West Nile virus has been detected in UK mosquitoes for the first time, a finding the British Equestrian Veterinary Association (BEVA) said is “notable” but the current risk to horses and humans “remains very low”.

The UK Health Security Agency (UKHSA) announced yesterday (21 May) that fragments of West Nile virus genetic material had been identified in mosquitoes collected in Britain for the first time.

The virus is mosquito-borne and can affect birds, humans and horses. No equine or avian cases of West Nile virus reported in the UK have been associated with local transmission. All confirmed equine cases to date have been linked to recent travel abroad.

The majority of infected horses do not show signs. For those that do, signs of West Nile virus typically include muscle tremors, weakness, stiffness, ataxia – wobbly gait – and potentially recumbency. Its effects can range from being relatively mild to fatal.

The British Horse Society (BHS) has further information on the virus on its website and states that once a horse or human has been infected, they cannot pass the virus directly to another. It adds that many horses recover and return to full health, although a small minority of cases report long-term effects such as gait abnormalities. The BHS website also states that the mortality rate for horses which show clinical signs is reported to be between 35%-45%.

The UKHSA said “there is no evidence to suggest ongoing circulation of the virus in birds or mosquitoes in the UK and the risk to the general public is very low”.

How was the virus detected in UK mosquitoes?

The discovery came about through a UKHSA and Animal and Plant Health Agency research programme. Fragments were detected in two samples of Aedes vexans mosquitoes (commonly known as inland floodwater mosquito) from wetlands on the River Idle near Gamston (Retford) in Nottinghamshire, in July 2023.

The mosquitoes were pooled into 200 groups of 10 for testing, and fragments of West Nile virus genetic material were identified in two of the pools. The other 198 were negative.

BEVA board member Prof James Wood, infectious disease epidemiologist and co-director of Cambridge Infectious Diseases at the University of Cambridge, said: “It is very interesting that UKHSA and APHA have published evidence of West Nile virus infection in mosquitoes in England, although it is not especially surprising as it has been known for some time that previously infected migratory bird species have been travelling to and in some cases then spending summers in the UK.

“West Nile virus has become more prominent across Europe over the last 10-20 years. Risk mapping has identified parts of England, in particular in the south, as sites where West Nile virus transmission might be expected. It is not clear whether this finding is simply a new observation of something that has been occurring for years, or a new occurrence.

“APHA has monitored dead birds for West Nile virus for some time, but not detected it and while this surveillance is to be applauded, surveillance in healthy wild birds has never been undertaken.”

Professor Wood added that flaviruses, such as West Nile virus, more generally do not cause disease in their avian hosts.

He said that the natural cycle for transmission for many of these does not involve mammalian hosts. But that spillover into horses and humans, with clinical disease consequences, is a concern for West Nile virus when it does happen.

“Encephalitis is seen occasionally in both humans and horses,” said Professor Wood. “Veterinary and public health authorities have known for some time about the risks of West Nile virus transmitting to humans and horses and this new finding does emphasise the importance of medical and veterinary surveillance in cases of unexplained encephalitis.

“When West Nile virus is known to be circulating locally, enhanced mosquito avoidance approaches (stabling of horses, especially at dawn and dusk in high-risk areas) may be warranted.”

West Nile virus: ‘Currently no cause for alarm’

A BEVA statement said that the presence of the virus in mosquito vectors highlights the importance of continued surveillance, particularly as climate conditions shift.

It added that veterinary practitioners should include West Nile virus in their differential diagnosis for acute neurological presentations, especially in horses with tremors or ataxia, even when travel history is unclear.

“Horse owners travelling to endemic regions with their horses should be advised to consider West Nile virus vaccination, especially during mosquito season,” said the BEVA statement.

“Seasonality may no longer be a reliable predictor of mosquito activity. Climate change has increased the complexity and unpredictability of vector-borne disease patterns.

“While this is the first known detection of West Nile virus in UK mosquitoes, there is currently no cause for alarm.”

BEVA also supports “proactive awareness and preparedness in equine practice”; further details on this are on its website.

“We will continue to monitor any further developments and share relevant updates with our members and the wider equine community,” added the statement.

Meera Chand, UKHSA deputy director for travel health, zoonoses, emerging infections, respiratory and tuberculosis, said: “While this is the first detection of West Nile virus in mosquitoes in the UK so far, it is not unexpected as the virus is already widespread in Europe.

“The risk to the general public is currently assessed as very low. Vector research of this kind is designed to give us early warning of potential threats so that we can enhance our disease surveillance and control activities and ensure patients receive appropriate testing.”

West Nile virus is a notifiable disease, so must by law be reported to Defra in England and Wales, and in Scotland and Northern Ireland to the equivalent government authority.

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A confirmed case of West Nile virus in the UK “need not prompt alarm”, the British Equine Veterinary Association (BEVA) says, but is a reminder of being vigilant for signs of disease.

The viral infection, which can be fatal and can also affect people, was confirmed in an unvaccinated warmblood mare, who had recently returned to the UK having been competing in southern Spain, by the Animal and Plant Health Agency, on 6 November. The mare started showing muscle twitches and gait abnormalities on her return journey, and her signs worsened after she reached home, including extensive twitching, abnormal posture, ataxia and hindleg weakness. Her signs improved and she was discharged from the Hertfordshire hospital on 8 November.

“The timeline was such that the infection was almost certainly contracted in Spain,” a BEVA spokesman said. “The horse was transported straight to a leading equine hospital and having now been discharged, is recovering well at home.”

BEVA stressed that there is no risk of the virus being transmitted. It is a notifiable disease as it is a threat to human health.

“However, this case need not prompt alarm as the infection does not spread from horses to other animals or people,” the spokesman said.

“Although horses can become very sick, and even die, when they become infected, they have very low levels of virus circulating in their bodies, insufficient for the infection to be transmitted.”

This is not the first time the virus has been confirmed in the UK – it was diagnosed in October 2013, in a horse who had arrived here from Europe. The disease is becoming increasingly common in southern and eastern Europe, where it is spread by mosquitos.

“With rising temperatures spread of West Nile virus to northern European countries, including the UK, is increasingly possible,” the BEVA spokesman said. “Cases in horses have been identified this year in multiple countries, including France and Germany. This case serves as a reminder of the value of vaccinating horses travelling to countries where the virus is common, such as southern and eastern Europe and the Americas.

“Vaccines are available in the UK. The risk is highest in summer and autumn when mosquitoes are more prevalent. It is also a reminder of the importance of remaining vigilant for signs of the disease in horses that are travelling. Signs typically include muscle tremors, weakness, stiffness, ataxia – wobbly gait – and potentially recumbency.”

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West Nile Virus has been confirmed in three horses in Washington State — the first cases in the state this year.

Last year Washington State had the highest number of West Nile Virus cases in the US, with 41 horses testing positive for the disease.

On Friday (31 July) the Washington Animal Disease Diagnostic Laboratory and the state Department of Agriculture confirmed one-year-old gelding quarter horse in Kittitas county had the disease. It was put down.

And at the end of last month (24 July) West Nile Virus was confimed in two other horses in the state.

An eight-year-old quarter horse mare in foal in Toppenish was euthanised, while a nine-year-old quarter horse gelding in Prosser is expected to recover.

None of the horses had been vaccinated against the disease.

Meanwhile also this week a horse near Hermiston in Oregon was the first to test positive to the virus in the state this year.

West Nile Virus is spread by mosquitos and can cause neurological damage and death in horses and humans.

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A vaccine to control the spread of West Nile virus has been licensed for use in Europe.

Produced by Fort Dodge Animal Health the vaccine, Duvaxyn WNV, is the first equine West Nile vaccine to be licensed in Europe.

Fort Dodge hopes the vaccine will provide reassurance to horse owners after an outbreak in Northern Italy last Septmber that killed five horses and one human.

Helen Barnes from Fort Dodge said: “Following the recent Italian outbreak, West Nile virus is now regarded as endemic in Italy — a fact that confirms the threat it poses to the equestrian sector in Europe where the consequences of a major outbreak could be devastating.

The vaccine is known as West Nile-Innovator in the USA and has been used there for several years. It was the only vaccine available during the 2002 outbreak that affected 15,000 horses and the US Department of Agriculture has credited it with contributing to the 70 per cent decline in US cases since the height of the outbreak.

Ms Barnes added: “Our experience with West Nile Virus in horses in the United States has given us an exceptional understanding of the disease and the importance of the vaccination and we’re now working in partnership with European Governments, the EU Commission and the veterinary community to build knowledge of the disease.”

Scientists believe the increased numbers of horses travelling internationally to compete and effects of global warming on mosquito breeding is to be blamed for the growth of West Nile.

Professor Ann Cullinare, head of virology at the Irish Equine Centre said: “The virus was responsible for one of the worst arbovirus epidemics in the history of America and, since it was first identified in New York in 1999, has been responsible for thousands of horse fatalities.

“European outbreaks of West Nile have not yet been of the epidemic nature of the disease in the United States, but it’s essential that Europe is prepared. Our capability to mount an effective response to the virus has been significantly improved by the licensing of this vaccine.”

West Nile virus is transmitted by mosquitos and can cause neurological damage in horses and humans. There is no cure but horses can survive with aggressive therapy, although some suffer permanent brain defects.

The incubation period for West Nile virus is between 3-15 days and the mortality rate among horses showing symptoms is reported to be as high as 40 per cent.

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Vets from Liverpool University and the Equine Veterinary Hospital in Arundel, Sussex fly out next week to teach poor farmers in the west African nation proper care of their horses.

There are only seven vets in the whole of Gambia, a country of only 1.5m people and which was without horses until thirty years ago.

Horses were brought in from neighbouring Senegal as the foreshortening of the rainy season due to climate change meant farmers needed to harvest faster.

Heather Armstrong, chief executive of the Gambia Horse and Donkey Trust, who was born in Gambia, has organised the aid.

She said: “All the vets are volunteering. Rob Christley and Gina Pinchbeck of Liverpool University have previously paid for their own flights.

“There’s no tradition of horsemanship in Gambia. The Gambians are lovely people but they know nothing about horses. We’re trying to reduce rural poverty by improving the welfare of working horses and donkeys.”

Ms Armstrong had Defra do blood tests on samples she had previously brought back from Gambia.

These confirmed that West Nile disease, which can also affect humans, has struck the Gambia.

The trust has fourteen staff based upcountry in Gambia in an area without electricity and running water.

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African Horse Sickness

What is it? Viral infection transmitted by midges affecting heart and lungs. Infections can be acute, with animals dying suddenly without signs. Mortality rate up to 90%

Signs: fever, respiratory or circulatory distress, loss of appetite, excessive salivation

Where is it? Endemic in Africa, but hasn’t been seen in Europe since 1990, when it arrived in Spain via infected zebras. It has never been diagnosed in Britain

West Nile Virus

What is it? Viral infection transmitted by mosquitoes, causing inflammation of the brain and spinal cord. Mortality rate is about 35% in horses which show signs, but the ratio of clinical to sub-clinical cases is 1:2.5-1:5. Humans can be infected, not from horses but via mosquito bites

Signs: fever, encephalitis (inflammation of the brain) with nervous signs

Where is it? Africa, Europe, Asia, and — since 1999 — America, where it is now endemic. It has never been diagnosed in horses in Britain

• Read H&H’s full report on the exotic equine disease threat currently facing horse owners in the current issue (15 November, ’07)

Horse owners and vets should be increasingly alert for any signs of West Nile Virus, warns the Wildlife Information Network (WIN), an international charity based in the UK.

West Nile Virus (WNV) is a viral infection of birds, horses and humans, spread by the bite of infected mosquitoes that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

In horses, the infection can produce a wide range of symptoms, varying from no clinical signs to severe neurological abnormalities. Nearly two thirds of horses with clinical signs either die or have to be put down, while those that survive the disease often go on to suffer terminal problems at a later date.

Although the disease has not been identified in horses or humans in Britain to date, antibodies to the disease have been found in birds in the UK, inviting speculation about the prevalence of the disease. An outbreak of the virus in Southern France in 2000, which killed around 20 horses, adds to the concern.

The export of horses from the UK to the USA, and in particular the transportation of bloodstock, is a major concern, according to the WIN. A vaccine has been licensed in the US to try to combat the effects of the virus, which is present throughout most of mainland USA. However, the vaccine is not licensed for use in the UK and WIN is advising that all animals travelling to mainland America should be vaccinated as soon as they arrive.

Ian Boardman of the WIN explains: “We are of the view that anyone who is transporting bloodstock to the USA for stud or for competition should ensure that the animal is vaccinated against West Nile Disease as soon as they touch down in the USA.”

Alternatively, if horse owners are transporting animals to the USA, they can apply for Special Treatment Authorisation (STA) for exotic diseases through their vets, The Veterinary Medicines Directorate can then issue the vaccination as a one-off for use in this country before the horse travels.

Bert Burns, at Merial UK, clarified why horse-owners have to go to such lengths to protect their horses: “As far as the regulatory bodies are concerned, there isn’t a problem in this country with WNV, so there is no need to license the vaccine in this country. I am sure that this situation would be reviewed if and when WNV is considered to be a serious threat.”

For more information about the Wildlife Information Network and West Nile Virus visit: www.wildlifeinformation.org or (tel: 020 7388 7003).

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