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-
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