West Nile virus (WNV) is an RNA virus of the family Flaviviridae, which is endemic in various, mainly tropical regions of the world. Yet, through migratory birds, WNV also regularly occurs in non-tropical areas further north and has been detected in Germany since 2018.
Transmission primarily takes places through mosquitoes (mostly species of Culex) between wild birds. However, the mosquitoes can also transmit WNV to people, horses and other mammals. Due to the low viral load, horses and humans are dead-end hosts and do not represent a source of the virus for mosquitoes.
The incubation period for WNV encephalitis in horses is 3 – 15 days. Most infections are subclinical, only a small percentage of horses develops neurological signs, such as stumbling, hind-leg paralysis, ataxia, tremor or weakness up to recumbency of the animals.
In birds, infections vary from asymptomatic to lethal, depending on the species. Passerine birds, birds of prey and owl species are most susceptible to the disease. They may develop severe epidemics with central nervous signs (e.g. vertigo, tremor, inability to fly) and death rates may increase.
In Germany, the disease is notifiable upon suspicion in birds and horses.