Legionellosis
(Legionnaires' Disease
and Pontiac Fever)
Medical Author: George Schiffman, MD, FCCP
Medical Editor: William C. Shiel Jr., MD, FACP, FACR
What is legionellosis?
Legionellosis is an infection that is caused by the bacterium
Legionella pneumophila. The disease has two distinct forms:
- Legionnaires' disease is the more severe form of the infection,
which may involve pneumonia. The onset of this form of the disease is usually two to 10 days after infection but can occur up to 16 days later. Legionnaires' disease acquired its name in 1976 after an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. Later, the bacterium causing the illness was named
Legionella pneumophila.
- Pontiac fever is a milder illness that develops from hours to
two days after initial infection and resolves spontaneously.
How common is legionellosis?
It is estimated that approximately 10,000 to 15,000 people
contract Legionnaires' disease in the United States each year. An
additional unknown number of people are infected with the Legionella
bacterium but have only mild symptoms or no symptoms at all.
Outbreaks of Legionnaires' disease have received the most media
attention. However, the disease most often occurs as single, isolated
cases not associated with any identified outbreak. Outbreaks are
usually recognized in the summer and early fall, but cases may occur
year-round. About 5%-15% of known cases of Legionnaires' disease
have been fatal.
Since the bacterium of Legionnaires' disease was identified in
1976, numerous hospital-acquired outbreaks of the disease have been
reported. These outbreaks have enabled researchers to study epidemics
of legionellosis.
What are the usual symptoms of legionellosis?
Patients with Legionnaires' disease usually develop a fever, chills, and a
cough. The cough may either be dry or produce sputum. Some patients with
Legionnaires' disease also have muscle aches, headache, tiredness, loss of
appetite, and occasionally diarrhea.
Legionnaires' disease at its worst can cause a severe pneumonia, seriously affect breathing, even lead to respiratory failure and adult respiratory distress syndrome (ARDS).
People with Pontiac fever experience a self-limiting influenza-like illness with fever, chills, headache, and muscle aches but, by definition, do not have pneumonia. Affected individuals generally recover in
two to five days without treatment.
Next: How is the diagnosis of Legionnaires' disease made? »
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