One evening in July 2003, Dr. Lyle Petersen stepped out of his home in Fort Collins, Colo., to pick up the mail. He ended up staying out longer than expected to chat with his neighbor. But when they both got being bitten by mosquitoes, they rushed back indoors.
Three days later, Dr. Petersen began to feel extremely fatigued. He developed a fever and a severe headache, and his muscles began to ache. “I was in bed for a week,” said Dr. Petersen, the director of the division of vector-borne diseases at the Centers for Disease Control and Prevention. “It’s the sickest I’ve ever been.”
Around day 7, a rash appeared on his skin. Given his expertise, Dr. Petersen suspected the cause. A blood test later confirmed he had been infected with West Nile virus, the leading cause of mosquito-borne disease in the country.
What is West Nile virus disease?
The virus is largely carried by Culex mosquitoes in the United States. Female mosquitoes spread the virus when they bite a bird infected with the virus and then bite a person. Many people who are infected don’t get sick. But about 20 percent develop a fever, “which ranges from pretty mild to pretty awful,” Dr. Petersen said. Other common symptoms include headaches, fever, tiredness, muscle aches, eye pain and skin rashes.
Most people recover within a few days, but some continue to experience extreme exhaustion for weeks to months. Dr. Petersen, for example, couldn’t walk up the stairs of his house without feeling fatigued for nearly three months after his infection.
In rare cases, the illness can lead to a severe neurological disease, causing paralysis, meningitis and brain damage. Those who are severely affected tend to be older and often immunocompromised. About 1 in 10 people who develop these neurological problems die from the disease.
Human cases of the virus typically peak in August and September. The number of people who develop West Nile virus disease varies widely each year: There have been between 712 and 5,674 cases annually from 2005 to 2023, according to the C.D.C. Since the virus was first detected in the United States in 1999, more than 27,000 people have been hospitalized with infections, and about 3,000 have died. But experts say that the number of infections may be far higher.
“For every person that’s diagnosed in the hospital, there’s probably another 150 out in the community that may or may not be having symptoms and may or may not be getting diagnosed,” said Dr. Desiree LaBeaud, an infectious disease physician and epidemiologist at Stanford University.
Where and when does the virus spread?
The mosquitoes hibernate in the winter, emerge in the spring, and remain active through the fall. To track them, local public health or environmental protection agencies set out mosquito traps each year and test the insects for West Nile virus. In New York City, for example, health officials placed traps all across the city in April and have continued to monitor them weekly. (They have identified mosquitoes infected with the virus in all five boroughs in recent weeks.)
Human cases have occurred most frequently in the Great Plains and some western states including California and Arizona. One reason could be the large tracts of rural areas and irrigated agricultural land that potentially provide habitat that is hospitable for mosquitoes to breed, said Anita Bharadwaja, a vector-borne epidemiologist with South Dakota’s Department of Health.
But scientists worry that climate change will create new hot spots for the virus as more regions experience longer warm seasons and wetter conditions. These previously less hospitable areas could see more mosquitoes that are active longer. Outbreaks of West Nile virus disease may also become more frequent with climate change, said Morgan Gorris, a scientist at the Los Alamos National Laboratory who studies the effects of climate on the spread of pathogens.
Can you treat or prevent a West Nile infection?
There are no treatments specifically for the disease. Instead, doctors focus on managing a patient’s symptoms with pain medications, anti-inflammatory drugs to help treat fever and reduce swelling, and intravenous fluids to prevent dehydration.
While there are vaccines approved to reduce the risk of infection in horses, there is no such option for humans.
Researchers have developed potential vaccines for humans that have gone through early clinical testing. But they have proven difficult to test in larger trials because the timing and location of West Nile virus disease outbreaks are unpredictable, Dr. Petersen said, which makes it hard to select an area to conduct research. “It’s very frustrating from our standpoint, because we know a human vaccine would work really well,” he added.
For now, experts recommended wearing long-sleeved shirts and pants to protect yourself and using insect repellent when you spend time outdoors. Look for ones that contain DEET, picaridin or IR3535 chemical. Experts also advise limiting outdoor activity during dusk and dawn, when mosquitoes are most active, and getting rid of any standing water around your home to prevent mosquitoes from breeding nearby.
