Legionnaires' disease

Legionnaires' disease is a serious form of atypical pneumonia. This illness is caused by bacteria from the Legionella genus. While several species exist, the most common is Legionella pneumophila.

This bacterium causes over 90% of all Legionnaires' disease cases. The term legionellosis describes any disease caused by these bacteria. This group includes both Legionnaires' disease and a milder illness called Pontiac fever. Pontiac fever causes fever and muscle aches but does not lead to pneumonia. Legionnaires' disease is much more severe and requires careful medical attention.

The bacteria enter the human body through the respiratory system. Most people become infected by breathing in aerosolized water. An aerosol is a fine mist of water droplets suspended in the air. This mist can come from cooling towers, hot tubs, or even decorative fountains. In some rare cases, the bacteria enter the body if contaminated water is swallowed or enters a surgical wound. Once the bacteria reach the lungs, they target specific immune cells called macrophages. These white blood cells are meant to destroy invaders. However, Legionella bacteria are clever parasites. They enter the macrophage and prevent it from working correctly. Specifically, they block the fusion of phagosomes with lysosomes. Normally, a lysosome uses enzymes to break down bacteria inside a cell. By stopping this process, the bacteria multiply inside the macrophage until the cell dies. The bacteria are then released to infect more cells.

Legionella bacteria live naturally in fresh water environments like ponds and creeks. They thrive in specific conditions, particularly in man-made water systems. The bacteria grow best in warm water between 77 and 113 degrees Fahrenheit.

They can survive as intracellular parasites inside amoebae, which are tiny water-dwelling organisms. When bacteria and amoebae live together in a biofilm, they become very hard to destroy. A biofilm is a layer of microbes that sticks to the inside of pipes. Because of this, many different objects can harbor the bacteria. These include hot water tanks, humidifiers, nebulizers, and large air conditioning systems in hotels or hospitals. Even grocery-store misting systems used for produce can be a source.

The history of this disease began with a major outbreak in 1976. This event occurred during an American Legion convention in Philadelphia. Investigators traced the source of the illness to the cooling system of a hotel. This discovery led to the name we use today. Since then, cases have been found all over the world. In the United States alone, between 8,000 and 18,000 people are hospitalized for this disease every year. It is estimated that Legionnaires' disease causes between 2% and 9% of all pneumonia cases acquired outside of a hospital.

Symptoms of the disease usually appear 2 to 10 days after a person is exposed. In rare cases, symptoms may take up to 20 days to show. Common signs include high fever, chills, and a cough. The cough may be dry or produce sputum, which is thick mucus. About one-third of patients may cough up blood. Many people also experience muscle aches, headaches, and tiredness. About half of the patients suffer from gastrointestinal symptoms like vomiting or diarrhea. Neurological symptoms, such as confusion, can also occur in nearly half of those infected.

Diagnosis can be difficult because the symptoms look like other types of pneumonia. Doctors use several tools to confirm the disease. A urine antigen test is a common method. This test looks for specific proteins from the bacteria in urine. Richard Kohler developed a version of this test in 1982. While fast and reliable, it mostly only detects the most common subtype, Legionella pneumophila serogroup 1. Doctors may also use a sputum culture to grow the bacteria from mucus. Chest X-rays are also used to look for signs of pneumonia in the lungs.

Prevention focuses on managing water systems to stop bacteria from growing. One way to do this is by controlling water temperature. Keeping water either very hot or very cold can prevent Legionella from thriving. It is also important to prevent water stagnation. Stagnation happens when water sits still in pipes for a long time. Regular disinfection using chemicals like monochloramine can also help. This chemical is effective because it can penetrate biofilms. While there is no vaccine, good maintenance of building water systems is the best defense.