West Nile Virus yet to get toehold in horses in Britain, study finds

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A micrograph of the West Nile Virus, appearing in yellow.
A micrograph of the West Nile Virus, appearing in yellow. Image: Cynthia Goldsmith, P.E. Rollin, USCDCP, public domain

Analysis of samples taken from nearly 1000 horses in Britain found no evidence of the mosquito-borne West Nile Virus, researchers report.

The findings are good news for Britain’s horse owners, but they provide no grounds for complacency.

First identified in Uganda in 1937, West Nile Virus is now found in many regions of the world, including the United States.

The emergence of West Nile Virus has also been seen across southern Europe, with repeated seasonal outbreaks in countries of the Mediterranean Basin, and most recently in Germany.

The ability of the virus to emerge is likely facilitated by the movement of birds carrying the virus in their blood.

Its presence in a region is usually detected following infection in dead-end hosts, such as horses or humans after bites from mosquitoes carrying the virus.

West Nile Virus can cause severe neurological problems in horses, with a fatality rate of about 33% in unvaccinated animals.

The study by Arran Folly and his colleagues at the Animal and Plant Health Agency in Surrey involved the testing of 988 serum samples from horses living in southeast England, considered the part of Britain at greatest risk for introduction of the virus.

The samples were tested for antibodies indicating previous exposure to West Nile Virus.

Two of the horses tested positive for WNV-specific antibodies. However, following a review of the clinical records of the two horses, the researchers were satisfied that the positive results were a result of vaccination against the virus within seven days of the blood samples being drawn.

The researchers concluded there was no evidence of underlying West Nile Virus transmission in England among the tested horses during the active season of 2019.

The findings are bolstered when considered in tandem with the results of the annual surveillance of bird samples from Britain in 2019, which found no West Nile virus RNA.

The researchers noted that three vaccines have been licenced for use in equines for West Nile Virus in Europe.

Discrimination between vaccinated and infected horses can be complex, they said.

The researchers said that while Britain is currently free of locally acquired cases — with only occasional human cases attributed to infection while abroad — the virus could emerge.

“Given that species of mosquito from the UK are competent vectors of West Nile Virus, and that there is migration of potentially viraemic birds from northern Europe, it is becoming increasingly feasible that WNV could emerge in UK mosquito populations.”

The full study team comprised Folly, Elisabeth Waller, Fiona McCracken, Lorraine McElhinney and Nicholas Johnson, all with the Virology Department at the Animal and Plant Health Agency in Surrey; and Helen Roberts with the Department for Environment, Food and Rural Affairs (Defra).

The study was funded by Defra, and the Scottish and Welsh governments.

Folly, A.J., Waller, E.S.L., McCracken, F. et al. Equine seroprevalence of West Nile virus antibodies in the UK in 2019. Parasites Vectors 13, 596 (2020). https://doi.org/10.1186/s13071-020-04481-9

The study, published under a Creative Commons License, can be read here

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