Omsk hemorrhagic fever (OHF) is caused by the Omsk hemorrhagic fever virus (OHFV), a member of the Flaviviridae virus family. OHFV is mostly spread to rats by the bite of an infected tick. OHFV seems to be highly persistent in varied habitats; therefore, transmission may occur without any direct tick or rodent contact. Tick bites or contact with the blood, excrement, or urine of an infected or dead animal may cause infection.
Human exposure may also occur via contaminated water, in a laboratory environment, or through infected goat or sheep milk. Omsk hemorrhagic fever (OHF) is mostly found in Omsk, Novosibirsk, Kurgan, and Tyumen in western Siberia.