Legionnaires' disease

Legionnaires' disease is a form of atypical pneumonia caused by any species of Legionella bacteria, quite often Legionella pneumophila. Signs and symptoms include cough, shortness of breath, high fever, muscle pains, and headaches. Nausea, vomiting, and diarrhea may also occur. This often begins 2–10 days after exposure. A legionellosis is any disease caused by Legionella, including Legionnaires' disease (a pneumonia) and Pontiac fever (a related upper respiratory tract infection), but Legionnaires' disease is the most common, so mentions of legionellosis often refer to Legionnaires' disease. Legionella is found naturally in fresh water. It can contaminate hot water tanks, hot tubs, and cooling towers of large air conditioners. Typically, it is spread by breathing in mist that contains Legionella, and can also occur when contaminated water is aspirated. It typically does not spread directly between people, and most people who are exposed do not become infected. Risk factors for infection include older age, a history of smoking, chronic lung disease, and poor immune function. Those with severe pneumonia and those with pneumonia and a recent travel history should be tested for the disease. Diagnosis is by a urinary antigen test and sputum culture. No vaccine is available. Prevention depends on good maintenance of water systems. Treatment of Legionnaires' disease is commonly conducted with antibiotics. Recommended agents include the fluoroquinolones levofloxacin and moxifloxacin, and the macrolide azithromycin. Hospitalization is often required. The fatality rate is around 10% for previously healthy people, but up to 25% in those with underlying conditions. The numbers of cases that occur globally is not known. Legionnaires' disease is the cause of an estimated 2–9% of pneumonia cases that are acquired outside of a hospital. An estimated 8,000 to 18,000 cases a year in the United States require hospitalization. Outbreaks of disease account for a minority of cases. While it can occur any time of the year, it is more common in the summer and autumn. The disease is named after the outbreak where it was first identified, at a 1976 American Legion convention in Philadelphia.

Signs and symptoms The length of time between exposure to the bacteria and the appearance of symptoms (incubation period) is generally 2–10 days, but more rarely can extend to as long as 20 days. For the general population, among those exposed, between 0.1 and 5.0% develop the disease, while among those in hospital, between 0.4 and 14% develop the disease. Those with Legionnaires' disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Almost all experience fever, while around half have cough with sputum, and one-third cough up blood or bloody sputum. Some also have muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia), chest pain, or diarrhea and vomiting. Up to half of those with Legionnaires' disease have gastrointestinal symptoms, and almost half have neurological symptoms, including confusion and impaired cognition. "Relative bradycardia" may also be present, which is low to normal heart rate despite the presence of a fever. Laboratory tests may show that kidney functions, liver functions, and electrolyte levels are abnormal, which may include low sodium in the blood. Chest X-rays often show pneumonia with consolidation in the lower portion of both lungs. Distinguishing Legionnaires' disease from other types of pneumonia by symptoms or radiologic findings alone is difficult; other tests are required for definitive diagnosis. People with Pontiac fever, a much milder illness caused by the same bacterial genus, experience fever and muscle aches without pneumonia. They generally recover in 2–5 days without treatment. For Pontiac fever, the time between exposure and symptoms is generally a few hours to two days.

Cause

Over 90% of cases of Legionnaires' disease are caused by Legionella pneumophila. Other types include L. longbeachae, L. feeleii, L. micdadei, and L. anisa.

Transmission Legionnaires' disease is usually spread by the breathing in of aerosolized water or soil contaminated with the Legionella bacteria. Experts have stated that Legionnaires' disease is not transmitted from person to person. In 2014, one case of possible spread from someone sick to the caregiver occurred. Rarely, it has been transmitted by direct contact between contaminated water and surgical wounds. The bacteria grow best at warm temperatures and thrive at water temperatures between 25 and 45 °C (77 and 113 °F), with an optimum temperature of 35 °C (95 °F). Temperatures above 60 °C (140 °F) kill the bacteria. Sources where temperatures allow the bacteria to thrive include hot water tanks, cooling towers, and evaporative condensers of large air conditioning systems, such as those commonly found in hotels and large office buildings. Before 1988, energy conservation programs from the late 1970s and early 1980s still mandated a maximum hot water generation, storage, and distribution temperature of 110 °F (43 °C), unknowingly Legionella bacteria's ideal breeding temperature. To minimize risks of bacterial growth, the American Society of Heating, Refrigerating and Air-Conditioning Engineers' 1988 ASHRAE Standard 188 and subsequent ASHRAE Guideline 12-2000 increased recommended hot water generation and storage temperatures to 135–140 °F (57–60 °C) with minimum distribution temperatures of 124 °F (51 °C). Though the first known outbreak was in Philadelphia, cases of legionellosis have occurred throughout the world.

Reservoirs

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