Legionella

Characteristics/Epidemiology

Normal habitat is water and soil

Can colonise cooling towers and water distribution system

85-90% of infections caused by Legionella pneumophila

Chlorine-tolerant - survives water treatment procedures

Pathogenesis/Transmission

Transmission: Inhalation of aerosolised organisms, swimming in contaminated water

Ability to reach the lung (due to decreased cough etc)

   -> phagocytosis by alveolar macrophages

   -> failure to fuse with lysosome

   -> multiply within phagosome

   -> cell ruptures, releasing bacteria

Clinical significance

Legionnaires' disease

Atypical acute lobar pneumonia

Develops in 1-5% of people exposed

Cause 1-5% of community-acquired pneumonia (after S. pneumoniae, H. influenzae, S. aureus)

Mortality rate - 5-30%

Early symptoms

Nonspecific

• Fever
• Malaise
• Myalgia
• Anorexia
• Headache

Later symptoms

• Range from cough to respiratory compromise
• Diarrhoea (watery, not bloody) in 25-50%
• (sometimes) Nausea, vomit, neurologic symtpom

 

Pontiac fever

Influenza-like illness

Attack rate 90% in exposed

Recovery - within 1 week.

No specific therapy required.

Laboratory identification

Facultative intracelluar parasite

Unencapsulated, slender rods

Aerobic

Fastidious, require L-cysteine

Gimenez stain more useful than Gram stain.

Definitive

Culture using buffered (pH6.9) charcoal yeast extract enriched with L-cysteine, iron, alpha-ketoglutarate

Other tests

• Direct fluorescent antibody test
• Radioimmunoassay for bacterial antigen in urine
• DNA probe for hybridization with legionella ribosomal RNA
• Detection of antibodies to Legionella (not useful because antibody develops slowly)

Treatment

First line: azithromycin (macrolides)

Second line: Levofloxacin (fluoroquinolones)

No treatment needed for Pontiac fever

 

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Bibliography:

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