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Kristy Murray was there from the very beginning. In 1999, now an epidemiologist and expert in tropical medicine a professor of pediatrics at Emory University, was part of the Centers for Disease Control and Prevention (CDC) team that responded to the first U.S. outbreak of West Nile Virus in New York City. “It was my very first breakout assignment,” says Murray Popular science. Thirty cases of unexplained encephalitis had been reported in the city, and it was up to Murray and her colleagues to find out why. The cause was initially baffling. People had symptoms of paralysis, “which is very unusual in encephalitis,” she explains, and older adults made up the majority of those worst off, despite viral paralysis often being most common in children. None of the patients had any relationship or apparent connection with each other.
To figure out what was going on, Murray said she and the rest of the CDC team acted as “disease detectives.” The first clue came from interviewing relatives of those who were sick. “The one thing that kept coming up is that many of them were active and spent a lot of time outside,” Muray said. From there, and through home visits, a CDC entomologist narrowed down the potential sources to Culex mosquitoes. More false leads and confusing test results eventually gave way to an identification of West Nile virus, after birds at the Bronx Zoo also began falling ill with encephalitis. In total, the investigation took about three weeks, Murray says.
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Although the initial mystery was solved relatively quickly (“especially before 1999,” Murray notes), uncertainties surrounding West Nile have persisted. When and where the worst outbreaks will occur remains unpredictable. It is unclear exactly why some people have no symptoms, while other infections prove fatal. There is still no vaccine or proven treatment available.
It’s been 25 years since the mosquito-borne virus was first found in the US. In that quarter century, the disease has spread from New York City to all 48 contiguous states. “It’s everywhere, literally all over the map,” Murray says. “There is no room in the [lower 48] where you can really hide from this pathogen.” Cases of West Nile virus have been reported every year, including 2024, with a peak between late July and October. Here’s what you need to know as this year’s season progresses, what we still don’t know and how experts recommend you protect yourself.
How does the West Nile virus spread?
Birds are the main host and reservoir for West Nile virus. The pathogen is transmitted from host to host mainly through mosquito bites. Culex Mosquitoes, a genus found worldwide and especially common in large cities, are the main vector that transmits the virus between birds or from birds to humans or horses. Humans and other mammals infected with the disease do not produce a sufficient concentration of virus particles to act as a reservoir and then infect additional mosquitoes. “Humans are what we call a dead-end host,” says Gonzalo Vazquez-Propkopeca disease ecologist and professor of environmental sciences at Emory University. Only a small proportion of cases are transmitted between people through blood transfusions and organ transplants.
But while we generally can’t pass the virus to each other, mosquitoes do a lot of work spreading it themselves. “It is without question the most widespread viral vector-borne disease in the United States,” Murray said. “It surpasses all others.” Other non-viral vector-borne diseases, such as tick-borne Lyme disease, may affect more people each year. But Lyme is a bacterial disease that requires effective treatment with antibiotics. There is no approved therapeutic agent for the treatment of West Nile.
Is 2024 a bad year for West Nile?
The CDC is tracking cases of West Nile, along with other arthropod-borne diseases, through ArboNET. As of August 13, the federal agency has confirmed 174 West Nile cases in 30 different states, with double digits in Texas, Louisiana, Nebraska, Nevada and Arizona. Of these, 113 were ‘neuroinvasive’, or the more serious variant of an infection that causes neurological symptoms such as encephalitis (swelling of the brain) or meningitis, which is swelling of the membrane surrounding the brain. So far, eight of the reported cases have proven fatal.
If you look recent years“The number of cases in West Nile, less than 200 cases nationwide, may not sound like much. However, it’s relatively early in the season and every confirmed case at this point likely represents many more hidden cases, Murray says.
In general, cases are vastly underreported because many cases are asymptomatic and many symptomatic infections are mild and difficult to distinguish from other viral infections, she explains. Fever, rash on the trunk, fatigue, pain and malaise are the causes of the majority of symptomatic West Nile cases. Often those infected do not seek treatment or testing. A small proportion of infections, less than one percent, become more serious, affecting the brain and nervous system and becoming ‘neuroinvasive’. These cases can be life-threatening. Survivors of neuroinvasive disease often end up with lifelong disabilities, says Kiran Thakura professor of neurology at Columbia University who studies neuroinfectious diseases.
But even those serious cases are undercounted because providers don’t always test and tests aren’t always positive, she says. In 2022, 827 confirmed neuroinvasive cases were reported to the CDC, but the agency estimates that between 24,810 and 57,890 neuroinvasive infections occurred. It is estimated that up to 15 percent of neuroinvasive cases are fatal, Thakur notes.
Delays in testing and reporting also mean it takes time for the CDC to learn of a confirmed case. “There is a lag in reporting cases, typically about two weeks,” Murray says, and we are just now entering peak transmission time.
Given these caveats, “we see a few more cases than we do [usually] That would happen this time of year, and in some earlier cases as well,” said Erin Staples, a physician and medical epidemiologist with the CDC’s Division of Vector-Borne Diseases. The biggest wave of illness starts in late August and early September, Staples says.
However, that doesn’t mean we’re guaranteed to have a terrible West Nile season across the country. Predicting how this year’s season will go in the coming months “is very difficult,” says Staples Popular science. Trends can change quickly and many variables contribute to the severity of an outbreak.
From year to year, the levels and epicenters of West Nile vary greatly. The virus can peak in the Northeast one season and in the Southwest the next. There was a major outbreak in 2003, and another in 2012. As a result, experts view the outbreak as “cyclical,” with peak waves occurring once every decade, Vazquez-Prokopec says. “It roughly looks like we’re in for a new peak,” he adds.
Climate and rainfall are important. Warm temperatures and the right moisture levels can contribute to a mosquito wave. Birds’ immunity levels also play a role, he says. If most birds in a region have antibodies and avoid disease in a given year, there will also be fewer human cases because the reservoir is smaller, Vazquez-Prokopec explains. “It’s a very complex cycle,” he adds, making accurate predictions difficult.
Regardless of what happens in the coming months, Staples notes that now is a crucial time to take preventative measures.
How can we control the West Nile virus?
Through surveillance of mosquito populations and birds, cities monitor the viral threat year after year. In addition, many municipalities also provide treatment Culex mosquitoes with pesticide sprays spread by fogging vehicles and by targeting the aquatic larvae. Mosquitoes need water to reproduce, so applying insecticides to drainage ditches and stream basins can help reduce their populations without accidentally killing beneficial insects such as pollinators, Vazquez-Prokopec says.
The CDC is researching preventative vaccines and antiviral treatments (and has been doing so for years), Staples says, although the development process, which requires large-scale human trials to prove efficacy, is challenging for such an unpredictable virus. One positive side of the Covid-19 pandemic is that alternative routes to FDA approval and licensure have become clearer, she adds.
But in the meantime, without a vaccine or drug to rely on, individuals can limit their own risk by eliminating sources of standing moisture around their homes (for example: emptying buckets and kiddie pools). Then there are behavioral interventions.
“We have to practice – not to panic, but to be careful,” says Vazquez-Prokopec. Mosquitoes are not only a nuisance, they are also a public health problem, he says. That’s why he advises people to take serious steps to prevent bites.
Insect repellents, especially those registered with the Environmental Protection Agency and recommended by the CDCare a crucial tool. Wearing loose-fitting shirts and long-sleeved pants also helps prevent bites. And people should be especially careful if they go out around sunset and sunrise, when mosquitoes are most active. “I keep a can of repellent by my front door and another by my back door, so I don’t forget that [apply] before I walk out,” says Staples.
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It is still not entirely clear why some people become very ill, while others have asymptomatic infections. However, some trends are clear and certain groups are known to be more vulnerable to severe West Nile virus. People with weakened immune systems, including those taking medications for autoimmune diseases, need to be more vigilant, Staples says. People over 50 are also at greater risk, Murray says. Serious neuroinvasive diseases are reported more frequently among men, but that could be because men share higher levels of other risk factors, such as working outside the home or comorbidities such as diabetes, Thakur notes. And eventually, anyone can develop a serious case.
West Nile virus may be benign for most people and the worst consequences may be rare, but preventative steps are simple and accessible. When the stakes are so high, it’s best to take the risk seriously, Thakur says. Additionally, the same West Nile avoidance strategies will also help minimize exposure to other vector-borne diseases such as West Nile Dengue of Powassan, Staples adds. “Another good reason to use your repellent,” she says.
Making it a habit now will be a good habit for our warming future, where we all want to take biting insects more seriously. Due to climate change, mosquito seasons are likely to lengthen and vector-borne diseases, including West Nile, will spread to new areas where people have not been previously exposed or have no immunity. As global warming continues, “it’s a disease category that I worry about a lot,” says Thakur.