis cases
has been reported in persons 45–64 years old, rather than the
?65-year-old age group as seen before 2000.
This study highlights the importance of considering Legion-
ella species as a cause of pneumonia in all age groups. A con-
tinued misperception that legionnaires disease is a disease of
elderly people may lead to preferential testing of older patients
and missed cases in children and young adults if legionellosis
is not considered in the differential diagnosis. For example,
McDonough et al.  recently reported 5 cases of legionnaires
disease in militaryrecruitsaged18–28yearsinthesametraining
company that were identified only retrospectively through PCR
analysis of throat swabs as part of a pneumonia surveillance
study. Pediatric cases comprise ∼1% of the cases reported from
1990 through 2005, yet evidence-based pediatric management
guidelines for community-acquired pneumonia [29, 30] do not
discuss legionnaires disease in the differential diagnosis or as
part of the testing recommendations,potentiallyleadingtomis-
diagnosis and underreporting of cases.
Legionella species are arguably the most important water-
borne organisms in the United States with regard to serious
morbidity and mortality. Legionnaires disease has been iden-
tified as a significant cause of community-acquired pneumonia
leading to hospitalization, identified in 2%–8% of cases in
North American and European studies [24, 31–33]. In several
studies of severe community-acquired pneumonia, Legionella
species have been the second most commonly identified or-
ganism, after pneumococcus [34, 35]. It is also a significant
cause of waterborne-disease outbreaks. In the most recent Wa-
terborne-Disease Outbreak Surveillance System summary ,
Legionella species were the most commonlyidentifiedinfectious
organisms in waterborne outbreaks associated with drinking
water and with water not intended for drinking (excluding
recreational water). Legionella species were also linked to all the
deaths associated with these outbreaks .
Given the significant morbidity associated with legionnaires
disease and its apparent rising incidence in recent years, le-
gionnaires disease is increasingly important as a public health
threat. Approximately 20%–25% of legionellosis casesaretravel
related [13, 36]. Because pneumonia caused by Legionella spe-
cies is clinically indistinguishable from other bacterial pneu-
monias [5, 31], clinicians should consider Legionella species in
the differential diagnosis of any patient with pneumonia, re-
gardless of age, especiallyforpatientswithimmunosuppression;
a recent history of travel, especially if it included stays in hotels
or on cruiseships; or exposure to environmental water sources,
such as whirlpool spas or decorative fountains. Current guide-
Increasing Legionellosis in the United States • CID 2008:47 (1 September) • 597
and 2003–2005 (B). Maps have been modified from the Census Regions and Divisions of the United States map prepared by the Geography Division,
US Census Bureau (http://www.census.gov/geo/www/maps/CP_MapProducts.htm).
Mean age-adjusted incidence rates for legionellosis (cases per 100,000) by US Census Bureau division during 2 periods: 1990–2002 (A)
lines for management of community-acquired pneumonia in
adults  recommend Legionella testing in patients with a
history of travel within 2 weeks before the onset of symptoms,
community-acquired pneumonia requiring admission to the
intensive care unit, failure of outpatient antibiotic therapy or
other nonresponding pneumonia, history of active alcohol
abuse, presence of a pleural effusion, or exposure as part of a
legionellosis outbreak or suspected outbreak. Current health
care–associated pneumonia guidelines  recommend that
clinicians maintain a high index of suspicion for legionnaires
disease in patients with health care–associated pneumonia, es-
pecially in those who have recently undergone transplantation,
who have immunosuppression, who have chronic underlying
diseases, or who are aged ?65 years. Clinical guidelines for
Legionella testing in pediatric pneumonia are lacking and
should be developed.
Further research is required to explain the recent increases
in legionellosis. Routine collection and dissemination to re-
searchers of more-comprehensive patient risk factor, laboratory
diagnostic testing, and other epidemiologic information by na-
598 • CID 2008:47 (1 September) • Neil and Berkelman
Monthly number of reported legionellosis cases by US Census Bureau region, 1990–2005
tional surveillance systems would aid these efforts. Programs
of routine environmental water monitoring for Legionella spe-
cies with reduction or elimination of the bacteria from water
systems when detected, are increasingly being implemented as
a prevention strategy [39, 40], and the impactoftheseprograms
requires further assessment. In addition, more research is
needed on the effectiveness of various water disinfection sys-
tems for reduction of Legionella species in water systems.
We thank Dr. Lauri Hicks (Centers for Disease Control and Prevention)
for legionellosis surveillance information and suggestions. We thank Dr.
Ben Silk (Emory University) for analytic guidance. We also thank Dr. Ruth
Jajosky (Centers for Disease Control and Prevention), the US State and
Territorial Health Departments, the National Notifiable Diseases Surveil-
lance System (Centers for Disease Control and Prevention), and the Di-
vision of Integrated Surveillance Systems and Services, National Center for
Public Health Informatics.
Potential conflicts of interest.
R.B. has served as a consultant on Le-
gionella epidemic investigations for Pathogen Control Associates, Atlanta,
Georgia. K.N.: no conflicts.
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