Reoviridae: The Orbiviruses (Colorado Tick Fever)

Abstract

Disease: Colorado tick fever. Etiologic Agent: Colorado tick fever virus. Source: Dermacentor andersoni ticks, possibly other ixodid tick species; certain wild rodents (tissues and blood); possibly human blood (by tick bite) or by inoculation or percutaneous contact with infected tick fluids, blood, or infected tissues of rodent hosts or human cases. Clinical Manifestations: Acute onset offever, becoming biphasic; chills; headache; photophobia; myalgias; arthralgias; lethargy; nausea and occasionally vomiting; rarely (especially in children) hemorrhagic tendency or signs of meningoencephalitis; fatalities very rare. Pathology: Leucopenia, mild anemia, persistent viremia; rarely thrombocytopenia and hemorrhagic manifestations (petechial rash, intestinal bleeding); focal necrosis in liver, myocardium, spleen, lymph nodes, intestine, and brain. Laboratory Diagnosis: Isolation of virus from blood cells (particularly erythrocytes) by suckling mouse or cell culture inoculation; direct fluorescent antibody staining of viral antigen in blood cells in peripheral blood smears; demonstration of significant antibody titer rise and/or of specific IgM antibody by indirect immunofluorescence staining, indirect enzyme immunoassay, neutralizing antibody or complement fixation methods. Epidemiology: Limited to 11 western and northwestern states of the United States and the southern parts of Alberta and British Columbia provinces in Canada, above 4,000 ft elevation at sites of natural tick-rodent foci (coinciding closely with distribution of the main tick vector); more common in males due to more frequent occupational or recreational exposure; seasonal occurrence (March to October) coinciding with tick activity; closely related viruses have been recognized elsewhere, but have not yet been shown to cause human disease. Treatment: Supportive only, no specific antiviral treatment known. Prevention and Control: A void tick bites in endemic areas; experimental vaccine for laboratory workers; cases should avoid donating blood for 6 months because of prolonged viremic period.