Scientists Warn: Swimming in Lakes Could Cause Legionnaires’ Disease

Legionnaires’ disease, a form of pneumonia caused by Legionella bacteria, can be contracted from stagnant natural water, making awareness and early diagnosis crucial for at-risk individuals.

Swimming in certain still-water lakes can expose individuals to Legionella bacteria, which can cause pneumonia. Authors of a practice article in the Canadian Medical Association Journal (CMAJ) advise open-water swimmers to be aware of this risk.

“Legionella infection represents a public health hazard owing to its ability to spread through exposure to natural water bodies and human-made water reservoirs,” writes Dr. Ashley Bryson, an internal medicine resident at the University of Manitoba, with coauthors.

Legionella infection is an atypical cause of community-acquired pneumonia. Referred to as legionnaires’ disease, it presents with fever, fatigue, respiratory symptoms, and sometimes diarrhea. Legionella bacteria thrive in the warm, stagnant water in plumbing systems, air conditioners, public spas, and even lakes and rivers. Risk factors for legionnaires’ disease include age older than 50 years, smoking history, chronic cardiovascular or kidney disease, diabetes, and a compromised immune system.

Case Study: Infection Following Lake Swimming

The 77-year-old patient described in the article had visited Iowa and swum in a natural lake 2 weeks earlier. Over 4 days of increasing weakness combined with fever, cough, and recurring sudden falls in his home, he visited the emergency department. Given his travel activity, clinicians suspected legionnaires’ disease after initial standard antibiotic therapy failed to treat his pneumonia.

Fewer than 100 cases of legionnaires’ disease are reported per year but, as diagnosis is challenging, cases may be underreported.

“Clinicians should consider legionnaires’ disease in patients with pneumonia that does not improve despite the use of broad-spectrum antibiotics, especially if the pneumonia is severe, occurring in immunocompromised patients or patients with recent travel history, or when only antimicrobials without activity against atypical pathogens have been administered,” the authors recommend.

Reference: “Legionnaires’ disease following lake swimming in Iowa” by Ashley Bryson, Martha Shepertycky, Terence Wuerz and Philippe Lagacé-Wiens, 18 February 2025, Canadian Medical Association Journal.