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Borrelia
ORGANISM:
- Genus: Borrelia
- Species: recurrentis, hermsii, burgdorferi
GENERAL CONCEPTS:
- Borreliae produce febrile diseases characterized by remittent fever.
- The organisms are transmitted to humans by lice or ticks.
- B. recurrentis produces epidemic relapsing fever (lice); B. hermsii causes endemic relapsing fever (ticks); B. burgdorferi is the agent responsible for Lyme disease (ticks).
DISTINCTIVE PROPERTIES:
- The Borreliae are similar to Leptospira but somewhat fatter and have more complex nutritional requirements.
- The cell wall contains various lipids including cholesterol.
PATHOGENESIS:
- Borreliae produce a generalized infection following an incubation period of about 1 week. Symptoms include fever, headache and muscle pain that lasts 4-10 days and subsides. An afebrile period lasting 5-6 days follows and then there is a recurrence of acute symptoms.
- Epidemic relapsing fever (transmitted by lice) is generally more severe than endemic relapsing fever (transmitted by ticks) and has an approximately 40% mortality if untreated. Also, the epidemic form is generally characterized by having a single relapse, while the endemic form may have several relapses due to cyclic antigenic variation of the Borrelia.
- Lyme disease (transmitted by ticks) involves the production of ulcerative lesions on the skin and may lead to arthritis or neurologic involvement.
HOST DEFENSES:
- Antibody is important in controlling disease because the organisms are capable of resisting non-specific defenses.
EPIDEMIOLOGY:
- B. recurrentis is transmitted via the human louse. Thus, this is a disease that occurs when people are crowded together under poor conditions; i.e. war, natural disaster, etc. Humans are the reservoir for B. recurrentis.
- B. hermsii is transmitted via the soft-shelled tick. This disease is maintained in (primarily) rat populations; humans acquire disease when bitten by an infected tick.
- B. burgdorferi is transmitted via the hard-shelled tick (deer tick). As with B. hermsii, the organisms are maintained within animal populations and humans are an accidental host.
DIAGNOSIS:
- Clinical: The symptomology of the recurrent fevers is not specific enough for accurate clinical diagnosis. With Lyme disease, however, the occurrence of a "bulls-eye" lesion on the skin (erythema chronicum migrans, ECM) is almost always (85%) associated with infection. This usually begins as a small red lesion that enlarges over several weeks to a reddened area that may cover several inches in diameter. Among cases that show ECM, about 20% progress to include arthralgia, about 50% involve intermittent episodes of arthritis and 10% progress to chronic arthritis.
- Laboratory: Darkfield smears can be used to observe the relapsing fever Borrelia but serologic tests (ELISA) are a better determinant for Lyme disease.
CONTROL:
- Sanitary: Measures to eliminate the tick and louse vectors are important for reducing the incidence of human disease.
- Immunological: A recombinant vaccine is available for Lyme disease, with a 75-90% efficacy.
- Chemotherapeutic: Tetracycline is the drug of choice.
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