Kristy Murray was there at the very beginning. In 1999, the epidemiologist and tropical medicine expert, now a professor of pediatrics at Emory University, was part of the Centers for Disease Control and Prevention (CDC) team responding to the initial U.S. outbreak of West Nile virus in New York City. “It was my very first outbreak assignment,” Murray tells Popular Science. Thirty cases of unexplained encephalitis had been reported in the city, and it was up to Murray and her colleagues to figure out why. The cause was initially baffling. People had symptoms of paralysis, “which is very unusual to see in encephalitis,” she explains, and older adults comprised the majority of those worst off, despite viral paralysis often being most common in children. None of the patients had any relation or apparent connection to one another.
To figure out what was happening, Murray says she and the rest of the CDC team acted as “disease detectives.” The first clue came from interviewing family members 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,” says Muray. From there, and through home visits, a CDC entomologist narrowed the potential sources down to Culex mosquitoes. More false leads and confusing test results finally gave way to a West Nile virus identification, after birds in the Bronx Zoo also began to fall ill with encephalitis. In total, the investigation took about three weeks, says Murray.
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Though the initial mystery was resolved relatively quickly (“especially for 1999,” notes Murray), uncertainties surrounding West Nile have lingered. When and where the worst outbreaks will occur remains unpredictable. Exactly why some people have no symptoms, while other infections prove deadly is unclear. There’s still no available vaccine or proven treatment.
It’s been 25 years since the mosquito borne virus was first found in the U.S.. In that quarter century, the disease has spread from New York City across all 48 contiguous states. “It’s everywhere–all over the map, literally,” says Murray. “There is no place in the [lower 48] where you can really hide from this pathogen.” Each year, 2024 included, West Nile virus cases are reported, with a peak between late July and October. Here’s what to know as this year’s season unfolds, what we still don’t know, and how experts recommend you protect yourself.
How does West Nile virus spread?
Birds are the primary host and reservoir for West Nile virus. The pathogen is mainly passed from host to host via mosquito bites. Culex mosquitos, a genus found worldwide and especially common in major cities, are the primary vector, transmitting the virus between birds or from birds to humans or horses. People and other mammals infected with the illness don’t produce a high enough concentration of viral particles to act as a reservoir and subsequently infect additional mosquitos. “Humans are what we call a dead end host,” says Gonzalo Vazquez-Propkopec, a disease ecologist and professor of environmental science at Emory University. Only a small proportion of cases are transferred between humans through blood transfusions and organ transplants.
Yet though we can’t generally pass the virus on to each other, mosquitos do plenty of work to spread it themselves. “It’s the most widespread viral vector borne disease in the United States, without a doubt,” says Murray. “It far surpasses any other.” Other non-viral vector-borne illnesses, like tick-borne Lyme’s disease, may affect more people each year. But Lyme is a bacterial disease with an effective antibiotic treatment. There is no approved therapeutic for treating West Nile.
Is 2024 a bad year for West Nile?
The CDC tracks West Nile cases, along with other arthropod-borne illnesses, through ArboNET. As of August 13, the federal agency has confirmed 174 West Nile cases in 30 different states, with double digit numbers in Texas, Louisiana, Nebraska, Nevada, and Arizona. Of these, 113 have been “neuroinvasive,” or the more severe variant of infection that causes neurological symptoms like encephalitis (brain swelling), or meningitis, which is swelling of the membrane surrounding the brain. So far, eight of those reported cases have proved deadly.
If you look at past years’ West Nile case numbers, fewer than 200 cases nationwide may not sound like much. However, it’s relatively early in the season and each confirmed case at this point likely represents many more hidden ones, says Murray.
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, a rash on the torso, fatigue, aches, and malaise are how the majority of symptomatic West Nile cases present. Often, those infected don’t seek any treatment or testing. A small proportion of infections, less than one percent, turn more serious, affecting the brain and nervous system and becoming “neuroinvasive.” These cases can be life threatening. Survivors of neuroinvasive illness often end up with lifelong disabilities, says Kiran Thakur, a neurology professor at Columbia University who studies neuroinfectious disease.
Yet even those severe cases are undercounted because providers don’t always test and tests don’t always come back 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. Up to 15 percent of neuroinvasive cases are estimated to be fatal, notes Thakur.
Delays in testing and reporting also mean that it takes time for the CDC to learn about a confirmed case. “There’s a lag in reporting cases, typically by about two weeks,” Murray says, and we’re just getting into the peak transmission time now.
Given those caveats, “we are seeing a few more cases than we [usually] would at this time of year, and some earlier cases,” says Erin Staples, a physician and medical epidemiologist with CDC’s Division of Vector-Borne Diseases. The biggest wave of illness onset tends to come at the end of August and beginning of September, Staples says.
However, that doesn’t mean we’re guaranteed to have a terrible West Nile season nationwide. Predicting how this year’s season will progress over the next couple of months “is very difficult,” Staples tells Popular Science. Trends can shift rapidly and lots of variables contribute to an outbreak’s severity.
Year-to-year, West Nile levels and epicenters vary a lot. The virus may spike in the Northeast one season and then the Southwest the next. In 2003, there was a major outbreak, another came in 2012. As a result, experts consider it “cyclic”, peaking in waves that come about once a decade, says Vazquez-Prokopec. “It seems, roughly, that we’re due for another spike,” he adds.
Climate and rainfall are important. Warm temperatures and the right level of moisture can contribute to a mosquito boom. Bird immunity levels also play a role, he says. If most birds in a region have antibodies and are avoiding illness in a given year, then there will also be fewer human cases, as the reservoir is smaller, Vazquez-Prokopec explains. “It’s a very complex cycle,” he adds– which makes accurate forecasting hard.
Regardless of what unfolds in the next couple of months, Staples notes that right now is a critical time to take preventative measures.
How can we manage West Nile virus?
Through surveillance of mosquito populations and birds, cities keep tabs on the viral threat year to year. In addition, many municipalities also treat for Culex mosquitos with pesticide sprays dispersed from fogging vehicles and by targeting the aquatic larvae. Mosquitoes need water to breed, so applying insecticide to drainage ditches and catchment basins can help reduce their populations without inadvertently killing beneficial insects like pollinators, says Vazquez-Prokopec.
The CDC is researching preventative vaccines and antiviral treatments (and has been for years), says Staples–though the development process, which requires large scale human trials to prove efficacy, is challenging for such an unpredictable virus. A silver lining of the Covid-19 pandemic is that it made alternate pathways to FDA approval and licensure clearer, she adds.
But in the meantime, without a vaccine or medication to rely on, iIndividual people can mitigate their own risk by eliminating sources of standing moisture around their homes (ex: emptying buckets and kiddie pools). Then, there’s behavioral interventions.
“We have to exercise–not panic, but caution,” says Vazquez-Prokopec. Mosquitoes are more than a nuisance, they’re a public health problem, he says. So, he advises that people take earnest steps to avoid bites.
Insect repellents, specifically ones registered with the Environmental Protection Agency and recommended by the CDC, are a critical tool. Wearing loose fitting long sleeve shirts and pants helps to prevent bites as well. And people should be particularly mindful when going out around dusk and dawn when mosquitoes are most active. “I have a can of repellent by my front door and another by my back door, so I remember to [apply] before I walk outside,” says Staples.
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It’s still not completely understood why some people become very sick 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 who are immunocompromised, including those who take medications for autoimmune diseases, should be more vigilant, says Staples. People over the age of 50 are also at higher risk, says Murray. Severe neuroinvasive illness is more commonly reported among men, though that could be because men share a higher level of other risk factors, like working outdoors or comorbidities such as diabetes, notes Thakur. And ultimately, anyone can end up with a severe 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, says Thakur. Plus, the same strategies for avoiding West Nile will also help to minimize exposure to other vector borne diseases like Dengue or Powassan, Staples adds. ” “Another great reason to use your repellent,” she says.
Getting in the habit now will be good practice for our warming future, where we’ll all want to take biting bugs more seriously. Under climate change, mosquito seasons are likely to grow longer, and vector–borne illnesses, including West Nile, are set to spread into new regions where people have no prior exposure or immunity. As global warming progresses, “it’s a disease category I worry about a lot,” says Thakur.
