Legionella - Pathogenesis

Pathogenesis

Legionella live within amoebae in the natural environment. however upon inhalation the bacteria can infect alveolar macrophages. subverting the normal host cell machinery to create a niche where the bacteria can replicate. This results in Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is via aerosols — the inhalation of mist droplets containing the bacteria. Common sources include cooling towers, swimming pools (especially in Scandinavian countries), domestic hot-water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources of Legionella include freshwater ponds and creeks. Person-to-person transmission of Legionella has not been demonstrated.

Once inside a host, incubation may take up to two weeks. Initial symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present.

However, the disease is generally not a threat to most healthy individuals, and tends to lead to harmful symptoms only in those with a compromised immune system and the elderly. Consequently, it should be actively checked for in the water systems of hospitals and nursing homes. The Texas Department of State Health services provides recommendations for hospitals to detect and prevent the spread of nosocomial infection due to legionella. According to the journal "Infection Control and Hospital Epidemiology," Hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the source is the water distribution system.

In the United States, the disease affects between 8,000 to 18,000 individuals a year.