Legionellosis, Singapore | CDC EID

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Volume 17, Number 7–July 2011
Research
Epidemiology and Control of Legionellosis, Singapore
Meng Chon Lam, Li Wei Ang, Ai Ling Tan, Lyn James, and Kee Tai GohAuthor affiliations: Ministry of Health, Singapore (M.C. Lam, L.W. Ang, L. James, K.T. Goh); and Singapore General Hospital, Singapore (A.L.Tan)

Suggested citation for this article

Abstract
To determine trends and clinical and epidemiologic features of legionellosis in Singapore, we studied cases reported during 2000–2009. During this period, 238 indigenous and 33 imported cases of legionellosis were reported. Cases were reported individually and sporadically throughout each year. Although the annual incidence of indigenous cases had decreased from 0.46 cases per 100,000 population in 2003 to 0.16 cases per 100,000 in 2009, the proportion of imported cases increased correspondingly from 6.2% during 2000–2004 to 27.3% during 2005–2009 (p<0.0005). The prevalence of Legionella bacteria in cooling towers and water fountains was stable (range 12.1%–15.3%) during 2004–August 2008.

Legionellosis (Legionnaires' disease and Pontiac fever) is an environment-related, acute respiratory infection caused by gram-negative, rod-shaped bacteria of the genus Legionella. Pontiac fever is a self-limiting influenza-like syndrome; Legionnaires' disease is more severe, has pneumonia as the predominant clinical finding, and is a potentially fatal illness. Currently, there are 52 Legionella species (1) and 70 serogroups (2). Of these species, 25 species are known to cause human disease (1). Most human infections are caused by Legionella pneumophila (3), and the predominant serogroup is serogroup 1 (4). Other species, which together with L. pneumophila, account for most human infections include L. longbeachae and L. micdadei. The mode of transmission of legionellosis is believed to be by inhalation of aerosols (5). Other possible modes of transmission such as aspiration of contaminated potable water have also been widely discussed (6–8).

Legionellosis was first identified in 1976 during an outbreak of severe pneumonia among delegates to the 1976 American Legion Convention in Philadelphia (9). Since then, several outbreaks linked to a variety of aerosol-producing devices, such as cooling towers (10–13), whirpool spas (14), decorative fountains (15), mist machines (16,17) and industrial air scrubbers (18), have been reported. The largest outbreak of the disease (449 confirmed cases) was attributed to cooling towers in a city hospital in Murcia, Spain, in July 2001 (10).

Risk factors for legionellosis include cigarette smoking, chronic lung disease, and immunosuppression (especially that caused by corticosteroid therapy and organ transplantation) (8). Environmental factors such as high humidity and increased rainfall also increase the risk for legionellosis (19).

In Singapore, a densely populated, tropical city-state with many high-rise commercial, office, and residential air-conditioned buildings, Legionnaires' disease was recognized as a potential public health threat because environmental surveys showed that cooling towers were heavily colonized by Legionella spp. bacteria. A study conducted in 1987 showed that Legionella spp. were present in 7 (46.7%) of 15 cooling towers sampled (20). Because air-conditioning systems are operated most of the year, a large heat load is imposed on water cooling systems, thus facilitating an increased rate of colonization and multiplication of Legionella spp. (21). Furthermore, the viability of Legionella spp. in contaminated aerosols is increased by high relative humidity (21).

Legionella spp. pneumonia accounts for 2%–7% of community-acquired pneumonia among hospitalized patients in Singapore (22). Legionnaires' disease was made a notifiable infectious disease in Singapore in 1985 and legally notifiable in 2000. A code of practice for prevention and control of Legionella spp. bacteria in cooling towers for building owners and water treatment contractors was published in 1992 and revised in 1994 and 1998. Subsequently, Environmental Public Health (Cooling Towers and Water Fountains) Regulations were enacted and implemented nationwide in 2001 (23).

We studied trends and clinical and epidemiologic features of legionellosis in Singapore during 2000–2009. We also reviewed the prevalence of Legionella spp. bacteria in cooling towers and water fountains during the same period to determine whether any relationship existed between disease incidence and prevalence of Legionella spp. bacteria in these artificial water systems during the past decade.

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Legionellosis, Singapore CDC EID


Suggested Citation for this Article
Lam MC, Ang LW, Tan AL, James L, Goh KT. Epidemiology and control of legionellosis, Singapore. Emerg Infect Dis [serial on the Internet]. 2011 Jul [date cited].
http://www.cdc.gov/EID/content/17/7/1209.htm

DOI: 10.3201/eid1707.101509
Comments to the Authors
Please use the form below to submit correspondence to the authors or contact them at the following address:
Meng Chon Lam, Ministry of Health, 16 College Rd, Singapore 169854; email: mengchon.lam@mohh.com.sg