Denmark: four horses ‘most likely’ infected with West Nile virus

In Denmark, four horses have most likely been infected with the mosquito-borne West Nile virus (WNV), according to the Danish agency for food, agriculture and fisheries. The four horses all had antibodies against WNV, had no prior travel history and were not vaccinated against WNV. Arrival of the virus in Denmark was ‘expected’, according to the authorities, as the disease is already endemic in other parts of Europe, and climate change increases the presence of vectors.

Cross reaction with the Usutu virus

The possible cases were detected thanks to a veterinary master’s thesis, which required the blood sampling of 822 clinically healthy horses in 22 veterinary clinics throughout the country. Four of these horses were found to be seropositive. However, there is a cross-reactive antibody binding between WNV and the Usutu virus, found to be present in blackbirds all over Denmark. Continued surveillance is needed to confirm presence of WNV.

WNV: mostly asymptomatic, but sometimes fatal

For the upcoming mosquito season, vets are encouraged to include WNV as a possible diagnosis in horses with compatible clinical signs, and are reminded of the existence of a vaccine for horses. While most cases are asymptomatic, in rare cases meningoencephalomyelitis develops with central nervous disturbances, such as impaired vision, compulsive movements, tremors and ataxia. The mortality rate in horses with severe clinical signs is high.