Introduction to West Nile Virus
West Nile virus infections are intense so far this year, with case counts running 40% higher than normal, health officials say. More than 770 cases, including about 490 severe cases, were reported as of early September, according to Centers for Disease Control and Prevention data posted this week. About 550 cases — 350 of them severe — are usually reported by this time of year.
Warning and Precautions
Health officials are ramping up warnings to the public, because most cases of the mosquito-borne disease are reported in August and September.
“West Nile virus can be a very serious disease and its presence in mosquitoes remains high right now in Massachusetts,” said the state’s public health commissioner, Dr. Robbie Goldstein, in a statement last week. People can protect themselves by wearing long-sleeved shirts and pants when possible, and using an EPA-registered insect repellent when spending time outdoors, health officials say.
History of West Nile Virus
West Nile virus was first reported in the United States in 1999 in New York, and then gradually spread across the country. It peaked in 2003, when nearly 10,000 cases were reported. Scientists say many people — perhaps tens of thousands each year — are infected but don’t know it because they have no symptoms, or only mild ones such as headaches, body aches, joint pain, vomiting, diarrhea and rashes.
Symptoms and Effects
In severe cases, damage to the central nervous system causes inflammation of the brain or spinal cord, and even death. In the last decade, health officials have fielded reports of 2,000 cases annually on average, including 1,200 life-threatening neurological illnesses and approximately 120 deaths. Deaths are on pace to be higher this year, CDC officials said, but they declined to discuss specifics, saying mortality statistics are too preliminary.
Cause of the Increase
The problem is not that there are more mosquitoes this year, but rather that a higher proportion of the bugs are carrying the virus, CDC officials said. Mosquito infection rates can be affected by such factors as temperature, rainfall, the amount of insect control going on, and how many nearby birds are infected. Colorado, which tends to see more West Nile virus, has reported about 150 of the nation’s cases — more than double what other states are reporting.
Hot Spots
Fort Collins is a hot spot. Monitoring last month in an area in the southwest part of the city found that 35 out of every 1,000 female mosquitoes were infected — far higher than the 8 per 1,000 that would be expected for that time of year — said Roxanne Connelly, a CDC entomologist who lives there. It’s not clear why, but she noted it’s been a wet and warm year.
Conclusion
The increase in West Nile virus cases this year is a cause for concern, and health officials are urging the public to take precautions to protect themselves from the disease. By understanding the history, symptoms, and causes of West Nile virus, individuals can take steps to prevent infection and stay safe.
FAQs
By MIKE STOBBE, AP Medical Writer
Q: What is West Nile virus?
A: West Nile virus is a mosquito-borne disease that can cause serious illness and death.
Q: How can I protect myself from West Nile virus?
A: You can protect yourself by wearing long-sleeved shirts and pants when possible, and using an EPA-registered insect repellent when spending time outdoors.
Q: What are the symptoms of West Nile virus?
A: Symptoms of West Nile virus can include headaches, body aches, joint pain, vomiting, diarrhea, and rashes. In severe cases, it can cause inflammation of the brain or spinal cord, and even death.
Q: Where are the most cases of West Nile virus reported?
A: Colorado, particularly Fort Collins, has reported a high number of cases this year.
Q: Is there a vaccine for West Nile virus?
A: There is no vaccine for West Nile virus, but health officials are working to develop one.
Originally Published: September 12, 2025 at 10:40 AM EDT
The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.
