Legionnaires' Disease

Legionnaires' disease is a form of atypical pneumonia caused by Legionella bacteria, most commonly Legionella pneumophila. This disease often manifests with symptoms such as cough, shortness of breath, fever, muscle pains, and headaches. Less frequently, nausea, vomiting, and diarrhoea can occur. Symptoms typically begin 2-10 days after exposure.

Chest X-ray of a severe case of legionellosis upon admission to the emergency department

Signs and Symptoms

The incubation period for Legionnaires' disease ranges from 2 to 10 days but can extend up to 20 days in rare cases. The disease primarily affects middle-aged and older individuals, especially those with a history of smoking, chronic lung disease, or weakened immune systems. Common symptoms include fever, chills, and a cough that may be dry or produce sputum. About half of the patients experience gastrointestinal symptoms, and nearly half present with neurological symptoms like confusion and impaired cognition. Laboratory tests might show abnormal kidney and liver functions, and chest X-rays often reveal pneumonia with consolidation in the lower lungs.

Cause and Transmission

Transmission electron microscopy image of L. pneumophila, responsible for over 90% of Legionnaires' disease cases

Over 90% of Legionnaires' disease cases are caused by L. pneumophila. The bacteria thrive in warm water environments like hot water tanks, cooling towers, and large air conditioning systems. The disease is primarily spread by inhaling aerosolized water or soil contaminated with Legionella. The bacteria grow best at temperatures between 25°C and 45°C (77°F and 113°F).

Diagnosis

Various stages of the disease: Chest radiograph (A) and high-resolution computed tomography (B) at hospital admission, repeat high-resolution computerised tomography of the chest a week after hospital admission (C, D), shown in a 42-year-old male with severe pneumonia caused by L. pneumophila serogroup 11a

Diagnosis of Legionnaires' disease can be challenging. The urine antigen test, which is quick and reliable, is commonly used but only detects L. pneumophila serogroup 1. Culturing the bacteria from sputum or other respiratory samples and comparing antibody levels in two blood samples taken weeks apart can also be diagnostic methods. Legionella spp. are difficult to stain with Gram stain but stain positive with silver and can be cultured on specific media like charcoal yeast extract with iron and cysteine.

Prevention

Preventing Legionnaires' disease relies on proper maintenance of water systems. Key measures include maintaining water temperatures outside the range of 20–55°C (68–131°F), preventing water stagnation, regular disinfection of water systems, and using biocides like monochloramine. Design considerations to reduce aerosol production and ensure proper training and communication among staff are also very important components of an effective water safety plan.

Treatment

Effective antibiotics for treating Legionnaires' disease include macrolides, tetracyclines, ketolides, and quinolones. The preferred agents are respiratory tract quinolones (such as levofloxacin) or newer macrolides (such as azithromycin). Treatment duration typically ranges from 5 to 10 days for levofloxacin or 3 to 5 days for azithromycin, but longer courses may be necessary for immunocompromised individuals or those with severe disease. Early initiation of appropriate antibiotics significantly reduces mortality.

Epidemiology

Legionnaires' disease was first identified in 1976 following an outbreak at an American Legion convention in Philadelphia. It primarily affects middle-aged and older adults. The disease is more prevalent in the summer and fall, and while outbreaks are highly publicised, most cases are isolated. In the United States, an estimated 8,000 to 18,000 cases occur annually, with significant under-reporting due to its rarity and the wide range of symptoms presented by infected individuals.

Outbreaks

A jug filled with chiller water taken from the Bellevue-Stratford Hotel's cooling system during the 1976 outbreak investigation on display at the David J. Sencer CDC Museum

Numerous outbreaks have been reported globally, often linked to complex water systems in hotels, hospitals, and other large buildings. Notable outbreaks have occurred in Stafford, England (1985), the Netherlands (1999), Murcia, Spain (2001), and Quebec City, Canada (2005). These outbreaks underscore the importance of stringent water safety and maintenance practices to prevent the spread of Legionnaires' disease.

Self-assessment MCQs (single best answer)

What is the primary cause of Legionnaires' disease?

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Which of the following is a common symptom of Legionnaires' disease?

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How is Legionnaires' disease most commonly spread?

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Which diagnostic test is quick and commonly used for Legionnaires' disease but only detects a specific serogroup?

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What temperature range is ideal for the growth of Legionella bacteria?

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Which antibiotic is preferred for treating Legionnaires' disease?

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What is a common preventative measure for Legionnaires' disease?

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Who is most at risk of developing Legionnaires' disease?

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What year was Legionnaires' disease first identified?

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Which imaging technique is often used to diagnose pneumonia in Legionnaires' disease?

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