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Rickettsia
ORGANISM:
- Genus: Rickettsia, Rochalimaea, Coxiella
- Species: Rickettsia prowazekii (epidemic typhus), Rickettsia typhi (endemic typhus), Rickettsia rickettsii (spotted fever), Rochalimaea quintana (trench fever), Coxiella burnetii (Q fever)
GENERAL CONCEPTS:
- The Rickettsia are Gram-negative, obligate intracellular bacteria that infect mammals and arthropods.
- R. prowazekii is the agent of epidemic typhus. During World War I, approximately 3 million deaths resulted from infection by this bacterium. In World War II, the numbers were similar. This agent is carried by the human louse; therefore, disease is a consequence of overcrowding and poor hygiene.
- Rocky Mountain spotted fever and Q fever remain relatively common.
DISTINCTIVE PROPERTIES:
- These organisms are small, pleomorphic coccobacilli about 2 µm in length. Their structure is typical of Gram-negative bacteria.
- Rickettsia replicate in the cytoplasm and nucleus of their host cell; Coxiella replicate only in the phagolysosome.
PATHOGENESIS:
- Typhus, spotted fever and trench fever are transmitted via arthropod vectors; Q fever is acquired via inhalation or ingestion of contaminated milk or food.
- Within minutes, the bacteria enter host endothelial cells via an induced phagocytosis. The enzyme phospholipase A may help penetration.
- Replication of the bacteria causes lysis of the host cell and consequent spread to other cells.
- Initial replication occurs at the site of entry producing a local lesion. This is followed by dissemination via the vascular system producing vasculitis and a skin rash. These lesions may become necrotic.
- Virulence is probably due to many factors including release of endotoxin, the production of immune complexes and hypersensitivity reactions.
- A characteristic triad of symptoms include fever, headache and rash (no rash with Q fever).
HOST DEFENSES:
- Humoral immunity may be important because, following recovery from disease, persons become immune to further infection.
EPIDEMIOLOGY:
- Epidemic typhus and trench fever are transmitted from human to human via the louse.
- Endemic (murine) typhus is primarily maintained in rodent populations and is transmitted via the flea. Humans are an accidental host.
- Spotted fever is found predominantly in animals and is transmitted by the tick. Humans are accidental hosts. Most cases of Rocky Mountain spotted fever in the US occur during the summer months in North and South Carolina, Kansas and Oklahoma.
- Q fever is found mostly in animals. Humans acquire disease primarily by inhalation of contaminated aerosols.
DIAGNOSIS:
- Clinical: These diseases present as febrile illnesses after exposure to arthropods or animal hosts or aerosols in endemic areas and are easily misdiagnosed. A delay in diagnosis may be partly responsible for the high mortality from Spotted fever. The spread of the rash is often characteristic: spread from the trunk to the extremities (centrifugal) is typical for typhus; spread from the extremities to the trunk (centripetal) is typical for spotted fever.
- Laboratory: The use of immunofluorescent antibodies to examine a biopsy can be diagnostic. The organism can be inoculated into tissue culture and grown over 4-7 days but this is very hazardous to personnel. The Weil-Felix test looks for the production of serum antibody that is reactive against Proteus OX19, OX2 or OXK antigens but it is not always reliable.
CONTROL:
- Sanitary: Arthropod and rodent control are possible but difficult.
- Immunological: No vaccines are currently available.
- Chemotherapeutic: Tetracycline or chloramphenicol are drugs of choice.
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