West Nile virus (WNV), a virus that is most commonly transmitted to humans by mosquitoes, may be much more deadly than previously believed, according to a new study, conducted by Kristy Murray, PhD, of Baylor College of Medicine and Texas Children’s Hospital.

Murray and her colleagues found that deaths from WNV occurred right after infection, as well as years later, well after patients seem to have recovered from the initial illness.

“While we understand the current focus on Zika virus, for many people in the United States today, West Nile virus is the much more serious mosquito-borne threat and that threat may persist even for patients who appear to have survived the infection unscathed,”said Murray in a press statement.

Murray and her colleagues studied 4,144 people in Texas who had WNV infections that occurred between 2002 and 2012. The researchers concentrated on “acute” deaths recorded in the first 90 days after infection, and also on WNV patients who died months to years later–yet far sooner than other people of similar age and overall health condition.

According to Murray, the delayed deaths appeared to be more common in patients who had suffered neurological complications during the acute phase of their illness. The cause of death was kidney disease.  

The researchers found there were 286 people who died in the acute phase of WNV. But after examining causes of deaths and symptoms from the initial infection, the researchers concluded that 268 people who survived infection subsequently died early due to the virus. Overall, counting both the acute and delayed group, the researchers attributed 554 deaths to WNV during the 10-year period, a 13 percent fatality rate. The Centers for Disease Control and Prevention (CDC) recorded a much lower national fatality rate of 4 percent between 1999 and 2015, but only for deaths that occurred during the acute phase of illness.

WNV, which also can infect birds that help spread the virus further, has been found in all of the lower 48 states since 1999, when it came to the U.S. The virus belongs to the same family of viruses as Zika and yellow fever. Like Zika, most people infected with WNV never experience symptoms. If they do, they have fever, nausea, fatigue or a rash. In rare cases, it can cause severe neurological complications that can lead to swelling of the brain and spinal cord.

There is no vaccine to prevent the virus, and there is no specific treatment.

The most effective way to avoid WNV is to prevent mosquito bites:

• Use insect repellents when you go outdoors. Repellents containing DEET, picaridin, IR3535, and some oil of lemon eucalyptus and para-menthane-diol products provide longer-lasting protection. Follow label instructions in order to optimize safety and effectiveness.

• Wear long sleeves, long pants, and socks when outdoors, if possible. Mosquitoes may bite through thin clothing, so spraying clothes with repellent containing permethrin or another EPA-registered repellent will give extra protection. Don’t apply repellents containing permethrin directly to skin. Do not spray repellent on the skin under your clothing.
• Take extra care to use repellent and protective clothing from dusk to dawn or consider avoiding outdoor activities during these times.
• Install or repair screens on windows and doors. Use air conditioning, if you have it.
• Empty all standing water from flowerpots, watering cans, gutters, buckets, children’s pools, pool covers, pet water dishes, discarded tires, and birdbaths on a regular basis.
• Support your local community mosquito control programs. They can provide information about the type of products being used in your area. Check with your local health department for more information.

Dead birds may be a sign that WNV is circulating between birds and the mosquitoes in an area. Report dead birds to state and local health departments. State and local agencies have different policies for collecting and testing birds. Check with your state health department to find information about reporting dead birds in your area.