West Nile virus (WNV) is a flavivirus belonging to the Japanese encephalitis virus group and was originally discovered in the West Nile District of Uganda in 1937. The virus was not detected in the United States until the summer of 1999, in New York City. Since then, WNV has significantly expanded its reach across North America.
WNV in Massachusetts
WNV was first confirmed in Massachusetts in 2000 and has since become endemic, with human cases occurring regularly throughout the Commonwealth. As of December 2024, there have been a total of 259 human cases in Massachusetts. This spread underscores the importance of ongoing surveillance and public health efforts to reduce the risk of transmission.
Symptoms and Risk Factors
Severe illness is very rare, and roughly 80% of people infected with the virus do not develop any symptoms. People over the age of 50 are considered at a higher risk for developing more severe symptoms of neuroinvasive disease, which include stiff neck, muscle tremors, seizures, changes in vision, and weakness or paralysis. Less severe symptoms of febrile illness include headache, fever, muscle aches, joint pain, fatigue, and rash. The onset of symptoms typically appears within 2 to 14 days of being bitten by an infected mosquito.
WNV Transmission Cycle
The WNV transmission cycle involves several key players: mosquitoes, birds, and mammals, including humans. The cycle begins when mosquitoes, particularly Culex species, feed on infected birds. Birds are the primary reservoirs of the virus, carrying high levels of the virus in their bloodstream. After feeding on an infected bird, the virus replicates within the mosquito and eventually reaches its salivary glands.
When the infected mosquito bites another bird or a mammal (such as a human or horse), it transmits the virus through its saliva. Humans and horses are considered dead-end hosts because they do not develop high enough levels of the virus in their bloodstream to infect other mosquitoes, thus not contributing to the spread of the virus.
English