Research article

Laboratory-based evaluation of legionellosis epidemiology in Ontario, Canada, 1978 to 2006

Victoria Ng^1,2 , Patrick Tang³ , Frances Jamieson^3,4 , Cyril Guyard^3,4 , Donald E Low^3,4,5 and David N Fisman^1,3,6,7

¹Child Health Evaluative Sciences, Research Institute of the Hospital for Sick Children, Toronto, Canada

²National Centre for Epidemiology and Population Health, The Australian National University, Canberra, Australia

³Ontario Agency for Health Protection and Promotion, Toronto, Canada

⁴Department of Pathobiology and Laboratory Medicine, University of Toronto, Toronto, Canada

⁵Department of Microbiology, Mount Sinai Hospital, Toronto, Canada

⁶Department of Epidemiology, Dalla Lana School of Public Health, University of Toronto, Toronto, Canada

⁷Department of Health Policy, Management, and Evaluation, University of Toronto, Toronto, Canada

author email corresponding author email

BMC Infectious Diseases 2009, 9:68doi:10.1186/1471-2334-9-68

Published:	21 May 2009

Abstract

Background

Legionellosis is a common cause of severe community acquired pneumonia and respiratory disease outbreaks. The Ontario Public Health Laboratory (OPHL) has conducted most testing for Legionella species in the Canadian province of Ontario since 1978, and represents a multi-decade repository of population-based data on legionellosis epidemiology. We sought to provide a laboratory-based review of the epidemiology of legionellosis in Ontario over the past 3 decades, with a focus on changing rates of disease and species associated with legionellosis during that time period.

Methods

We analyzed cases that were submitted and tested positive for legionellosis from 1978 to 2006 using Poisson regression models incorporating temporal, spatial, and demographic covariates. Predictors of infection with culture-confirmed L. pneumophila serogroup 1 (LP1) were evaluated with logistic regression models.

Results

1,401 cases of legionellosis tested positive from 1978 to 2006. As in other studies, we found a late summer to early autumn seasonality in disease occurrence with disease risk increasing with age and in males. In contrast to other studies, we found a decreasing trend in cases in the recent decade (IRR 0.93, 95% CI 0.91 to 0.95, P-value = 0.001); only 66% of culture-confirmed isolates were found to be LP1.

Conclusion

Despite similarities with disease epidemiology in other regions, legionellosis appears to have declined in the past decade in Ontario, in contrast to trends observed in the United States and parts of Europe. Furthermore, a different range of Legionella species is responsible for illness, suggesting a distinctive legionellosis epidemiology in this North American region.