Mosquito-Borne Illness: A Risk in Every Bite

For 21 years, Cleopatra ruled the Ptolemaic Kingdom of Egypt, informally becoming forever known as “Queen of the Nile.”

For the same amount of time now, the West Nile virus has ruled the consciousness of public health authorities in Texas every mosquito season. West Nile is this state’s queen of mosquito-borne illnesses, reigning since its arrival in 2002. It can be severe enough to cause hospitalization and even death.

But in the interminable Texas summer, other disease threats from mosquito bites are on public health authorities’ radar, too – even malaria, which was recently found in Texas after 20 years of no locally acquired U.S. cases.

Of course – along with multiple possible diseases and their accompanying potential complications – we know there’s always a healthy surplus of mosquitoes. And they’re crafty, Kelly Broussard, epidemiologist at the Texas Department of State Health Services (DSHS), told The Scope.

“Some mosquitoes have actually evolved to bite humans specifically,” Broussard said. “Because they’ve figured out how we build structures with little bits of water hanging around, and they hang out on the doorsill, and then when you open the door, the suction pulls them inside.

“They’ve evolved to sit there and wait until they can be just pulled in the house so they can bite you all they want. They will do anything to bite us.”

While local public health authorities deploy some methods to control mosquito-borne disease, prevention of such illnesses is predominantly an individual responsibility: people everywhere using the most common methods of preventing a mosquito bite.

West Nile: The Reigning Top Threat

Mosquito season, in DSHS’ eyes, unofficially runs from about May through November, with May serving as roughly the beginning of West Nile season. The vast majority of people who contract West Nile through a mosquito will never have any symptoms, Broussard said. Others will have symptoms but never be diagnosed. The cases that get reported to DSHS are usually people sick enough to seek medical care who are then tested, she said. So this year’s DSHS-recorded count of West Nile cases in Texas – eight as of last week – is likely nowhere close to the actual number.

In 2019, the last full year for which DSHS has completed its analyses, Texas had 32 recorded cases of West Nile with four deaths.

“Some years we have hundreds of West Nile cases; some years we have 50,” Broussard said. “But … it’s really very much an undercount of what we know is the true public health burden.”

West Nile is sometimes a year-round phenomenon and can affect Texans in any part of the state, making it a more pervasive threat than any other mosquito-borne illness. Neuroinvasive disease resulting from West Nile – which can manifest as serious illnesses such as encephalitis or meningitis – can cause death. According to the Centers for Disease Control and Prevention, less than 1% of those infected develop neuroinvasive disease.

Dengue and Malaria

While far less burdensome on the state than West Nile, Broussard says, the dengue virus and the recent emergence of malaria in Texas are also on the agency’s radar.

Texans virtually never contract dengue via mosquito bite, but Broussard says there are “certain times of the year, in certain parts of far south Texas, where there are sporadic little pockets of that particular mosquito and virus finding a way to infect a few people without [them] leaving Texas at all.” Through its July 29 weekly report, DSHS had recorded just one case of dengue in Texas in 2023. In 2019, there were 74, including one severe case. DSHS says dengue is “characterized by severe pains in the eyes, head, and extremities and accompanied by catarrhal symptoms.”

Malaria – long eradicated in the U.S. from the standpoint of acquiring it on American soil – is on state health authorities’ radar following a confirmed case in Cameron County. Seven cases were also recently identified in Florida. Malaria is a parasitic infection, Broussard notes, as opposed to the viral infections that represent the more common mosquito-borne diseases. That makes treatment of malaria important, whereas with the viral mosquito-borne infections, she said, “there’s essentially no [treatment] that can be provided to reduce the potential for severe disease.”

DSHS is “seeing a small pocket of transmission and [is] on the lookout to make sure we’re not missing anything else,” Broussard said.

Whatever Happened to Zika?

In 2016, the Zika virus became the predominant public health concern in the United States and worldwide – probably the last widespread public health scare in the U.S. prior to COVID-19. In 2016, DSHS recorded 315 Texas cases of Zika. The next year, the agency recorded 55 cases.

Today, however, Zika is not a major concern in Texas. In fact, Broussard says, there was a severe global downturn of the disease after about 2019. There’s no one straightforward explanation for that, she says, but Texas has not had a reported Zika case since 2021. And that single case – “as almost all of ours have always been,” Broussard says – was a person who acquired the virus in another country.

“But in terms of worldwide activity, there’s so little Zika happening that we just don’t see as much risk for travelers. We have thankfully learned a lot more from the long-term health follow-up side about what it means to be exposed to Zika, especially if you were exposed as a fetus. So there’s a lot of really helpful research that we’re learning about. But there’s so much less being transmitted throughout the world, and we don’t believe any in Texas. So it’s a very different picture from a few years ago.”

Prevention is Personal

Some local jurisdictions in Texas have what the state calls “mosquito control districts,” which Broussard says are essentially subdivisions of the local health department or connected to another local organization. Tools these districts may use to prevent the spread of mosquito-transmitted disease include “various equipment that they use to provide actual chemical control in mosquito breeding. But that is a pretty burdensome process.”

So ultimately, steering clear of mosquito-borne illness – and its most severe potential impacts – is a responsibility most effectively undertaken by people themselves, using the most tried-and-true methods: applying insect repellent containing deet, wearing long-sleeved shirts and long pants and removing standing water in and around your home.

“Mosquitoes can breed in an incredibly small amount of water,” Broussard noted. “There are certain types of mosquitoes that [need] just a few tablespoons of water, teaspoons even. So it’s really easy to forget to check for breeding habitats in your house, in your yard.”

More information on preventing both mosquito biting and breeding is available on the DSHS Mosquito-Borne Diseases webpage.