Denton County reported its first West Nile virus death of 2024 — the second local human case this year — on Tuesday.
The person, who was diagnosed with West Nile neuroinvasive disease, lived in the northwest quadrant of unincorporated Denton County, according to a news release from the county. The neuroinvasive form of West Nile is a rare but serious and sometimes fatal infection that attacks the nervous system.
Denton County Public Health is not releasing further information about the case to protect patient confidentiality.
“We’re seeing more positive mosquito traps in the last few weeks indicating increasing risk for contracting West Nile virus, just as we’ve seen in our second human case and first death of the season,” Juan Rodriguez, DCPH assistant director and chief epidemiologist, said in the release. “We ask community members to take preventive actions to lower the risk of contracting a mosquito-borne illness.”
Another person, who lives in Sanger, tested positive for the West Nile virus neuroinvasive disease in late July.
More than 30 mosquito traps throughout Denton County have yielded positive test results for West Nile virus so far this season.
With several months left of mosquito season in Texas — and August being one of the most active months — Denton County isn’t out of the woods yet.
This year is a worse year for mosquitoes and contracting West Nile virus, likely due to the weather, an expert says.
With that in mind, it’s important to know West Nile virus is, why it’s prevalent and what can be done to prevent its spread.
What is West Nile virus and its neuroinvasive form?
West Nile virus is spread to humans through the bite of an infected female mosquito that has fed on infected birds, according to the Centers for Disease Control and Prevention.
There are no vaccines to prevent or medicines to cure West Nile virus. So, preventing mosquito bites is crucial.
However, the CDC reports only about 1 in 5 people infected with West Nile virus will develop symptoms.
Symptoms include fever, headache, body aches, vomiting, diarrhea or rash.
The virus is rarely transmitted from person to person, the CDC says, although it could be transmitted through a blood transfusion or organ transplantation, or from mother to baby through pregnancy, delivery and breastfeeding.
West Nile virus neuroinvasive disease affects less than 1% of people who become infected with West Nile virus, according to the CDC.
The disease typically manifests as meningitis, encephalitis or acute flaccid myelitis.
The CDC states most patients with non-neuroinvasive West Nile virus or West Nile virus neuroinvasive meningitis recover completely. But they can experience fatigue, malaise and weakness for weeks or months.
Those who recover from West Nile virus encephalitis or acute flaccid myelitis, the CDC states, often have residual neurologic effects.
The CDC reports that more people infected with West Nile virus have lifelong immunity from getting it again but may have a weakened immune system or see their immunity wane over time.
Why is there an increase in positive mosquito traps this year?
West Nile virus is the most common mosquito-borne disease in the U.S. Cases are more prevalent when mosquito populations swell under the right environmental conditions.
Temperature and rainfall can have a significant impact on mosquito populations, said Vanessa Macias, a University of North Texas professor with expertise in mosquito biology.
Denton County has had significantly more rainfall so far this year compared to last year, according to the National Weather Service.
“The more rain we have, the more mosquitoes we’re going to have,” Macias said.
As a result, Denton County alone has announced it was taking action to cull mosquito populations on 15 occasions so far in 2024, compared to three occasions in 2023.
Denton County places mosquito traps in high population density areas of the county’s jurisdiction. It tests these traps weekly from May to October.
Cities in Denton County also regularly test mosquito traps for the virus.
Cities within the county and Denton County itself have been conducting ground fogging in their respective jurisdictions each time a trap yields a positive test result.
When ground fogging is done, it’s conducted three times in the area. It doesn’t have to be consecutively but close in time, said Jenica Bates, DCPH’s public information officer. So, it could occur three days in a row or three times within a week.
Ground fogging employs a fine mist of insecticide and water. When dispersed, the mist can kill adult mosquitoes on contact.
Killing adult mosquitoes is the best way to eradicate diseases like West Nile virus, Macias said.
Mosquitoes are different from other disease-transmitting insects, Macias said, in that they have to ingest the virus and the virus has to reproduce inside the mosquito first to be transmitted.
“So, it infects the mosquito truly,” Macias said. “Once the virus has infected the mosquito and gotten into her salivary glands, the next time she bites someone, she’ll spit the virus into them. So, the virus actually needs the mosquito in order to be transmitted. That’s really important, because that means if we get rid of the mosquito in an area, then we won’t have the virus.”
However, it’s possible that ground fogging will not kill every mosquito in an area.
Additionally, mosquito populations can grow quickly. Their typical lifespan, from egg to adulthood, is about two weeks, Macias said.
So, ground fogging is a temporary solution. However, Macias conducts research into a more permanent solution to eradicate mosquito-borne diseases: genetic engineering.
“You give mosquitoes a small set of new genes that enable them to do something new,” Macias said. “What we’re interested in giving them is either genes that cause them to essentially self-destruct, so it causes a population crash, or genes that help them to recognize a specific pathogen like West Nile so that when they inject the virus, they can recognize it and destroy it instead of passing it on to another human or bird.”
The genetically engineered mosquitoes would breed with others, passing the genes through a population.
It’s much easier to build the population crash genes in a laboratory, Macias said. That solution, Macias said, is being deployed in Brazil to reduce dengue fever and being tested in other places with promising results.
How can you avoid contracting West Nile virus?
Because there are no vaccines or medications to cure West Nile virus, the best way to prevent it is by protecting against mosquito bites.
Mosquitoes will bite during the day or at night. However, they are most active at dusk and dawn. Local authorities recommend staying indoors at these times.
When outside, use insect repellent containing 10% to 30% DEET, wear long-sleeved shirts and pants, and eliminate mosquito breeding grounds.
Mosquitoes breed in standing and stagnant water. Prevent such water from accumulating in yards and neighborhoods.
Some common neighborhood mosquito sources are:
- Uncovered boats
- Open trash bins
- Clogged rain gutters
- Fountains and bird baths
- Potted plant saucers
- Buckets and barrels
- Pet water bowls
- Wagons or other toys
- Areas beneath leaky hoses
- Neglected pools
- Tires
- Rot holes in trees
- Ponds
- Low ground areas
It’s important to check and clean these areas regularly, Bates said, because new mosquitoes could breed and hatch quickly after water accumulates.
Residents should avoid even the smallest accumulations of water if possible.
“Mosquitoes can lay their eggs and grow in very small quantities of water,” Macias said. “So, even if we’re proactive in getting rid of standing water, there’s always little pockets of water they can find.”
For standing water that can’t be dumped, Bates recommends using Bti briquettes, also known by the brand name Mosquito Dunks.
These contain a larvicide that releases a natural bacterium called Bacillus thuringiensis israelensis (Bti) that can kill mosquito larvae before they mature.
Bti briquettes are commonly available at major retailers.
The city of Denton also offers a free supply of two Bti briquets per resident as supplies last. Proof of residency, such as a driver’s license or utility bill, is required.
The city has two pickup locations:
- Customer Service, 601 E. Hickory St., Suite F. Open Monday through Friday from 8 a.m. to 5 p.m.
- City of Denton Composting, 1100 S. Mayhill Road. Open Tuesday through Saturday from 7 a.m. to 3 p.m.