Creeping closer

Climate change is bringing the emerging threat of West Nile virus ever closer to the UK. Richard Newton FRCVS and Fleur Whitlock MRCVS outline what you need to know

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WEST NILE VIRUS (WNV) can infect horses if they are bitten by infected mosquitoes, with various mosquito species known to transmit the virus. These mosquitoes most commonly feed on birds, which are the primary hosts of WNV, and this has significant implications for the global spread of WNV, as birds can migrate long distances and introduce the virus to new regions.

If infected birds arrive in an area with uninfected mosquitoes, the virus can be transmitted to the local mosquito population when they feed on the birds.

Some of these mosquitoes can act as so-called bridging vectors if they then go on to bite and infect horses or humans, as well as birds. However, unlike birds, infected horses and humans do not carry enough virus in their blood to infect mosquitoes, and therefore they are considered incidental or “dead-end” hosts.

WHAT DISEASE DOES WNV CAUSE?

WNV causes West Nile fever, a viral disease that can affect both horses and humans. In horses, the infection can vary widely in severity; some horses may show no clinical signs at all (subclinical infections), while others may display mild signs such as fever, reduced appetite and lethargy.

However, roughly 30% of infected horses develop neurological signs, which can range from mild to severe. These may include muscle tremors (fasciculations), head tilt, weakness, incoordination (ataxia), dragging limbs (limb paresis or paralysis), or even recumbency and inability to stand.

There is no specific treatment for WNV, management is supportive and may include anti-inflammatory drugs, intravenous fluids and intensive nursing care. While many horses recover fully, some may have long-term neurological deficits and in a small number of cases, the disease can be fatal.