The San Antonio Metropolitan Health District detected West Nile virus in a mosquito trap in early May, months earlier than usual. The city and county are urging residents to take precautions against mosquitoes as rainfall, high temperatures and humidity create the perfect conditions for the insects.
The positive mosquito pool was located near the intersection of Hillcrest Drive and Bandera Road in Northwest San Antonio. Metro Health workers fogged and treated standing water in the area shortly afterwards, according to the department’s website.
There have not been any human cases of West Nile virus in Bexar County this year, according to the latest data from the Texas Department of State Health Services.
Although no more mosquito samples have come back positive since May, city and county officials are ramping up efforts to inform the public about how to keep themselves safe from mosquito-borne diseases this summer.
“We have the warmer temperatures, we have the humidity, we have the recent rainfall,” said Joel Lara, senior environmental health officer at Metro Health. “Those are ideal conditions for mosquito development.”
Mosquitoes and West Nile virus
The majority of pesky mosquitoes that bother you in the warmer months are “nuisance mosquitoes.” Although they leave painful bites, they don’t pose much of a risk to humans, explained Molly Keck, an entomologist with Texas A&M AgriLife Extension.
“They’re not known to be good carriers or vectors of various diseases,” Keck said. “They’re just annoying, but they’re not ones that your city health officials would be highly concerned about.”
Only a handful of the over 80 species of mosquitoes in Texas, though, are known to be carriers of illnesses like West Nile virus, the most common mosquito-borne illness in the U.S. These mosquitoes typically get the disease by feeding on infected birds and then transmit the virus to humans and other animals through bites.
Most people infected with West Nile virus don’t experience any symptoms or complications, but roughly 1 in 5 people experience flu-like symptoms, according to the U.S. Centers for Disease Control and Prevention. There are no specific treatments for West Nile virus.
In very rare cases (less than 1% of infections), the virus affects the central nervous system, causing inflammation of the brain or spinal cord. Around 10% of people who develop neurological complications die. In Texas, there have been 881 cases of West Nile neuroinvasive disease reported to the state health department from 2020 through 2025, though some of the data is provisional.
Local surveillance and control efforts
Metro Health works to control and monitor the mosquito population in the city through traps, disease detection and treatment of water with insecticides. The efforts start in the spring and last into the fall. Mosquito traps placed in 20 locations around the city are checked and tested on a weekly basis for the presence of West Nile virus.
If a mosquito pool tests positive for the virus, the city deploys insecticide from a truck on an established route, what’s referred to as “fogging,” and may also treat specific pools of standing water. Bexar County’s Department of Public Works runs a similar vector control program for unincorporated areas.
Positive hits for West Nile virus typically don’t show up until mid to late summer in San Antonio, according to city data from recent years.
This year may have been an outlier due to the higher amounts of rainfall the area has experienced this spring, but there’s several factors behind how populations drift, including the migration of birds, which some researchers have theorized was a significant contributor to the 2012 Texas West Nile virus outbreak.
Protecting yourself from mosquitoes this summer
The single most important thing you can do to keep mosquitoes away is eliminating stagnant water around your home, especially dirty water, where mosquitoes reproduce. It doesn’t take much water — even an overturned bottle cap filled with water can breed mosquitoes, Keck said.
Lara advised checking rain gutters, animal food bowls, bird baths and anything else around your residence that could be a breeding ground for the insects. For water that can’t be eliminated, like a heavy bird bath, you could treat the water with insecticide, which is available at home improvement stores.
Keck advised not relying on broad insecticides in the long term since these can also kill natural predators of mosquitoes like dragonflies. Instead, look for Bti products, which target mosquito larvae without harming most other insects, Keck said.
If you have to be outside, especially during peak mosquito activity hours at dawn and dusk, wear long sleeves and pants, or spray yourself with insect repellent.
“Water is the big thing,” Keck said. “Stay indoors during dawn and dusk — that tends to be when the species of mosquito that more likely is to carry West Nile is more active. Wear insect repellent. Cover yourself up, which is really hard in the summer in Texas, or just use any form of an insect repellent when you know you’re going to be outside when you might be encountering mosquitoes.”
This story first appeared in the San Antonio Report.