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

West Nile virus infections in the United States are running roughly 40% higher than normal this year, federal health officials said, with more than 770 reported cases, including about 490 severe illnesses, as of early September.

Typically, about 550 cases — including roughly 350 severe cases — are reported by this point in the season, the Centers for Disease Control and Prevention data showed. Health officials have intensified public warnings because the majority of human infections occur in August and September.

"West Nile virus can be a very serious disease and its presence in mosquitoes remains high right now in Massachusetts," Massachusetts Public Health Commissioner Dr. Robbie Goldstein said in a statement. Officials have urged people spending time outdoors to wear long-sleeved shirts and pants when possible and to use an EPA-registered insect repellent.

Public health agencies cautioned that reported infections represent only a fraction of actual infections, because many people infected with West Nile virus have no symptoms or only mild, flu-like illness such as headache, body aches, joint pain, vomiting, diarrhea and rash. In severe cases, the virus can cause inflammation of the brain or spinal cord and lead to long-term neurological problems or death.

Over the past decade, health officials have received reports of about 2,000 West Nile cases per year on average, including roughly 1,200 life-threatening neurological illnesses and about 120 deaths annually. The CDC said deaths appear to be on pace to be higher this year but declined to provide specific mortality figures, saying preliminary data remain incomplete.

Officials said the increase in reported human cases this year does not necessarily reflect a larger mosquito population but a higher proportion of mosquitoes carrying the virus. Mosquito infection rates can be influenced by factors such as temperature, rainfall, the amount of mosquito control activity and the number of infected birds in an area.

Colorado has emerged as a notable hot spot this season, reporting about 150 of the nation’s cases — more than double what many other states have reported. Local monitoring in Fort Collins this summer found that 35 out of every 1,000 female mosquitoes were infected, far higher than the roughly 8 per 1,000 that would typically be expected for that time of year, Roxanne Connelly, a CDC entomologist who lives in the area, said. Connelly and local officials noted the region has experienced a wet and warm season, conditions that can affect mosquito breeding and virus transmission.

West Nile virus was first detected in the United States in New York City in 1999 and spread across the country in subsequent years, peaking in 2003 when nearly 10,000 cases were reported. Health authorities continue to monitor mosquito infection rates and human cases as the season progresses and encourage preventive measures, particularly for older adults and people with weakened immune systems who are at greater risk for severe disease.

The CDC and state health departments said surveillance, targeted mosquito control and public education remain central to limiting mosquito-borne transmission while investigations continue into environmental factors that may be driving higher infection rates in some regions.