Mosquito-borne diseases continue to pose a health concern across the United States, with several viruses and parasites transmitted by local mosquito species causing illness each year. While most infections result in mild symptoms or go unnoticed, some can lead to severe complications, particularly in vulnerable populations.

West Nile virus remains the most prevalent mosquito-borne disease nationwide, transmitted principally by Culex mosquitoes that commonly bite both birds and humans. Annually, approximately 2,000 cases are reported in the U.S., often during the summer months of July through September, though actual infections may be undercounted due to frequently mild or absent symptoms. Illness typically resembles a flu-like syndrome with headache, fever, and body aches. However, in rare cases, the virus can invade the nervous system, causing meningitis or encephalitis, which is especially dangerous for older adults or those with compromised immune systems.

Eastern equine encephalitis (EEE) is less common but more severe, circulating primarily in swampy regions along the Eastern and Gulf Coasts. Between 2003 and 2025, 218 cases were documented. While some patients experience mild fever and fatigue, others develop serious neurological symptoms such as seizures and behavioral changes. The disease carries a fatality rate of about 30 percent among severe cases, with many survivors facing lasting neurological damage.

Jamestown Canyon virus is typically found in the Upper Midwest, including states like Wisconsin and Michigan. It spreads between mosquitoes and deer, with sporadic human infections mostly causing flu-like symptoms. Severe neurologic complications such as encephalitis are rare. Confirmed cases numbered 362 from 2011 to 2025, with an observed increase in recent years, possibly due to enhanced awareness and improved diagnostic methods.

La Crosse encephalitis virus affects regions of the upper Midwest, Mid-Atlantic, and Southeast, often transmitted by mosquitoes that feed on small mammals like chipmunks. This virus mainly impacts children under 16, with annual neuroinvasive cases estimated between 30 and 90. Mild infections frequently go unreported, but more serious illnesses can involve brain inflammation. Researchers caution this virus might expand geographically, partly due to the spread of Aedes albopictus mosquitoes, which could act as an additional vector.

St. Louis encephalitis virus shares similarities with West Nile virus, also transmitted by Culex mosquitoes and circulating among birds. Cases occur predominantly in Central and Eastern states, though sporadic infections have been recorded in the South and western states such as California and Arizona. Annual case numbers range from one to 49. While the U.S. has not experienced a major outbreak in about two decades, isolated transmissions remain a possibility.

Other mosquito-borne illnesses such as dengue, chikungunya, malaria, and Zika virus are rare within the U.S. but present ongoing concerns primarily related to travel exposure. Malaria cases in the country mostly involve individuals infected abroad, with about 2,000 cases reported annually. Dengue, chikungunya, and Zika viruses are transmitted by the tropical Aedes aegypti mosquito, which has limited but expanding presence in parts of the southern U.S. Recent years have seen unusually high numbers of locally transmitted dengue cases, particularly in California, Florida, and Texas, raising concerns about potential increases linked to climate change.

Given that effective treatments are limited for many mosquito-borne illnesses, public health experts emphasize preventive measures such as using EPA-approved insect repellents, wearing protective clothing, and reducing mosquito breeding sites around homes. Climate change and increased travel are expected to influence the distribution and incidence of these diseases, underscoring the importance of continued surveillance and public awareness.