A Pathogen to Consider More Broadly in Patients with Pneumonia: Legionella

Abstract & Commentary

By Brian G. Blackburn, MD, Clinical Assistant Professor of Medicine, Division of Infectious Diseases and Geographic Medicine, Stanford University School of Medicine, is Associate Editor for Infectious Disease Alert

Dr. Blackburn reports no financial relationships relevant to this field of study.

Source: Neil K, Berkelman R. Increasing incidence of legionellosis in the United States, 1990-2005: changing epidemiologic trends. Clin Infect Dis. 2008;47:591-599.

Synopsis: The incidence of legionellosis in the United States increased significantly in 2003-2005 compared to previous years. This was due mostly to an upsurge of cases in the northeastern and southern United States and a shift of disease from elderly to middle-aged adults. Legionellosis should be considered as a potential cause of pneumonia in a broad range of patients, rather than a small subset with specific risk factors.

Legionella spp. are gram-negative bacteria found primarily in freshwater environments; they cause disease (including outbreaks) often linked to man-made water systems.1 They primarily cause two clinical syndromes: a self-limited, influenza-like illness known as Pontiac fever, and a form of serious bacterial pneumonia. Traditionally accepted risk factors for legionellosis include older age, alcohol use, smoking, diabetes, chronic lung disease, renal failure, and immunosuppression.1

An increase in the incidence of legionellosis has been noted in the United States since 2003. To investigate this recent increase, the authors analyzed legionellosis cases reported to the CDC from 1990-2005. These data were obtained through the voluntary National Notifiable Diseases Surveillance System. Both confirmed and probable cases were included from 1990-2003, but only included confirmed cases from 2004-2005.

In 2003, the number of reported legionellosis cases in the United States increased by 70%, compared to 2002 (from 1,310 cases to 2,223). This increase was sustained through 2005 (the last year for which complete data were available), with > 2,000 annual cases reported in 2005. During 1990-2002, the mean annual legionellosis case count was ~ 1,270, whereas for 2003-2005, the yearly mean was ~ 2,200 cases, a significant increase. The population-based US incidence rate of legionellosis increased 65%, from 0.45 cases per 100,000 in 2002 to 0.75 in 2003.

Although the 65- to 74-year-old age group had the highest mean number of reported annual cases from 1990-1999, the 55- to 64-year-old age group had the highest mean annual case count from 2000-2005, followed by the 45- to 54-year-old age group; males comprised 61% of the case-patients in the more recent years.

Overall, the Northeast region reported the largest percentage of cases (31.5%), followed by the Midwest (30.6%), the South (26.7%), and the West (11.2%). The increase in legionellosis cases after 2002 was mainly attributable to an increase in states east of the Mississippi River. The Northeast and Southern regions showed the greatest increase (104% in the Northeast and 113% in the South); there was little increase in the Midwest and almost none in the West. Regional population-based incidence rates revealed similar findings. Overall, legionellosis cases were most frequently reported in the fall and summer, although in the West there was little monthly variation.


These data support the hypothesis that the incidence of legionellosis in the United States increased significantly beginning in 2003, and that this increase has been sustained since that time. Other data suggest that this trend has continued through at least 2006.2 The data presented in this study suggest the increase in legionellosis has been driven, in part, by a change in the epidemiology of this disease. Although legionellosis was previously regarded as occurring most commonly in the elderly or debilitated, the trend since 2000 indicates that younger, otherwise healthy persons now comprise the majority of the cases. Middle-aged men are now the most commonly diagnosed patients with legionellosis. Data from Europe similarly suggest the need to consider legionellosis as a cause of community-acquired pneumonia in all hosts.3

Intriguing geographic data from this study suggest that Legionella spp. thrive particularly well in the Northeastern and Southern United States. Not only are incidence rates highest in these regions, but the increase in the post-2002 period was driven largely by higher rates there. Previous work has suggested that increasing Legionella incidence may be related to higher average monthly rainfall.4 However, in some areas, Legionella incidence subsequently continued to increase despite decreased rainfall, or even drought, calling into question the relationship between monthly rainfall totals and Legionella incidence. Other data suggest the relationship between climate and legionellosis may be more complex, as a study in Philadelphia identified an overall association between Legionella incidence and increased temperature (with most cases occurring in summer), but most notably, increased case clustering occurred 6-10 days after heavy rainfalls and with increased humidity.5 Although further study is needed, it seems possible that the increase in legionellosis is related, in part, to climactic changes that we are only beginning to understand.

Although the data in this report are subject to the inherent biases of a passive reporting system, there did not appear to be ascertainment bias from year-to-year, nor from region-to-region, to explain the findings. Overall, these data provide provocative insights into the regional and demographic trends surrounding the recent increase in reported Legionella cases, and suggest the need to consider this pathogen in all patients with pneumonia.


  1. Stout JE, Yu VL. Legionellosis. N Engl J Med. 1997;337:682-687.
  2. McNabb SJN, et al. Summary of notifiable diseases-United States, 2006. MMWR Morb Mortal Wkly Rep. 2008;55:1-92.
  3. von Baum H, et al. Community-acquired Legionella pneumonia: new insights from the German competence network for community acquired pneumonia. Clin Infect Dis. 2008;46:1356-1364.
  4. Hicks LA, et al. Increased rainfall is associated with increased risk for legionellosis. Epidemiol Infect. 2007;135:811-817.
  5. Fisman DN, et al. It's not the heat, it's the humidity: wet weather increases legionellosis risk in the greater Philadelphia metropolitan area. J Infect Dis. 2005;192:2066-2073.