West Nile Virus Cases Running About 40% Above Normal, CDC Says

Health officials say reported West Nile virus infections are running about 40% higher than normal this year, with more than 770 cases and roughly 490 severe neurological illnesses reported as of early September.

Data posted by the Centers for Disease Control and Prevention show the nation has recorded more than 770 West Nile virus cases so far this season, compared with an average of about 550 cases by this point in recent years. Officials warned that most human infections occur in August and September, and they have intensified public messaging urging measures to avoid mosquito bites.

The surge reflects not necessarily a greater number of mosquitoes but a higher proportion of insects carrying the virus, CDC officials said. Mosquito infection rates can be influenced by temperature, rainfall, levels of insect control, and how many local birds are infected, the agency said. Deaths are on pace to be higher than in recent years, CDC officials added, but they declined to provide detailed mortality figures, calling national mortality data preliminary.

Colorado has emerged as a notable hot spot, with the state accounting for about 150 of the nation’s cases — more than double what many other states are reporting. Officials in Fort Collins said monitoring in a southwestern part of the city found 35 infected female mosquitoes per 1,000, far exceeding the roughly 8 per 1,000 expected for this time of year. Roxanne Connelly, a CDC entomologist, said it was unclear why infection rates were elevated in that area but noted the year has been both wetter and warmer than usual.

State health officers in Massachusetts also cautioned residents that the virus remains widespread in mosquitoes. "West Nile virus can be a very serious disease, and its presence in mosquitoes remains high right now in Massachusetts," Public Health Commissioner Dr. Robbie Goldstein said in a statement. Health departments nationwide reiterated standard prevention guidance: wear long sleeves and pants when possible, reduce outdoor exposure during peak mosquito hours, and apply an EPA-registered insect repellent when outdoors.

West Nile virus first appeared in the United States in 1999 in New York and spread across the country in subsequent years, peaking in 2003 when nearly 10,000 cases were reported. Many infections produce no symptoms or only mild illness, and public health officials estimate that tens of thousands of infections may go unreported each year because people do not seek care. Typical mild symptoms include headache, body aches, joint pain, vomiting, diarrhea and rash.

In a minority of cases the virus attacks the central nervous system. Severe illness can take the form of encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes around the brain and spinal cord). Over the last decade, annual national totals have averaged about 2,000 reported West Nile virus cases, including roughly 1,200 life-threatening neurological illnesses and about 120 deaths.

Clinicians diagnose West Nile virus through a combination of clinical evaluation and laboratory testing, typically looking for specific antibodies in blood or cerebrospinal fluid. There is no antiviral treatment proven to cure West Nile virus; management is supportive and may include hospitalization, intravenous fluids and respiratory support in severe cases.

Public health agencies emphasized surveillance and mosquito control as key tools. Local vector-control programs monitor mosquito populations and test trapped insects and bird carcasses for the virus. Officials said the current pattern — higher infection rates among mosquitoes rather than simply larger mosquito populations — points to ecological and environmental drivers that may vary by region and are being investigated.

With most human cases occurring in late summer and early fall, officials urged continued vigilance through the season. Those who experience symptoms consistent with West Nile virus, particularly fever with headache or sudden neurological changes such as confusion, muscle weakness or difficulty speaking, should seek medical care promptly and tell clinicians about recent outdoor mosquito exposure.

The CDC continues to update national surveillance data and to coordinate with state and local agencies on testing, prevention guidance and public messaging as the season progresses.