West Nile Virus
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.
The risk of contracting West Nile fever in Austria is currently still very low. Since transmission occurs through mosquitoes, West Nile virus infections occur in the summer months. In Austria, a total of 55 domestically acquired West Nile virus cases were confirmed between 2010 and 2022. The likely sites of infection are found in Vienna, Lower Austria and Burgenland. There have been no human deaths in Austria to date.
In animals, clinical WNV infections were first detected in Austria in 2008 in birds of prey. Since then, surveillance programmes have been carried out in wild birds and since 2011 also in horses.
With the Gelsen-Monitoring of the AGES for the surveillance of vectors (= carriers) of human pathogenic viruses such as the West Nile Virus, mosquitoes are collected and classified at predefined locations. The infestation rate of mosquito populations with West Nile Virus and other pathogens is determined using molecular biological methods.
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 terminal hosts and can become ill but cannot transmit the virus to other mosquitoes. WNV strains are divided into subtypes 1 (WNV-1) and 2 (WNV-2).
A variety of different mosquito species are possible vectors, most notably 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 animal-to-human or bird-to-mammal chain of transmission. The virus can survive winter in the mosquito Culex and undergo a multiplication cycle in the mosquito the following year in spring. After 10-15 days of incubation in the mosquito, WNV is transmitted by biting. Viral transmission is also thought to occur via direct contact. The significance of oral or faecal transmission is still unknown; ingestion of the virus via aerosols is also under discussion.
80 % of infections are asymptomatic. 20 % of the infected patients show a flu-like illness with sudden, high fever (= mild West Nile fever), muscle pain, headache, gastrointestinal symptoms, possibly cold symptoms, lymph node swelling and skin spots. The incubation period is 2-8 days. Within 7-10 days, the main symptoms usually subside. In individual cases (0.7%), West Nile meningitis or encephalitis occur. Neuroinvasive disorders with clouding of consciousness, coordination disorders, difficulty swallowing, extreme fatigue 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 individuals over the age of 50 are at 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. In 2015, eight more cases were found, including five blood donors. Human-to-human transmission is extremely rare, but is reported. Transmission through blood transfusion, organ transplantation, intrauterine exposure and/or breastfeeding is possible.
Symptomatology in birds
Birds usually show few or no clinical signs, eventually diagnosed with CNS symptoms with corresponding accompanying symptoms: Lethargy, coordination difficulties, ataxia, depression, torticollis, opisthotonus, and bleeding in beak and cloacal region. Young birds (1-11 day old chicken chicks, goslings) show much more pronounced viremia in contrast to older birds. High mortality is found in crows and other corvids. Myocarditis and encephalitis are frequently diagnosed in dead birds. Experimentally infected chickens and turkeys showed no symptomatology. Symptoms in geese included loss of appetite, weight loss, depressed behavior, myocarditis, and neurologic disturbances. Rhythmic sideways head movements ("sideways oscillation") were commonly seen.
Symptomatology in mammals
Horses have a higher risk of infection than humans; therefore, clinical disease usually occurs earlier in horses in an affected area. The disease is asymptomatic in 90% of cases. Clinical signs in horses are often nonspecific and similar to those of other primarily CNS diseases. Common signs include fever, mild ataxia, depression or lethargy, propping of the head, weakness of the hindquarters, generalized 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 is no known case of direct dog- or cat-human transmission.
In sheep, neurological disorders, ataxia, fever, and behavioral disturbances (teeth grinding, depression) occur. Abortion or stillbirth may occur in ewes. Death may also occur in young lambs shortly after birth. Infected ewes may die.
Therapy and prophylaxis
Specific therapy for WNV infection is not currently available. Clinically ill horses require general intensive medical care with symptomatic therapy.
The best therapy here is prevention of infection. For prophylaxis, vaccines against West Nile virus for horses have been available in Austria since 2011 and have been used in the USA for several years. Protection exists at the earliest three weeks after the second vaccination. In addition to internationally competing show horses, recreational horses should also be adequately protected against this dangerous disease.
Furthermore, it is very effective to protect the horses against the mosquitoes - often very simple measures are sufficient:
- Taking the horses out of the pasture at certain times, when the insects are particularly active (dawn and dusk), and stabling them in a sheltered place
- Use of mosquito repellents on the animals themselves, which are suitable for horses and commercially available on the market.
- Installing mosquito screens and insect repellent lamps in the barns
Avoiding standing water on the property. Standing shallow puddles, such as those often found in used tires, near manure piles or drainage structures, are ideal breeding grounds for gnats.
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.
Suitable sample materials are:
- Organs (WNV RT-PCR, Flavivirus RT-PCR, histology, immunohistochemistry, virus cultivation, sequencing).
- Serum (IgG and IgM ELISA, SNT)
- CSF (RT-PCR)
An IgG ELISA, specific for flavivirus antibodies, which cannot differentiate between WNV, tick borne encephalitis virus (TBEV/FSME) or Usutu virus antibodies in the blood, is used for equine sera testing. Sera positive in the first IgG ELISA are then differentiated for WNV using a second WNV IgM specific ELISA test or a specific WNV serum neutralization test (SNT). If only antibodies are found, it must be ensured that it is not a vaccination antibody titer (vaccination proof of the horse, WNV antibody titer rise) or whether the examined animal has been in endangered areas abroad (tournament sport) before the examination. Only in this way can an autochthonous infection of horses in Austria be excluded or confirmed with a high degree of certainty.
Lethal cases of WNV encephalitis are confirmed by neuropathohistological examination and direct detection methods (RT-PCR, virus isolation and immunohistochemistry).
A WNV PCR examination is performed from the organ material of fallen animals and in the positive case the WNV strain is subtyped by means of sequencing.
All clinical forms of equine encephalomyelitis are notifiable in Austria. Differential diagnoses in horses include infections such as Japanese encephalitis virus (JEV), TBE, equine herpes virus 1 (EHV-1), Borna disease, rabies, etc. If there is molecular biological and histopathological evidence for the presence of viral American equine encephalitis virus (Eastern equine encephalitis virus (EEEV), Western equine encephalitis virus (WEEV) or Venezuelan equine encephalitis virus (VEEV)), the samples are forwarded to the European Reference Laboratory in France. Differential diagnosis in birds should primarily consider avian influenza and Newcastle disease viruses and septicemic bacterial infections.
Last updated: 01.06.2023