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(American Tick Typhus, Tick-borne Typhus Fever) AGENT:Rickettsia rickettsii. RESERVOIR AND INCIDENCE:Dogs, wild rodents and rabbits. Reported from most of continental U.S., highest incidence in S. Atlantic and South Central States. 2/3 of human cases are reported in children. TRANSMISSION:Ixodid ticks (especially Dermacentor) or their host species. Most rickettsias are obligate intracellular parasites of the gut cells of invertebrates and can only survive briefly outside living cells. Crushed ticks or mites and their feces may infect through broken skin. Transmission from tick bite occurs only after several hours of attachment. DISEASE IN ANIMALS:Subclinical only. DISEASE IN MAN:Fever has a sudden onset, with chills, headache, severe muscle pains, photophobia and meningism for four weeks. A red, morbilliform rash develops within 3-5 days of onset of fever and with hemorrhages spreading on limbs. Enlarged liver and spleen, myocarditis, renal tubular necrosis and bronchopneumonia occur. Damage to endothelial cells of blood vessels by invasion of rickettsias causes thrombi and hemorrhages. Focal liver necrosis, hemorrhages in genitalis and gangrene of the scrotum may occur. The case fatality rate in untreated cases is 15-20%, but with prompt treatment is about 5%. DIAGNOSIS:Rickettsiae can sometimes be isolated in special laboratories from blood obtained in the first few days of illness. A rise in antibody titer during the second week of illness can be detected by specific CF, IFA, and microhemagglutination tests or by the Weil-Felix test. Antibody response may be suppressed if antimicrobial drugs are given very early. TREATMENT/PREVENTION/CONTROL:Treatment of human disease with tetracycline or chloramphenicol. Control ticks on newly arrived animals. ADDITIONAL INFORMATION: http://www.cdc.gov/ncidod/dvrd/rmsf/Natural_Hx.htm
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