Kovalev SY and Mazurina EA.
Omsk hemorrhagic fever virus is a tick-borne encephalitis virus adapted to muskrat through host-jumping.
J Med Virol. 2022;94(6):2510-2518. doi: 10.1002/jmv.27581.

Omsk hemorrhagic fever virus (OHFV) is endemic in Siberia (Russian Federation) and was recently also found in Kazakhstan (see Snapshot week 26/2022). OHFV is usually benign and has a low mortality rate of about 1%.

OHFV is closely related to TBE virus and was isolated in 1947. The virus consists of three subtypes (OHFV-1, -2 and -3) with a genetic distance of 9%–11% between each other. There are several theories about its evolutionary history, and one hypothesis is that the virus descended directly from the Far Eastern subtype of TBE-virus by host-jump or rapid adaption of the virus to a new species of arthropod host.

Phylogenetic analyses of the E gene fragments and the clusteron approach as well as amino acid sequence analyses indicate that OHFV derived from TBE-FE virus.

The main host for OHFV is the muskrat (Ondatra zibethicus), which was introduced in Siberia in 1935/1936, where both TBE-FE and TBE-Sib are present. In this region, some strains of TBE-FE have been identified which can cause hemorrhagic symptoms and it can be assumed that OHFV originated from such strains. The emergence of OHFV most likely happened between 1937 and 1939, and OHFV-1 and -2 probably appeared simultaneously. While OHFV has become widespread in Siberia, OHFV-2 disappeared, and OHFV-3 emerged much later than the other subtypes.

The main factor for the emergence of OHFV and its spread was human activity by introducing the muskrat in Siberia and the development of Siberia (by road and railways construction), and this may be an example of the critical role of humans in the emergence of new viruses.

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