Distribution
In Europe, the virus was first detected in a human patient in Albania in 1985. Today, however, these are one of the most widespread flaviviruses in Africa, the Middle East, and Europe. In Hungary, it was first identified in rodents in the 1970s, but it did not cause clinical disease in the country until 2003. Data on human cases in Hungary have been available since 2003, and since 2012, West Nile virus infections have been classified as notifiable diseases. Until 2017, the number of reported cases per year was well below fifty, but in 2018, there were already 225 cases, fifteen of which were fatal.
Host Species
Birds are the primary hosts of the virus, but it can also infect mammals, causing disease in horses and humans (with a steadily increasing number of cases). In rarer instances, infections have also been observed in amphibians and reptiles. However, mammals, including humans, as well as amphibians and reptiles are regarded as dead-end hosts, meaning they do not transmit the virus onward; only birds can sustain the transmission cycle through mosquitoes. Migratory birds may play an important role in spreading the pathogen during their migrations, particularly because some bird species can carry very high levels of the virus with extremely low mortality rates, allowing them to transport it silently even across continents.
Transmission and Vectors
The virus is primarily transmitted via mosquito vectors, but it can also spread, more rarely through bodily fluids, blood transfusion, organ transplantation, and predator–prey interactions. The main vectors are mosquitoes of the genus Culex (family Culicidae), though the virus has been detected in species from ten other genera as well. In Europe, the common house mosquito (Culex pipiens) is considered the most important vector.
Symptoms
In birds, infection manifests mainly as neurological symptoms. Young birds are often affected, and the infection can cause encephalitis and meningitis, leading to movement coordination problems, muscle spasms, neck stiffness, and paralysis. Acute myocarditis and heart muscle damage are also significant, sometimes leading to sudden heart failure and death.
Among mammals, horses are highly susceptible to the virus. Infected horses may develop skin hypersensitivity and fever, and in more severe cases, neurological symptoms and encephalitis.
In humans, approximately 75% of infections are asymptomatic. In less than 20–25% of cases, symptoms resemble influenza, with general fever, headache, muscle weakness, nausea, and loss of appetite. Only about one case of encephalitis or meningitis (neurological disease) occurs per 4,000–5,000 infections.
Treatment and Prevention
There is currently no vaccine available for humans, but a vaccine for horses is available in Hungary.