In the UK, West Nile Virus (WNV) in horses remains rare, but cases have been identified in horses imported from countries where the virus is more common, like southern Europe. The name refers to the place of origin, as the virus was first characterised in 1937 in the West Nile district of Uganda (East Africa). Around 25 years later, the first cases of the disease occurred in Europe. Today, the West Nile virus is found on all continents. Currently, there have been no confirmed cases of West Nile Virus (WNV) in horses within the UK, although it has been reported in other parts of Europe.
The virus is mainly transmitted by mosquitoes (mainly Culex species) and primarily affects birds but can also infect horses and humans. The highest risk of infection is during the warm months when mosquitoes are active. WNV is a notifiable disease in the UK, meaning any suspected cases must be reported to the Animal and Plant Health Agency (APHA) due to its potential health implications and to enable effective monitoring and containment of any potential outbreak.
In the UK, the total number of West Nile Virus (WNV) infections in horses, based on an estimated equine population of around one million, remains very low.
- A few isolated cases have been reported.
- Typically linked to horses returning from affected regions like Spain, without any ongoing spread within the country.
- In Germany, infections rose from two cases in 2018 to annual averages of 17–31.
- Cases between 2019 and 2023, highlighting a more significant exposure level in continental Europe.
- Monitoring and preventive measures continue in the UK to mitigate any potential spread.
- In 2024, no new cases of infected horses have been reported so far.
The number of reported infections has therefore currently stabilised.
Infection process
The WNV is transmitted by the bite and saliva of infected mosquitoes. The incubation period, i.e. the time between infection and the onset of the first symptoms of the disease, is 3-14 days. Horses can become infected if they live in areas with a high population of infected mosquitoes. Birds serve as the main host of the virus, while horses and humans are so-called ‘dead-end hosts’, which means that they cannot transmit the virus further and are also much less likely to become ill after an infection.
Accordingly, not all infected horses show symptoms, with only around 8-10% of infected animals developing severe neurological symptoms. Approximately 20% of animals with the symptomatic disease will suffer from lifelong neurological deficits, as the inflammatory processes in the brain and meninges have caused lasting damage. In 30-50% of cases, the infection can be fatal.
Symptoms of infection
About 90% of infected horses show no symptoms at all, so in most cases the disease progresses without causing any problems. If the disease does break out, the following signs may occur:
- Fever
- Lethargy, general weakness
- Lack of appetite, colic symptoms
- Trembling or muscle twitching
- Stumbling, staggering, coordination disorders (ataxia)
- Paralysis of the hindquarters
- Swallowing disorders
- Head tilt, facial paralysis
- Getting cast
If WNV infection is suspected, the veterinarian must be informed immediately. The veterinarian will examine the horse, accordingly, verify the suspicion by laboratory diagnostics and, if necessary, also report the case to the disease registry and initiate treatment.
Treatment
There is no specific antiviral treatment for West Nile virus in horses. Treatment is symptomatic, which means that the symptoms are alleviated by the veterinarian with anti-inflammatory and pain-relieving medication. In severe cases, intensive care is needed to control the neurological symptoms. If necessary, the horses must be accommodated in a fully padded stable with deep bedding to prevent injuries from falls. The prognosis for clinically ill patients must be made very cautiously.
Prophylaxis and prevention
There are approved vaccines for horses that provide good immunity against the virus. Horses are vaccinated for this once a year, ideally in spring about 4-6 weeks before the start of the mosquito season. According to the manufacturer, the vaccination should be boosted annually.
In addition, prophylactic measures should always be taken:
- Mosquito repellent: Use of mosquito repellent, fly rugs and nets (e.g. in front of stable windows in box stables) to protect horses from mosquito bites.
- Do not let horses out to pasture at dusk or at night, when there is the highest mosquito activity.
- Avoid mosquito breeding sites: standing water where mosquitoes breed should be avoided or eliminated near stables and horse pastures.
Summary
West Nile Virus (WNV) infections in horses remain uncommon in the UK, unlike Germany, where cases have increased since 2018. The main risk areas are in the east and northeast of Germany. In about 90% of infected horses, the disease is asymptomatic. A clinical disease is associated with severe neurological symptoms and deficits and can be fatal in individual cases. There is no treatment for WNV; the affected horse can only be treated symptomatically by a veterinarian with anti-inflammatories and painkillers. Therefore, prophylactic measures such as WNV vaccination and mosquito repellent should be considered in affected areas.
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