Publication
West Nile Virus
From Department of Agriculture, Food and the Marine
Published on
Last updated on
From Department of Agriculture, Food and the Marine
Published on
Last updated on
West Nile Virus (WNV) is a vector-borne infectious flavivirus that can affect a wide range of animals, including birds and horses leading to the disease known as ‘West Nile Fever’.
West Nile Virus is a virus which predominantly affects birds and is transmitted via the bite of infected vectors (mosquitoes). Other mammals, including horses and humans can become incidentally infected via insect bites. WNV is considered to be a zoonotic disease/zoonosis (an animal disease affecting humans). In most cases horses and humans that get infected with WNV won’t show signs of illness, though a small number can potentially become very sick. An outbreak in animal populations could have a significant impact on animal welfare.
Currently, West Nile Virus is not present in Ireland. In several European countries, WNV is present and endemic in wild birds (meaning infection is continually present or maintained at baseline levels) which occasionally results in other mammals, including humans becoming infected.
It is possible that the disease profile of West Nile Virus may change in the future. Several factors including the distribution of virus reservoirs, vector factors (distribution, survivability), host susceptibility, virulence of the virus strains and increasing temperatures may change the transmission risk.
Current climate trends, global trade and travel may increase the risk of establishment of mosquito species capable of spreading diseases such as West Nile Virus in previously unaffected regions.
West Nile Virus does not pose a risk to food safety.
West Nile Fever is a notifiable disease in Ireland, meaning that any suspected cases must be reported to the Department of Agriculture, Food and the Marine without delay by contacting your local Regional Veterinary Office (RVO) . Outside of office hours, suspect notifiable disease can be reported to the National Disease Emergency Hotline on 01 492 8026 (which is operational 365 days of the year).
There is currently no licenced WNV vaccine available in the European Union.
West Nile Virus primarily affects wild birds, who are the natural hosts of this virus. Susceptibility to disease caused by WNV varies greatly amongst bird species, ranging from subclinical infection to death. Species within the family Corvidae (e.g. cows, ravens, magpies) are most susceptible to disease.
However, WNV can infect a broad range of domestic and wild animals e.g. bats, squirrels, reptiles.
Infection in horses, and other mammals including humans are considered incidental or ‘spillover’ events. Mammals are considered ‘dead end hosts’, meaning they do not develop enough virus in the bloodstream to spread the disease to other biting mosquitoes, but they can still present with clinical signs of WNV infection.
West Nile Virus is spread by vectors (mosquitoes). Although WNV is not currently present in Ireland, the mosquitoes capable of spreading the disease are found throughout the island of Ireland in large numbers. Mosquitoes become infected by feeding on infected birds, which can go on to infect other birds at their next blood feed. The virus replicates in large numbers (‘amplifies’) within infected birds.
WNV is maintained in nature between bird-mosquito transmission cycles. Transmission of the virus occurs primarily when mosquitoes bite an infected bird. The mosquito becomes infected via the infected bloodmeal after biting, and then acts as a carrier (vector) by spreading the virus to subsequent wild birds/hosts they bite. Wild birds are both ‘amplifier’ and ‘reservoir’ hosts for the virus.
Just like WNV transmission in birds, infection in mammals (equine species and humans) can be caused by a bite from an infected mosquito. These infections are considered incidental or ‘spillover’ events in comparison to the transmission cycle in birds. Mammals are considered ‘dead end hosts’, meaning they do not develop enough virus in the bloodstream to spread the disease to other biting mosquitoes, but they can still present with clinical signs of WNV infection.
The higher-risk period for the spread of West Nile Virus is when mosquitoes are most active. In Ireland this period is usually between April and November. Milder winters can result in the mosquitoes that are responsible for spreading disease remaining active for longer periods. Wind speed and direction can also affect how far mosquitoes can travel and could potentially facilitate the spread of the disease if it were to occur in Ireland.
West Nile Fever is a zoonotic disease/zoonosis (an animal disease that can affect humans). The main route of human infection with West Nile Virus is through a bite from an infected mosquito.
Although people are susceptible to West Nile Virus (like equine species), they are considered dead-end hosts, meaning they do not develop enough virus in the bloodstream to spread the disease, but they can still present with clinical signs. Most human cases of WNV occur between July and September when mosquitoes are most active. Symptoms range in severity from mild to severe illness. Medical conditions leading to immunosuppression and increased age are risk factors for increased disease severity. Most people infected with WNV recover spontaneously.
Please consult the HPSC website for up-to-date information on West Nile Virus infections in people.
Ireland and the UK are currently free from West Nile Virus.
WNV has circulated in Europe since the 1950s. It is currently present in southern, eastern and western European countries. This presents a potential for spread across mainland Europe, towards the UK and Ireland. The virus has also been identified in the US. The transmission season during warmer months has also become longer in recent years, likely due to factors involved in climate change.
In 2022, nine European (Italy, Germany, Greece, Spain, Croatia, France, Hungary, Portugal, and Austria) reported 101 WNV outbreaks among equine species and 323 WNV outbreaks among birds. Portugal was the only one of these named countries which did not report any human cases of WNV in 2022.
In 2023, 709 locally acquired human cases of WNV (+19 travel associated), including 67 deaths were reported by 9 European countries (Italy, Greece, Romania, France, Hungary, Spain, Germany, Croatia, Cyprus). Cases occurred between 30/05/23-05/11/23, with the majority (96%) occurring between July and September. To date, the HPSC reported human travel related cases in Ireland in 2013 (1) and 2023 (1).
In 2024, the first human case of WNV was reported on 01/03/24 in Seville, Spain.
A map detailing countries affected with West Nile Virus is available here: West Nile fever (europa.eu)
Weekly case updates reported in Europe are available here: Weekly updates: 2024 West Nile virus transmission season (europa.eu)
WNV infections in equids are usually asymptomatic (no clinical signs), with no illness noted in most infected animals. In asymptomatic cases, most animals recover spontaneously.
All horses are susceptible to the virus. Adult animals are most frequently affected. Older and unvaccinated populations may develop more severe disease.
The proportion of infected horses developing West Nile Fever is low (<10%).
Horses who do show clinical signs following West Nile Virus infection, may develop West Nile fever/ encephalomyelitis. This clinical condition is characterised by central nervous system dysfunction as the virus crosses the blood-brain barrier and causes inflammation of the brain (encephalitis) and/or the spinal cord (myelitis). It can result in prolonged residual effects or be fatal, with mortality reaching 50% in horses that develop West Nile encephalomyelitis.
There is a wide range in the severity of neurological signs that can develop in West Nile encephalomyelitis. If present, clinical signs of WNV are non-specific and can resemble other neurological conditions. Clinical signs may include loss of appetite, discomfort, depression, anxiety, malaise, fever, lameness, ataxia, circling stumbling, muscle twitching, partial paralysis, impaired vision, head pressing, teeth grinding, inability to swallow, poor coordination, aimless wandering, convulsions, circling, weakness, coma and death.
The differential diagnoses of WNV in horses may include but is not limited to: Equine herpes virus 1 (EHV-1), Equine protozoal myeloencephalitis (EPM), Rabies, Eastern, Western and Venezuelan equine encephalomyelitis (EEE, WEE, VEE), Cervical Vertebral Myelopathy (CVM, wobbler syndrome), Equine degenerative myelopathy (EDM) and Borna disease. Diagnosis of WNV is through serologic testing (presence of IgM antibodies against WNV).
Veterinary treatment and supportive care are indicated for clinically affected horses. There are no specific treatments other than supportive care. Once fully recovered, horses can resume normal activities and do not pose a risk to humans. Unfortunately, if infected horses have severe clinical signs or fail to recover, euthanasia may be warranted.
In birds there is a wide range in the severity of neurological signs from sub-clinical/mild/severe caused by West Nile Virus between species. Avian Influenza is considered the main differential for WNV in birds.
Clinical signs may include paralysis, reluctance to move and incoordination. Many species of birds are resistant to the disease. Certain bird species, including raptors, geese and corvids, tend to be more sensitive to WNV infection and frequently develop clinical signs that lead to fatal outcomes. Recovery rates are dependent on disease severity.
If you have any concern that one of your animals has West Nile Virus, do not delay in acting. Equine suspects should be notified immediately to the DAFM.
Equine West Nile Virus suspects include horses:
• Animals presenting with clinical signs of (viral) encephalitis, with an unknown aetiological cause and no recent history of travel
• OR any horse with neurological signs that develop within 4 weeks of return/travel to Ireland
To report a suspect case, please contact your local Regional Veterinary Office (RVO) (9am-5pm). Outside of office hours, you can ring the National Disease Emergency Hotline on 01-492-8026 (which is operational 365 days of the year).
If you are responsible for horses travelling to and from Ireland, it is important to be aware of the prevalence of WNV in the area you are travelling to and to discuss precautions with your vet that can be put in place to protect your animals while abroad. If upon your return to Ireland a horse shows any clinical signs associated with WNV infection, you should seek prompt advice from your vet.
West Nile Virus (WNV) is defined as a category E disease under EU legislation (Regulation (EU) 2018/1882). In the event of a case of WNV occurring in Ireland, surveillance and notification to the EU. West Nile Fever is also a disease listed in the World Organisation for Animal Health (WOAH) Terrestrial Animal Health Code, and any outbreak must also be reported to WOAH. Ongoing surveillance programmes in wild or sentinel birds allow competent authorities of EU Member States to take appropriate protective control measures. Other control measure to minimise disease spread could also be implemented by DAFM. Public health advice will follow from public health authorities.
There are 3 main ways that West Nile Virus could enter the country:
• Introduction of an infected horse
• Infected migratory wild birds.
• Infected mosquitoes.
West Nile Virus could enter Ireland through movement of an infected horse or human. However, horses and humans do not spread the disease onwards.
Sustained onward transmission of West Nile Fever requires infected wild birds and mosquitoes.
When abroad in areas where WNV is present, horses should wherever possible be protected from mosquito exposure. Although it does not guarantee complete protection from contracting WNV, precautions to minimise infection risk including the use of mosquito screens, insect repellents and stabling (during periods of peak mosquito activity (dawn/ dusk)) may help reduce the likelihood of contracting WNV infection.
Vaccination of horses is an effective control measure in areas where disease prevalence is high, such as in the US. It is currently not used as a control measure in Europe.
Surveillance is conducted in Ireland for West Nile Virus.
• Wild bird surveillance
• Equine-routine flavivirus screening (WNV/USU/LIV/TBE)
Surveillance is also conducted in Europe. Confirmed cases of WNV must be reported to the European Commission by the competent authorities of Member States.
• In 2023 there were a total of 153 outbreaks of WNV in horses, reported in 7 European countries: France (44), Spain (38), Hungary (26), Italy (25), Germany (14), Portugal (5), Austria (1).
• In 2023 there were a total of 251 outbreaks of WNV in birds, reported in 8 European countries: Italy (200), Germany (19), Spain (19), Bulgaria (6), Hungary (3), France (2), Austria (1), Greece (1).
• Except for Portugal, Austria and Bulgaria, all countries that reported outbreaks among equines or birds also reported locally acquired human WNV infections in 2023.