Jamestown Canyon virus is spread to people by infected mosquitoes. The virus is found throughout much of the United States, but most human cases are reported from the upper Midwest. Cases occur from late spring through mid-fall. You can reduce your risk by taking steps to prevent mosquito bites.
In 2025, the Vermont Department of Health started monitoring mosquito populations for Jamestown Canyon virus.
Most people with Jamestown Canyon don’t develop symptoms. Some may experience flu-like symptoms a few days to two weeks after a mosquito bite.
Common symptoms include:
You may also have:
People who have weak immune systems or who are older may be more likely to develop severe disease from Jamestown Canyon virus. About 50% of people who get sick are hospitalized.
Severe symptoms may include:
There are no vaccines to prevent or medicines to treat Jamestown Canyon. For mild symptoms, your health care provider may recommend:
If you develop severe symptoms, you may need to be hospitalized to receive IV fluids, medication and monitoring.
Jamestown Canyon virus circulates in the environment between mosquitoes and animals, typically deer. Infected mosquitoes can then spread the virus to people and other animals by biting them. People are considered "dead-end" hosts. This means they do not spread the virus, even when they are sick, to mosquitoes that bite them. Jamestown Canyon virus is not spread by coughing, sneezing, touching, or other contact with someone who is infected.
Most cases of Jamestown Canyon virus disease occur during April through September. Many Jamestown Canyon virus infections are asymptomatic, but the specific symptomatic-to-asymptomatic ratio is not known. Among patients who are symptomatic, initial symptoms are non-specific, including fever, fatigue, or headache. Some patients also report respiratory symptoms, such as cough, rhinitis, or pharyngitis. The incubation period for Jamestown Canyon virus disease is unknown.
Some patients with Jamestown Canyon virus disease might develop meningitis or encephalitis with possible meningismus, seizures, altered mental status, and cerebrospinal fluid (CSF) pleocytosis.
About half of known Jamestown Canyon virus disease patients have been hospitalized. However, deaths are rare and have been primarily reported in immunocompromised patients.
