West Nile fever spreads north as SSI tracks European cases

West Nile fever has spread to new parts of Europe this year, with the highest number of cases recorded so far, DR reports, citing the Danish Serum Institute (SSI), which is monitoring the development.

There have been no locally acquired cases in people in Denmark, SSI chief physician Lasse Vestergaard said. He said there was no reason for people in the country to be worried, but that the spread of the virus further north in recent years was important to follow.

By October 21, 1,965 locally acquired human cases had been recorded in 20 European countries, according to the European Centre for Disease Prevention and Control. SSI reports rising numbers in Belgium and more cases in recent years in northern Germany, not far from the Danish border.

The virus circulates mainly between wild birds and mosquitoes. Infected mosquitoes can pass it to people and horses. Up to 80 percent of people infected have no symptoms. Others may develop flu-like symptoms, including fever and headache. The World Health Organization estimates that about one in 150 infected people develop a severe illness involving inflammation of the brain. Older people and those with weakened immune systems face the greatest risk.

Mosquito activity and the speed at which the virus develops in mosquitoes are affected by temperature. Warmer conditions and longer summers can therefore create favourable conditions for the virus and extend the period when transmission can occur. Earlier this year, the first signs of local West Nile virus in Denmark were found in Danish horses.

Vestergaard said the European trend did not mean SSI expected an outbreak in Denmark now, but that the institute was following developments.