What causes Legionnaires' disease?Legionnaires' disease is caused by inhaling Legionella bacteria from the environment. Typically, the bacteria are dispersed in aerosols of contaminated water. These aerosols are produced by devices in which warm water can stagnate, such as air-conditioning cooling towers, humidifiers, shower heads, and faucets. There have also been cases linked to whirlpool spa
baths and water misters in grocery store produce departments. Aspiration of contaminated water is also a potential source of infection, particularly in hospital-acquired cases of Legionnaires' disease. There is no evidence of person-to-person transmission of Legionnaires' disease.
The bacteria that cause Legionnaires' disease are found in warm, stagnant water and the soil it seeps into. People inhale the bacteria when it becomes airborne, usually through air conditioners, humidifiers, shower heads and faucets, whirlpool spas, and even the water misters found in grocery stores. The bacteria has also been found in soil and groundwater at construction sites. Some people can be exposed to the Legionella bacteria, but not develop the infection.
Once the bacteria are in the lungs, cellular representatives of the body's immune system (alveolar macrophages) congregate to destroy the invaders. The typical macrophage defense is to phagocytose the invader and demolish it in a process analogous to swallowing and digesting it. However, the Legionella bacteria survive being phagocytosed. Instead of being destroyed within the macrophage, they grow and replicate, eventually killing the macrophage. When the macrophage dies, many new Legionella bacteria are released into the lungs and worsen the infection.
Legionnaires' disease develops 2-10 days after exposure to the bacteria. Early symptoms include lethargy, headaches, fever, chills, muscle aches, and a lack of appetite. Respiratory symptoms such as coughing or congestion are usually absent. As the disease progresses, a dry, hacking cough develops and may become productive after a few days. In about a third of Legionnaires' disease cases, blood is present in the sputum. Half of the people who develop Legionnaires' disease suffer shortness of breath and a third complain of breathing-related chest pain. The fever can become quite high, reaching 104°F (40°C) in many cases, and may be accompanied by a decreased heart rate.
From the onset of symptoms, the condition typically worsens during the first 4 to 6 days, with improvement starting in another 4 to 5 days. Most infection occurs in middle-aged or older people, although it has been reported in children. Typically, the disease is less severe in children.
Risk factors include cigarette smoking; underlying diseases such as renal failure, cancer, diabetes, or chronic obstructive pulmonary disease; people with suppressed immune systems from chemotherapy, steroid medications, or diseases such as cancer and leukemia; alcoholism; being middle-aged or elderly, and in people on a ventilator for extended periods.