Originally from Africa, the virus has spread in Europe, Asia and North America. It is mainly transmitted through mosquito bites and can lead to a fatal neurological disease in humans, although most people infected never develop any symptoms.

Scientists have said that milder temperatures attributed to climate change could cause diseases such as the West Nile virus to become more widespread.

“In light of favourable climatic conditions this year – an abundance of precipitation… a warm and long autumn, a high number of (virus) carriers could be observed in the autumn,” Rospotrebnadzor, Russia’s consumer health watchdog, said.

The virus causes West Nile fever in around 20% of cases, the World Health Organization (WHO) has said. It is related to the Zika, dengue and yellow fever viruses.

More than 80% of Russia’s West Nile fever cases are recorded in its southwest region.

There is no vaccine against the virus in humans although one exists for horses, the WHO says.