Horses infected with the virus cannot infect other horses or humans, but the discovery is important because humans can also be infected via mosquito bites and develop the disease.
The virus is moving north as a result of climate change, and last year a total of 14 European countries reported cases of locally acquired West Nile virus in humans - with the northernmost case in Berlin.
Symptoms of West Nile virus are flu-like and include fever, headache, muscle aches, nausea and vomiting.
