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Legionella
ORGANISM:
- Genus: Legionella
- Species: pneumophila
GENERAL CONCEPTS:
- Legionellae are ubiquitous aquatic saprophytes
- Legionella is a relatively recent discovery in terms of human disease.
- L. pneumophila, in particular, is the etiologic agent of Legionnaire's disease, first described in Philadelphia in 1976.
- Legionnaire's disease is characterized by cough and fever with radiologic evidence of pneumonia.
- Legionella is a unique, previously unrecognized bacterium. One might say it is a new organism for an old disease.
DISTINCTIVE PROPERTIES:
- Legionella are motile, catalase-positive, Gram-negative bacilli. Some strains produce a yellow fluorescence under 366 nm ultraviolet light. Most are fastidious, requiring iron and L-cysteine for growth.
- There are 25 species of Legionella and 42 serogroups. Most (85%) clinical isolates are L. pneumophila serotype 1.
- The Legionella cell wall is mostly diaminopimelic acid (DAP) and contains little peptidoglycan. The LPS appears to lack the endotoxic lipid A moiety.
- Several species produce hemolysins and there are some reports of exotoxins.
- Legionella are capable of intracellular (macrophage) multiplication.
PATHOGENESIS:
- Legionella have, in general, a low potential for virulence and most infections occur in persons having compromised immunity or
pulmonary function.
- There are two forms of disease produced by Legionella:
- Pontiac fever: An epidemic flu-like condition described in Pontiac Michigan in 1968 was later found to be due to Legionella. This condition was marked by fever, chills, headache and malaise that lasted 2-5 days and resolved.
- Legionnaire's disease: A severe pneumonia characterized by fever, chills and a non-productive cough. This multi-organ disease has significant mortality if not treated promptly.
- The virulence of Legionella is dependent upon their ability to survive and multiply within macrophages.
HOST DEFENSES:
- Most healthy individuals resist infection by Legionella but the mechanisms are not well understood.
EPIDEMIOLOGY:
- Legionella are typically associated with aerosolized water (central air conditioning, cooling towers, showers, whirlpools).
- Disease is generally waterborne; transmission occurs via airborne droplets.
- The organisms exist in nature; humans are an accidental host.
DIAGNOSIS:
- Clinical: Symptoms include headache, malaise, rapid fever, nonproductive cough, pneumonia. Generally, disease is difficult to diagnose but might be suspected in middle aged to older men who smoke and drink.
- Laboratory: Bacteria can be grown on Buffered Charcoal-Yeast Extract (BCYE) agar. Direct immunofluorescent may be used to visualize the organisms. An increase in Legionella-specific serum antibody is evidence of infection.
CONTROL:
- Sanitary: Regular maintenance of air conditioning or the inclusion of biocidal compounds into water cooling towers reduces the reservoir. Similarly, hyperchlorination of the water supply eliminates the source.
- Immunological: None available.
- Chemotherapeutic: Erythromycin is the drug of choice.
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