West Nile virus (WNV) infections have sharply increased in 2018 as compared to the previous 4 years. This is largely due to the early start of the 2018 transmission season in the WHO European Region, which normally lasts from July to October.

This year’s season has been characterized by high temperatures and extended rainy spells, followed by dry weather. Such weather conditions have been conducive to mosquito breeding and propagation. A total of 401 autochthonous human WNV infections have been reported by European countries this year, as of 16 August, including 22 deaths, according to data from the European Centre for Disease Prevention and Control (ECDC). The most affected countries include Serbia (126 cases), Italy (123), Greece (75), Hungary (39) and Romania (31). These areas have all been affected by WNV in the past.

WNV is maintained in nature in a cycle involving transmission between birds and mosquitoes. Humans, horses and other mammals can be infected. West Nile Virus is a member of the flavivirus genus and belongs to the Japanese encephalitis antigenic complex of the family Flaviviridae and it is commonly found in Africa, Europe, the Middle East, North America and West Asia.

Residents of or travellers to WNV-affected areas are at risk of WNV infection. Only 20 per cent of infected individuals will show WNV symptoms. Certain population groups (e.g. the elderly, pregnant women, immunocompromised people) are at higher risk of developing a neuroinvasive form of the disease.

Given the favourable weather conditions this season, the virus may spread to new areas, thus affecting new populations never exposed to the virus before. In these areas, low general awareness about WNV among the public and both human and animal health professionals might challenge early detection of cases.