What You Need to Know About West Nile Virus: Symptoms and Prevention

One evening in July 2003, Dr. Lyle Petersen left his Fort Collins, Colorado, home to get the mail. He stayed outside longer than he expected to talk to a neighbor. But when both were bitten by mosquitoes, they ran back inside.

Three days later, Dr. Petersen began to feel unusually tired. He developed a fever, a severe headache, and his muscles began to ache. “I was in bed for a week,” said Dr. Petersen, director of the division of vector-borne diseases at the Centers for Disease Control and Prevention. “It was the worst I’ve ever been in.”

Around day 7, a rash appeared on his skin. Given his knowledge, Dr. Petersen suspected the cause. A blood test later confirmed that 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 spread by Culex mosquitoes in the United States. Female mosquitoes spread the virus when they bite a bird infected with the virus, which then bites a human. Many people who are infected do not get diseased. But about 20 percent of people experience a fever“which range from fairly bland to quite nasty,” Dr. Petersen said. Other common symptoms include headaches, fever, fatigue, muscle aches, eye pain and skin rashes.

Most people recover within a few days, but some continue to feel extremely exhausted for weeks or months. Dr. Petersen, for example, couldn’t walk up the stairs of his home without feeling tired for almost three months after he was infected.

In uncommon cases, the disease can lead to a grave neurological condition, causing paralysis, meningitis, and brain damage. People who are severely affected are usually older and often have weakened immune systems. About 1 in 10 people who develop these neurological problems die due to illness.

The number of human cases of the virus typically peaks in August and September. The number of people who develop West Nile disease varies greatly each year: from 712 to 5,674 cases per year between 2005 and 2023, According to the CDC Since the virus was first detected in the United States in 1999, more than 27,000 people have been hospitalized with the infection and about 3,000 have died. But experts say the number of infections could be much higher.

“For every person diagnosed in the hospital, there are probably 150 others in the community who may or may not have symptoms and may or may not receive a diagnosis,” said Dr. Desiree LaBeaud, an infectious disease physician and epidemiologist at Stanford University.

Where and when does the virus spread?

Mosquitoes hibernate, emerge in the spring, and remain vigorous through the fall. To track them, local public health or environmental agencies set out mosquito traps every year and test the insects for West Nile virus. In Recent York City, for example, health officials deployed traps across the city in April and continue to monitor them weekly. (In recent weeks, they have identified mosquitoes infected with the virus in all five boroughs.)

Human cases have been most common in the Great Plains and some Western states, including California and Arizona. One reason may be huge swaths of rural areas and irrigated farmland, which potentially provide habitat conducive to mosquito breeding, said Anita Bharadwaja, a vector-borne epidemiologist with the South Dakota Department of Health.

But scientists worry that climate change will create up-to-date hotspots for the virus as more regions experience longer toasty seasons and wetter conditions. These previously less hospitable areas could see more mosquitoes that are vigorous longer. Outbreaks of West Nile virus could also become more constant with climate change, said Morgan Gorris, a scientist at Los Alamos National Laboratory who studies the effects of climate on the spread of pathogens.

Can West Nile virus infection be treated or prevented?

There is no specific treatment for this disease. Instead, doctors focus on treating the patient’s symptoms with pain medications, anti-inflammatory drugs to facilitate fight fever and reduce swelling, and intravenous fluids to prevent dehydration.

While there are approved vaccines to reduce the risk of infection in horses, there is no similar option for humans.

Scientists have developed potential human vaccines that have passed early clinical trials. But they have proven challenging to test in larger studies because the timing and location of West Nile virus outbreaks are unpredictable, Dr. Petersen said, making it challenging to choose a region to study. “From our perspective, it’s very frustrating because we know that a human vaccine would work really well,” he added.

For now, experts recommend wearing long sleeves and pants to protect yourself and using insect repellent when you spend time outside. Look for ones that contain DEET, picaridin or the chemical IR3535. Experts also recommend limiting outdoor activity at dusk and dawn, when mosquitoes are most vigorous, and getting rid of standing water around your home to prevent mosquitoes from breeding nearby.