Diagnostic testing for Legionnaires’ disease: Trends in BC

Legionnaires’ disease is often severe and is a potentially fatal form of bacterial pneumonia, particularly in individuals with compromised health status. Legionella pneumophila serogroup 1 accounts for the majority of cases worldwide.[1] Other species of Legionella may be similarly pathogenic. This organism may also cause the milder syndrome referred to as Pontiac fever. In the pneumonic form, nonproductive cough, abdominal pain, diarrhea, and confusion/delirium are common.

Pontiac fever, in contrast, is a self-limiting, influenza-like illness that can last 3 to 5 days after exposure in healthy individuals. Exposure to Legionnaires’ disease is commonly associated with contaminated whirlpools/spas, water towers, and home humidifiers. Outbreaks due to Legionnaires’ disease have been identified globally, including in Canada, and are most often associated with cooling towers.[1-3] Transmission does not occur from person to person. Risk factors in adults include age (being over 50 years old), cigarette smoking, alcohol consumption, diabetes, chronic heart and lung disease, and immune suppression. Pediatric cases of Legionnaires’ disease are uncommon.

The BC Public Health Microbiology and Reference Laboratory offers a wide range of laboratory tests for Legionella including urine antigen, molecular, culture, and serology. The Figure shows testing trends in BC from 2010 to 2014 and the number of positive cases identified during that period. Although requests for urine antigen tests have been steadily increasing since 2011, the number of positive cases remains stable. In 2014 a small cluster of positive cases in one health authority likely contributed to the increase in cases during that year. This is in contrast to requests for respiratory PCR tests, which remained fairly consistent over the 5-year period and for which the number of positive cases has remained stable. Culture tests are less commonly ordered, with few positive cases identified, but culture testing is required for subtyping purposes. Legionella serology has been offered since September 2012, with one positive case of Legionella maceachernii detected in 2014.

Rapid and accurate detection of positive Legionella cases is important for patient care and public health follow-up. The follow-up requires either a culture or PCR amplicon for molecular subtyping to allow for cluster detection, which in turn assists in source detection and public health intervention.

Appropriate diagnostic tests should be ordered for patients with clinical presentations consistent with Legionnaires’ disease. For critically ill patients it is reasonable to submit urine for antigen testing and a lower respiratory sample (e.g., broncheo-alveolar lavage) for PCR. Serology testing should be reserved for patients whose urine antigen and PCR tests are negative but whose symptoms are consistent with Legionnaires’ disease, suggesting the possibility of Legionella species other than L. pneumophila. A blood sample in a gold-top serum separator tube is needed for serology testing. Health care professionals are asked to refer to the BC Public Health Microbiology and Reference Laboratory Guide to Programs and Services for specimen and collection system details.

Legionnaires’ disease is a reportable communicable disease in BC, and both physicians and laboratories are reminded to report this infection to public health authorities.
—Muhammad Morshed, PhD, SCCM
—Yin Chang, MSc
—Linda Hoang, MD, FRCPC

Acknowledgments
Thanks to Yvonne Simpson, Jonathan Laley, and Rob Azana for assistance with data extraction for this study, and to the BC Legionella Working Group for reviewing this article.

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This article is the opinion of the BC Centre for Disease Control and has not been peer reviewed by the BCMJ Editorial Board.

References

1.    Bartram J, Chartier Y, Lee JV, et al. (eds). Legionella and the prevention of Legionellosis. Geneva: Switzerland, WHO Press; 2007.
2.    Newton HJ, Ang DK, van Driel IR, et al. Molecular pathogenesis of infections caused by Legionella pneumophila. Clin Microbiol Rev 2010;23:274-298.
3.    Lévesque S, Plante PL, Mendis N, et.al. Genomic characterization of a large outbreak of Legionella pneumophila serogroup 1 strains in Quebec City, 2012. PLoS One 2014;9:e103852.