Omsk hemorrhagic fever is a tick-borne encephalitis of the Flaviviridae family endemic to Russia, specifically the Omsk region and other areas of western Siberia. It is related to yellow fever and Kyasanur Forest disease. Natural infection may occur as a result of infected tick bites (Dermacentor reticulatus, Dermacentor marginatus, Ixodes persulcatus [taiga tick]), contact with body fluids, or inhalation of an aerosol from infected water voles and muskrats. Workers with exposure to animal pelts or muskrats are at higher risk as most cases have occurred in muskrat trappers. Infection leads to immunity. The mortality rate of Omsk hemorrhagic fever is about 10%.

Its incubation period is typically 2-8 days. Initial symptoms include the sudden onset of flu-like symptoms including fever, chills, headache, myalgia, arthralgia, severe prostration, flushing of the upper body, conjunctival redness, vesicles on the soft palate, petechiae, cervical lymphadenopathy, and a relative bradycardia. Within two weeks it may progress to pneumonia, central nervous system involvement (delirium, seizures, coma), or gastrointestinal (GI) and mucosal bleeding. Thrombocytopenia, increased liver function tests, and leukopenia may be seen.

Person-to-person transmission has not been documented, but infection via exposure to infected body fluids has been documented. A vaccine (inactivated TBE) is available, but only in Russia.