Mosquitoes become infected when they feed on infected wild birds, which may circulate the virus in their blood for a few days. After an incubation period of 10 to 14 days, infected mosquitoes can then transmit the disease to other birds and mammals. The WNV is located in the mosquitoes salivary glands. During a blood meal, the virus is injected into a new animal or even a human being. The virus multiplies within its new host and may cause illness.
Most infected humans have no symptoms.
The elderly or people with a weakened immune system can develop encephalitis which can be serious and even lead to death. The WNV was first identified in the US in New York City in 1999. In addition to NYC, this year infected crows have been found in New Jersey, Connecticut, and Massachusetts. The WNV was isolated from a sick stray kitten found in a storm drain in New Jersey in 1999. One location with infected horses also had resident chickens.