Legionella
Legionella refers to a group of bacteria, most notably *Legionella pneumophila*, which is responsible for Legionnaires' disease, a severe form of pneumonia. These gram-negative, rod-shaped bacteria thrive in warm water environments, including natural settings like streams and man-made systems such as air conditioning units, hot tubs, and decorative fountains. Transmission typically occurs through inhaling aerosolized water droplets containing the bacteria, with potential sources also including potting compost. Those most at risk include individuals with compromised immune systems, such as patients with cancer or HIV, and those recently hospitalized for other illnesses.
In the United States, approximately 18,000 people are hospitalized annually due to Legionnaires' disease, with a significant mortality rate. Symptoms of infection often include high fever, cough, chills, and gastrointestinal issues. Diagnosis can involve sputum cultures or urinary antigen tests. Treatment usually employs macrolide or fluoroquinolone antibiotics, as early misdiagnosis can occur due to symptom overlap with other pneumonia-causing bacteria. Historical outbreaks, such as the significant one in 1976 at a Philadelphia hotel, have underscored the importance of awareness and testing for Legionella in healthcare settings.
Legionella
- TRANSMISSION ROUTE: Ingestion, inhalation
Definition
Legionella pneumophila, the bacterium that causes Legionnaires’ disease, leads to severe pneumonia and is the most commonly known form of Legionella disease worldwide.
Natural Habitat and Features
Legionella are gram-negative rod-shaped bacteria that proliferate in water, mud, streams, and within some aquatic devices, such as decorative water fountains. Whirlpool spas have also been found to harbor Legionella, as have air conditioning systems and water heaters. Some forms of Legionella are known to infect such amoebas as Acanthamoeba castellanii or Hartmannella species. Several cases of L. longbeachae have been identified in potting compost in Scotland. The transmission was believed to occur through inhaling droplets from the compost.
When not infecting humans or animals, Legionella reside within fresh-water protozoa and amoebas and within a biofilm, an aggregate of different types of bacteria that link together, as on a water surface. Legionellae have been proven to reproduce with many different types of amoebas, multiple species of protozoa, and one species of slime mold. Legionellae alternate between two states, the reproductive state and the transmissive state.
Legionella thrives best at temperatures ranging from 77 to 113 degrees Fahrenheit (25 to 45 degrees Celsius). The incubation period for Legionella ranges from two to nineteen days. Most outbreaks occur in warm weather.
Pathogenicity and Clinical Significance
Legionella invades the host lungs, and the microbes are immediately attacked by the host macrophages. However, Legionella converts these macrophages into compartments in which the bacteria then multiply. This is similar to the means Legionella employ when they grow within protozoa, which are usually their hosts. Each year, up to 18,000 people are hospitalized for Legionnaires’ disease in the United States. Up to 30 percent of these cases are fatal.
The presence of this pathogen in humans can be detected with sputum cultures or urinary antigen assays. A second test may be needed several days later if the first test is negative and patients still present with symptoms that are indicative of Legionella infection, such as vomiting, high fever, chills, cough, diarrhea, and confusion. Legionella infection is more likely to occur in persons who have already been hospitalized for another illness and who then contract Legionella; it is also common in those persons with compromised immune systems, such as persons with cancer or with human immunodeficiency virus (HIV) infection. In addition, persons who take immunosuppressive drugs, such as those who have had organ transplants, have an elevated risk of Legionella infection.
Legionella infection may present in hospitals where patients live in close quarters. All patients diagnosed with pneumonia while in the hospital should be tested for Legionella.
Guinea pigs were the first species in which Legionella was isolated in 1943, but the genus was not established until 1979, three years after the 1976 outbreak, when more than 200 persons attending an American Legion conference at a hotel in Philadelphia became severely ill; twenty-nine attendees died. The pathogen was traced to the air conditioning system of the hotel. A milder form of this infection is referred to as Pontiac fever, named after the city in Michigan where an outbreak of Legionella infection occurred in 1968, infecting more than sixty county health workers. This discovery was made retrospectively after the 1976 identification of Legionella. The source of the infection in the Pontiac case was found to be the air conditioning system. In contrast to Legionnaires’ disease, Pontiac fever may develop in healthy persons.
Legionella can be isolated in the blood, lung tissue, sputum, and stool. It has also been cultured from bone marrow.
Drug Susceptibility
Erythromycin was once a common drug of choice for eradicating infection with Legionella, although other macrolide antibiotics, such as azithromycin, or fluoroquinolone antibiotics, such as levofloxacin, have been approved by the US Food and Drug Administration for the treatment of this infection and become first-line drug treatments of choice. Legionella infection may initially be misdiagnosed as Streptococcus pneumoniae infection, the most common cause of pneumonia, if laboratory testing is not performed.
Bibliography
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