Legionella

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:
1. 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.
2. 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.

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.