Other Names
WNV, West Nile fever, West Nile encephalitis, West Nile encephalomyelitis in horses.
What is Equine West Nile Virus?
West Nile virus (WNV) is a mosquito-borne flavivirus that causes neurological disease in horses, humans, and a wide range of bird species. Originally described in Uganda in 1937, the virus has spread progressively across Africa, the Middle East, Europe, Asia, and dramatically into the Americas since its arrival in New York in 1999. It is now considered the most widely distributed arbovirus in the world.
In horses, WNV infection most commonly causes a febrile illness that resolves without complication, but in a significant minority of infected horses the virus invades the central nervous system, causing encephalomyelitis (inflammation of the brain and spinal cord) that can be severe or fatal. The case fatality rate in horses showing neurological signs varies by study but ranges from 30–40%, and a proportion of survivors have permanent neurological deficits.
In Europe, WNV circulates in an expanding geographic range — established in southern and central Europe (Italy, Greece, Hungary, Romania, and beyond) and has been detected in the UK on several occasions, though large UK outbreaks remain rare. As climate warms and the range of competent mosquito vectors expands northward, WNV poses an increasing threat to UK horses.
WNV is zoonotic — it can infect humans, typically causing a flu-like illness (West Nile fever) and, in a small proportion of cases, serious neurological disease (West Nile encephalitis or meningitis). Severe WNV disease in humans carries mortality rates of 10–15% in affected individuals. Horses are dead-end hosts and do not transmit the virus to humans, but they serve as sentinels for virus circulation in an area.
What Causes West Nile Virus in Horses?
WNV is a flavivirus (family Flaviviridae), related to dengue, Zika, and Japanese encephalitis viruses. It is transmitted through the bite of infected mosquitoes, primarily members of the Culex genus (especially Culex pipiens in Europe).
The transmission cycle:
- Wild birds — particularly corvids, raptors, and passerine species — serve as amplifying reservoir hosts
- Culex mosquitoes feed on viraemic birds and become infected
- Infected mosquitoes bite horses (and humans), transmitting the virus
- Horses and humans develop infection but do not produce sufficient viraemia to infect mosquitoes — they are dead-end hosts
Outbreaks typically peak in late summer and early autumn when mosquito populations are at their height. Areas near water bodies, wetlands, and flooded land carry the highest risk. Horse-to-horse transmission does not occur.
Symptoms and Signs
The majority of WNV-infected horses (approximately 70–80%) show no clinical signs at all or only mild, non-specific signs that go unnoticed. When neurological disease develops, signs typically appear 5–15 days after the infective mosquito bite:
Early signs:
- Fever — often the first sign, may be mild or intermittent
- Anorexia and depression
- Muscle twitching or tremors
Neurological signs (when CNS invasion occurs):
- Weakness and incoordination (ataxia) — often more pronounced in the hindlimbs
- Stumbling, tripping, or difficulty walking
- Muscle fasciculations (fine trembling of individual muscles)
- Hypersensitivity to touch and sound
- Altered mentation — dullness, apparent drowsiness (“sleeping sickness” appearance)
- Head tilt, circling, or pressing the head against a surface
- Cranial nerve dysfunction — facial droop, tongue weakness, difficulty swallowing
- Inability to rise from recumbency in severe cases
Neurological signs in WNV tend to have a gradual onset compared to the rapid progression seen with EEE. However, cases can deteriorate rapidly, particularly in older or immunocompromised horses.
Diagnosis
Diagnosis requires laboratory confirmation as neurological signs are non-specific. Key tests include:
Serology: IgM capture ELISA is the preferred ante-mortem test, detecting IgM antibodies that indicate recent (acute) infection. A single positive IgM result is highly indicative; paired sera showing seroconversion (rising titres) confirms recent infection. IgG antibodies persist longer and indicate past exposure or vaccination.
PCR: WNV PCR on blood is most useful in the early viraemic phase (first 1–3 days after infection, before neurological signs develop in most horses) and may be negative by the time the horse is presented with neurological disease.
Cerebrospinal fluid analysis: CSF may show inflammatory changes (increased white cells) consistent with viral meningitis or encephalitis.
Post-mortem examination: Brain histopathology demonstrates characteristic perivascular cuffing and neuronal necrosis. Immunohistochemistry or PCR on neural tissue confirms WNV.
Important differentials to exclude include equine herpesvirus myeloencephalopathy (EHM), equine protozoal myeloencephalitis (EPM), equine encephalomyelitis (EEE/WEE), ragwort hepatoencephalopathy, and cervical vertebral malformation (Wobblers).
See our entry on equine encephalomyelitis for comparison with related mosquito-borne neurological diseases.
Treatment
There is no specific antiviral treatment for WNV in horses. Supportive and symptomatic care is the mainstay:
- Rest and nursing care in a safe, padded environment
- NSAIDs or corticosteroids to reduce fever and CNS inflammation (the relative merits of steroids versus NSAIDs are debated; discuss with your vet)
- Fluid therapy and nutritional support — horses that cannot eat normally may require nasogastric feeding or IV fluid support
- Anti-convulsant medication for horses with seizures
- Physical therapy and assisted standing for horses that are recumbent but potentially recoverable
- Vitamin E supplementation is widely used by equine neurologists — it is an antioxidant that may support neurological recovery, though evidence is largely empirical
- DMSO (dimethyl sulfoxide) IV infusions may reduce cerebral oedema
Horses that remain recumbent and fail to improve with intensive care may require euthanasia on welfare grounds.
Prevention and Vaccination
Vaccination is the most effective preventive measure in endemic or high-risk areas. Inactivated WNV vaccines are licensed in several European countries and are widely used in the USA. Vaccination is available in the UK, though not routinely recommended for all horses — discuss risk assessment with your vet if you are in an area where WNV has been detected or if your horse travels internationally.
Mosquito control and avoidance:
- Reduce standing water on premises — buckets, water troughs, blocked drains, and poorly draining areas should be managed
- Change water troughs at least twice weekly during peak mosquito season
- Stable horses during peak mosquito activity (dawn and dusk, warm still evenings)
- Apply insect repellent (DEET-based products or permethrin-treated rugs) during peak mosquito activity periods
- Fit fine-mesh screens to stable windows and doors
- If dead birds are found on your property, report to APHA — dead corvids and raptors are sentinel events for WNV surveillance in the UK
The British Equine Veterinary Association provides current surveillance updates on WNV detection in the UK and Europe.
When to Call the Vet
- Any horse showing neurological signs — incoordination, weakness, unusual behaviour, head pressing, seizures — requires urgent veterinary assessment
- Fever followed by progressive weakness or altered mentation in summer or early autumn
- Horses that have been in areas where WNV has been detected (southern Europe, USA, Middle East) developing neurological signs on return
- Any unexplained fever during mosquito season, even without neurological signs — early treatment support may improve outcomes
Prognosis
The prognosis for horses showing neurological signs from WNV is guarded. Case fatality rates of 30–40% are reported in horses with clinical neurological disease. Of survivors, approximately 40% recover fully, 40% have residual mild neurological deficits that improve over months, and around 20% have more significant lasting deficits affecting their working ability. The severity of neurological signs at presentation and response to treatment in the first 48–72 hours are the best prognostic indicators. Early diagnosis and supportive care improve outcomes.
Frequently Asked Questions
Is West Nile virus present in the UK?
WNV has been detected in the UK on several occasions — principally in migratory birds. The competent mosquito vector (Culex pipiens) is established in England. As of 2024, large-scale equine outbreaks have not occurred in the UK, but the virus is increasingly well-established in southern and central Europe and climate-related northward expansion of both the virus and its vectors is expected. UK horse owners, particularly those in eastern and southern England, should be aware of WNV and consider vaccination if their horse travels to Europe or endemic regions.
Can I get West Nile virus from my horse?
No. Horses are dead-end hosts — they do not carry sufficient viral levels in their blood to infect mosquitoes, and the virus cannot be transmitted from horse to human through direct contact, blood, or handling. The risk to humans comes from infected mosquitoes in the same environment, not from the horse itself. Standard hygiene practices when handling sick animals are always advisable.
How long does recovery from West Nile neurological disease take?
Horses that survive the acute phase and begin to improve typically show continued recovery over weeks to months. Neurological recovery in horses is slow — subtle deficits may continue to improve for 6–12 months post-infection. Physio, controlled exercise programmes, and time are the main supports for recovery. Most significant improvement occurs within the first 6 months.
Should I vaccinate my horse against West Nile virus in the UK?
Risk-based vaccination decisions should be made with your vet. Current risk in most of the UK is low but increasing. Key situations where vaccination is recommended: horses travelling to continental Europe (particularly Mediterranean and Balkan regions), horses competing internationally, horses in areas where local WNV activity has been confirmed, and horses that are elderly or immunocompromised. As surveillance data improves and the virus spreads, vaccination guidance is evolving — discuss with your vet annually.