Legionella infection

Background and epidemiology: The Legionella bacterium was first identified in 1976 when 34 members of the American Legion died at a conference in Philadelphia. It is a ubiquitous aerobic gram-negative organism that lives in soil and water. Although Legionella pneumophila is the most common disease-causing species, others such as L. longbeachiae and L. micdadei also cause disease.

Figure. Dieterle stain of lung tissue biopsy reveals small, brownish black intra- and extracellular bacilli. Photo by: CDC

Legionella pneumophila infection is most commonly associated with water-droplet transmission from cooling towers for large air-conditioning systems. It has also been found in spas, hot tubs, humidifiers and shower heads. In contrast, L. longbeachiae is usually associated with exposures to potting soil. Direct transmission of Legionella between people has not been documented.

Clinical symptoms of Legionella infection occur 2–10 days after exposure. Infections are more common among people who smoke, are elderly, have existing lung or kidney disease or diabetes, or are otherwise immunocompromised. Case fatality rates range from 5% to 30%.1

Each year, 8000– 18 000 people with Legionnaires' disease are admitted to US hospitals. A recent outbreak of Legionnaire's disease in a Toronto nursing home was responsible for 20 deaths among 127 infected residents and workers. Other large outbreaks have occurred; 125 people were infected with Legionella from contaminated cooling towers of the Melbourne Aquarium in Australia,2 and 188 became ill in Netherlands after a flower show where 2 whirlpool spas and a sprinkler later tested positive for Legionella.3

Clinical management: Legionella infection (also known as Legionellosis or Legionnaire's disease) usually presents as a severe atypical pneumonia. A rare, mild, self-limiting nonpneumonic illness called Pontiac fever has also been reported.1

Patients with Legionnaire's disease usually have high fever, chills and a dry cough. Systemic features, including myalgia, anorexia, headache, diarrhea, confusion, renal impairment and hyponatremia, are also common.

Current recommendations to treat undiagnosed atypical community or institutional acquired pneumonia include antibiotics that are effective against Legionella, such as erythromycin, ciprofloxacin and rifampicin.

Diagnosis: When a patient is suspected to have a Legionella infection, the clinician has a choice of possible diagnostic tests, including urinary antigen testing, serology and culture. Polymerase chain reaction (PCR) testing for Legionella DNA is also available in some labs.

Urinary antigen testing is the most rapid and sensitive test, but only detects infection with L. pneumophila. Samples may not show test-positivity for up to 5 days after the start of symptoms; testing should therefore be repeated if the specimen is taken early in the illness.

Positive serology (immunoglobulins IgM and IgG) for Legionella is common in the adult population. To demonstrate recent infection, serology testing requires 2 samples taken 3–6 weeks apart to show a fourfold rise in titre.

Culturing Legionella in vitro is difficult, and conventional media cannot be used; testing for Legionella must be specifically requested. Appropriate samples include sputum, pleural fluid or, from deceased patients, lung tissue. Identification of L. pneumphila serotypes through sputum culture can be used to trace environmental sources of infection.

Prevention: Reducing the risk of Legionella infection is possible with good maintenance, disinfection and testing of water-cooling towers. Owners of home humidifiers and spas should follow the manufacturer's instructions for maintenance and disinfection.