West Nile Virus (WNV), a flavivirus spread by mosquitoes, hit the news headlines when it unexpectedly appeared in the United States in 1999, causing the death of thousands of wild birds and disease in humans and horses.
Since then over 29,000 people have been affected in the US and over 1100 deaths have occurred, with human cases reported from almost every US state. Canada has seen over 4500 human cases since it appeared there in 2002. Most infected humans show no clinical signs, but a small proportion develops mild fever, headaches and sometimes a skin rash. Less than one percent of infected humans show neurological signs associated with meningitis or encephalitis.
The virus is spread between wild birds by biting mosquitoes, which can also bite and infect other hosts such as geese, horses and humans. Some species of mosquito found in Great Britain are capable of transmitting WNV between birds and other hosts, raising concerns that disease caused by WNV could arise in this country.