From the beginning of the year through this Tuesday, 771 cases of West Nile virus were reported in the U.S., according to the U.S. Centers for Disease Control and Prevention. That’s trending up higher than usual.
Per the Associated Press, cases up 40% more than is typical for this point in the year. Historical data shows that at least 712 cases have been reported each year since 2003, with most cases reported in August and September.
West Nile Virus is a mosquito-borne illness that was first reported in the U.S. in 1999, though it was previously found in Africa, West and Central Asia, and the Middle East. Until 2003, under 70 cases were reported annually, but then they shot up to nearly 10,000 cases in a single year. Though lower numbers of annual cases have been reported in subsequent years, it is the leading cause of mosquito-borne disease in the contiguous U.S. today, with an average of 1,300 cases and 130 deaths each year.
There have been cases reported in all contiguous U.S. states since the 1999 onset. This year, cases have been reported in 39 states, with the highest rates (25 cases or more) reported in Arizona, California, Colorado, Illinois, Louisiana, Minnesota, North Dakota, South Dakota and Texas.
Most people infected with the virus never develop any symptoms. Some people experience mild, flu-like symptoms such as fever with a headache and body aches, joint pain, vomiting diarrhea or rash. Those with mild symptoms often recover completely, but may experience fatigue for weeks or months.
Severe illness related to West Nile Virus can affect the central nervous system or require hospitalization. Symptoms of severe illness with West Nile include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, or paralysis. Recovery can take weeks or months and some effects might be permanent. Usually, symptoms begin within two to six days of being bitten. However, they can begin later, especially in people with weakened immune systems.
CDC data for 2025 shows that 490 neuroinvasive cases of West Nile have been reported as of Tuesday.
Most cases are mosquito-borne, though sometimes it is spreads through blood transfusions or in orher medical or lab settings. There are no licensed vaccines or medicines available for West Nile disease, though infection is believed to bring lifelong immunity in many cases. According to the CDC, the best way to avoid illness is to avoid mosquito bites by staying inside, wearing protective clothing and using bug spray.