Legionnaires Disease in Emergency Medicine
- Author: Frank C Smeeks lll, MD; Chief Editor: Rick Kulkarni, MD more...
Background
An outbreak of serious pulmonary infections among people attending a convention of the American Legion in Philadelphia during the US Bicentennial celebration in July 1976 prompted the description of Legionnaires disease and its causative organism, Legionella pneumophila.
A nonpneumonic variant caused by the same species is called Pontiac fever, named for an outbreak of the described disease in Pontiac, Michigan, in 1968.
Pathophysiology
Legionella species are poorly staining, obligate, aerobic, gram-negative bacilli. Serogroup 1 is most frequently identified in clinical disease, but 18 serogroups of L pneumophila are presently known. Other than L pneumophila, 34 species of Legionella have been identified, mostly from immunocompromised patients with pneumonia.
Although pneumonia is the most common presenting problem, other pulmonary manifestations are frequent. The constitutional, cardiac, gastrointestinal, neurologic, renal, musculoskeletal, hepatic, and hematologic abnormalities variably seen with this disease are elaborated in Clinical.
This electron micrograph depicts an amoeba, Hartmannella vermiformis (orange), as it entraps a Legionella pneumophila bacterium (green) with an extended pseudopod. After it is ingested, the Legionella pneumophila bacterium can survive as a symbiont within what then becomes its protozoan host. The amoeba then becomes what is referred to as a "Trojan horse," for by harboring the pathogenic bacteria, the amoeba can afford them protection, and in fact, in times of adverse environmental conditions, are able to metamorphose into a cystic-stage enabling it, and its symbiotic resident pathogens to withstand such environmental stresses. Image courtesy of the Centers for Disease Control and Prevention and Dr. Barry S Fields. Epidemiology
Frequency
United States
Since the initial identification of 235 cases in 1976,[1] Legionnaires disease has become recognized as the most common cause of atypical pneumonia in hospitalized patients. It is the second most common cause of community-acquired bacterial pneumonia. Legionnaires disease is reportable in the United States. The Centers for Disease Control and Prevention (CDC) received reports of 3,181 total cases in 2008.[1] Prevalence reports of Legionella have increased with time likely due to the availability of more effective testing modalities. However, it is also possible that Legionella infections are increasing in frequency for environmental, population, or behavioral reasons.
International
Outbreaks have been recognized throughout North America, Africa, Australia, Europe, and South America.
Mortality/Morbidity
- Legionnaires disease has a 25% mortality rate. However, this figure should be interpreted cautiously because of possible underreporting of comorbid disease.
- Legionnaires disease is frequently associated with gastrointestinal symptoms, including nausea, vomiting, diarrhea, and abdominal pain.
- Neurologic symptoms are also common, including lethargy, altered mental status, and nonfocal neurologic examination findings.
- Hyponatremia is common in Legionnaires disease.
- Rhabdomyolysis and renal failure may be seen in this disease.
Sex
Men are affected more frequently than women.
Age
- The weighted mean age for patients with Legionnaires disease is 52.7 years, with increasing incidence until age 79.
- The incidence in persons younger than 35 years is less than 0.1 cases per 100,000 people.
- Older patients have higher mortality rates.
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