Legionellaceae are intracellular aerobic gram-negative bacilli that grow on buffered charcoal yeast extract (BCYE) agar. L. pneumophila causes 80-90% of cases of human Legionella disease and includes 16 serogroups; serogroups 1, 4, and 6 are most common.
- Legionella is found in fresh water and human-constructed water sources. Outbreaks have been traced to drinking water systems and rarely to cooling towers.
- The organisms are transmitted to individuals primarily via aspiration, but can also be transmitted by aerosolization and direct instillation into the lungs during respiratory tract manipulations.
- Legionella is an underestimated cause of community-acquired pneumonia, with only ∼3% of such cases actually diagnosed. It causes 10-50% of cases of nosocomial pneumonia if the hospital's water system is colonized with the organism.
- Pts who have chronic lung disease, who smoke, and/or who are elderly, immunosuppressed, or recently discharged from the hospital are at particularly high risk for disease.
Legionellosis manifests as either an acute, febrile, self-limited illness (Pontiac fever) or pneumonia (Legionnaires' disease).
- Pontiac fever is a flulike illness with a 24- to 48-h incubation period. Malaise, fatigue, and myalgias occur in 97% of cases. Fever, chills, and headaches are also very common, but pneumonia does not develop. The disease is self-limited and does not require antimicrobial treatment. Recovery takes place in a few days.
- Legionnaires' disease is more severe than other atypical pneumonias and is more likely to result in ICU admission.
- After a usual incubation period of 2-10 days, nonspecific symptoms (e.g., fever, malaise, fatigue, headache, anorexia) develop and are followed by a cough that is usually mild and only slightly productive. Chest pain and GI difficulties can be prominent.
- Radiologic findings are nonspecific, but pleural effusions are present in 28-63% of pts on hospital admission.
- Legionnaires' disease is not readily distinguishable from pneumonia of other etiologies on the basis of clinical manifestations, but diarrhea, confusion, temperatures >39°C (>102.2°F), hyponatremia, increased aminotransferase levels, hematuria, hypophosphatemia, and elevated CPK levels are documented more frequently than in other pneumonias.
- Extrapulmonary infection results from hematogenous dissemination and most commonly affects the heart (e.g., myocarditis, pericarditis).
The use of Legionella testing-especially the Legionella urinary antigen test-is recommended for all pts with community-acquired pneumonia.
- Sputum or bronchoscopy specimens can be subjected to direct fluorescent antibody (DFA) staining and culture.
- DFA testing is rapid and specific but is less sensitive than culture.
- Cultures on BCYE medium (with antibiotics to suppress competing flora) require 3-5 days to become positive; up to 2 weeks may be needed for non-pneumophila species.
- Serologic confirmation requires comparison of acute- and convalescent-phase samples. Detection of the necessary fourfold rise in titers often requires 12 weeks, but a single titer of 1:256 is presumptive evidence for Legionnaires' disease.
- Urinary antigen testing is rapid, inexpensive, easy to perform, reasonably sensitive (70-90%), and highly specific (95-100%). It is useful only for L. pneumophila serogroup 1, which causes 80% of disease cases.
- Urinary antigen is detectable shortly after disease onset and for up to 10 months, even during antibiotic treatment.
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LegionellaInfections
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Mortality rates approach 80% among immunocompromised pts who do not receive timely therapy. Among immunocompetent hosts, mortality can approach 31% without treatment but ranges from 3% to 11% with appropriate and timely therapy. Fatigue, weakness, and neurologic symptoms can persist for >1 year.