West Nile fever is caused by the West Nile virus (WNV). The virus is transmitted by gnats (mosquitoes of the genus Culex), and birds (over 300 bird species) act as the natural reservoir. Humans and other mammals, especially horses, can also become ill. However, humans and horses represent a dead end for the virus - they can become ill but cannot transmit the virus to other mosquitoes. Thus, the virus cannot be transmitted from horse to horse, nor from horse to human.

West Nile Virus is endemic in various regions such as Africa, Israel, Western Turkey, Middle East, India, and North and Central America. In Europe, individual cases are reported each year, as well as seasonal outbreaks from southern, eastern, and western European countries. In Europe, West Nile virus is mainly transmitted by "indigenous" Gelsen species.

The website of the European Centre for Disease Prevention and Control (ECDC) provides an overview of the geographical distribution of West Nile virus infection reports in humans and animals - ECDC - WNV Dashboard.

The natural reservoir of West Nile virus is birds, in particular migratory birds (especially Passeriformes such as corvids and sparrows), and it can be spread over long distances via these birds. Birds of prey such as hawks and falcons also contract West Nile Virus and are often indicator animals for virus presence in Central Europe.

Furthermore, various mammals (monkeys, horses, cattle, goats, sheep, deer, buffalo, camelids, pigs, dogs, wolves, foxes, bears, cats, bats, skunks, squirrels, hares and other rodents), reptiles (alligators, snakes), amphibians as well as humans can be infected. Mammals, reptiles and amphibians do not play a significant role in the further spread of the virus as final hosts.

West Nile virus is transmitted by a bite from an infected mosquito. The virus can be transmitted by a large number of different species of gnats, in Central Europe mainly by the native common gnat.

2 to 14 days

80 % of infections are asymptomatic. 20 % of infected patients show a flu-like illness. In individual cases (0.7 %), West Nile meningitis or encephalitis develop.

The treatment of West Nile fever in humans and animals is purely symptomatic, i.e. there is no medication directed against the virus.

There is no vaccine for humans that can be used in prevention. Avoiding gel bites is the simplest means of preventing infection. There are several approved vaccines for horses. Horses are to be vaccinated twice at intervals of three to five or four to six months and thereafter annually

The causative agent of West Nile fever is the West Nile virus (WNV). WNV is an enveloped RNA virus that is transmitted from infected birds to humans and mammals via mosquito bites of the genus Culex. Both humans and horses are dead-end hosts for the virus - they therefore act as final hosts and can contract the disease but cannot transmit the virus to other mosquitoes. The WNV strains are divided into subtypes 1 (WNV-1) and 2 (WNV-2).

A large number of different mosquito species are possible vectors, especially the genera Culex, Aedes and Ochlerotatus. In the genus Culex, these are mainly Culex quinquefasciatus, C. molestus, C. pipiens, C. restuans, C. salinarius and C. tarsalis. Culex pipiens, as a parasite of birds and humans, is one of the possible vectors in the transmission chain from animals to humans or birds to mammals. The virus can survive the winter in the mosquito Culex and undergo a reproduction cycle in the mosquito the following year in spring. After 10-15 days of incubation in the mosquito, the WNV is passed on by biting. The virus is also transmitted via direct contact. The significance of oral or faecal transmission is still unknown; ingestion of the virus via aerosols is also under discussion.

Symptoms

80 % of infections are asymptomatic. 20 % of infected patients show flu-like symptoms with sudden, high fever (= mild West Nile fever), muscle aches, headaches, gastrointestinal symptoms, possibly cold symptoms, swelling of the lymph nodes and skin spots. The incubation period is 2-8 days. The main symptoms usually subside within 7-10 days.

In individual cases (0.7 %), West Nile meningitis or encephalitis occurs. Neuroinvasive disorders with clouding of consciousness, coordination disorders, difficulty swallowing, extreme tiredness and dizziness combined with behavioural and personality changes are typical signs. Hepatitis, myocarditis, nephritis, pancreatitis and splenomegaly coupled with a long convalescence are the result. Immunocompromised patients and people over the age of 50 have a higher risk of developing the severe form of the disease.

In 2014, West Nile virus was diagnosed for the first time in a blood donor in Austria. Eight further cases were found in 2015, including five blood donors. Human-to-human transmission is extremely rare, but transmission through blood transfusion, organ transplantation, intrauterine exposure and/or breastfeeding is possible.

The risk of contracting West Nile fever in Austria is currently still very low. As the virus is transmitted by mosquitoes, West Nile virus infections occur in the summer months. A total of 55 domestically acquired West Nile virus cases were confirmed in Austria between 2010 and 2022. The most likely places of infection are in Vienna, Lower Austria and Burgenland. One positive human case was reported in 2023. This was a case imported from Italy. There have been no human deaths in Austria to date.

In animals, clinical WNV infections in birds of prey were first detected in Austria in 2008. Since then, a surveillance programme has been carried out in wild birds and, since 2011, also in horses. Information on current West Nile fever cases in animals can be found in the animal disease radar.

The AGES mosquito monitoring programme for the surveillance of vectors (= carriers) of human pathogenic viruses such as West Nile virus collects and classifies mosquitoes at predefined locations. Molecular biological methods are used to determine the infestation rate of mosquito populations with West Nile virus and other pathogens.

Symptoms in birds

Birds usually show few or no clinical symptoms; CNS symptoms with corresponding accompanying symptoms may be diagnosed: Lethargy, coordination difficulties, ataxia, depression, torticollis, opisthotonos and haemorrhages in the beak and cloacal region. Young birds (1-11 day old chicken chicks, goslings) show a much more pronounced viraemia than older birds. A high mortality rate is found in crows and other corvids. Myocarditis and encephalitis are frequently diagnosed in dead birds. Experimentally infected chickens and turkeys showed no symptoms. Symptoms in geese were loss of appetite, weight loss, depressive behaviour, myocarditis and neurological disorders. Rhythmic sideways head movements ("sideways oscillation") were frequently observed.

Symptoms in mammals

Horses have a higher risk of infection than humans, so in an affected area clinical disease usually occurs earlier in horses. The disease is asymptomatic in 90% of cases. The clinical signs in horses are often non-specific and similar to those of other diseases, especially CNS diseases. Common symptoms include fever, mild ataxia, depression or lethargy, propping of the head, weakness of the hindquarters, generalised muscle weakness, loss of appetite, visual disturbances, partial paralysis, convulsions and coma. In clinically ill horses, the infection is lethal in up to 40 % of animals.

In dogs and cats, the course of the disease is usually asymptomatic. There are no known cases of direct dog or cat-human transmission.

In sheep, neurological disorders, ataxia, fever and behavioural disorders (teeth grinding, depression) occur. In ewes, abortion or stillbirth can occur. Death can also occur in young lambs shortly after birth. Infected ewes can die.

Therapy and prophylaxis

Specific treatment of a WNV infection is currently not possible. Clinically ill horses require general intensive medical care with symptomatic therapy.

The best therapy here is the prevention of infection. Vaccines against West Nile virus for horses, which have been authorised in Austria since 2011 and have been used in the USA for several years, are available for prophylaxis. Protection is effective at the earliest three weeks after the second vaccination. In addition to international competition horses, leisure horses should also be adequately protected against this dangerous disease.

It is also very effective to protect horses from mosquitoes - very simple measures are often sufficient:

• Take the horses out of the pasture at certain times when the insects are particularly active (dawn and dusk) and stable them in a protected area
• Use mosquito repellents on the animals themselves that are suitable for horses and commercially available on the market
• Install mosquito screens and insect repellent lamps in the stable

Standing water on the property should be avoided. Standing shallow puddles, such as those often found in used tyres, near manure heaps or drainage systems, are ideal breeding grounds for mosquitoes.

If possible, birds should not be able to enter a horse stable or chickens should be kept in the same stable room, as these can also be a source of infection.