The West Nile virus is detected in Houston and other parts of the U.S. Should you be worried about mosquito-borne illnesses?

What to know about dengue fever, West Nile virus and malaria. (Illustration by Kyle McCauley; Photo: Getty Images)
What to know about dengue fever, West Nile virus and malaria. (Illustration by Kyle McCauley; Photo: Getty Images)

This week public health officials in Harris County, Texas, confirmed that a mosquito sample collected in Houston had tested positive for West Nile virus . While mosquitos — and mosquito-borne illnesses — are more prevalent in the summer, local officials noted that recent severe storms in the Houston area had led to increasing mosquito activity.

"After the recent rains and warmer first months of the year, we are seeing an increase in mosquito populations," Dr. Maximea Vigilant, director of Harris County Public Health's mosquito and vector control division, told the media. "We remind our residents to enjoy the outdoors but remember to protect themselves and their families from diseases transmitted by mosquitoes."

West Nile virus has also been detected in mosquitos this year in other parts of the U.S., from Illinois to California, and Tennessee reported a human case earlier this month. While numbers currently remain low, mosquito-borne illnesses — including West Nile virus, malaria and dengue fever — are cause for concern.

It’s important to note that you can’t get these illnesses without being bitten by an infected mosquito, but mosquito bites are also incredibly common in the warmer months. So, how worried should you be about West Nile virus, malaria and dengue in the U.S.? Infectious disease experts break it down.

In 2023, there were 2,406 cases of West Nile virus detected in the U.S. over 47 states, according to data from the Centers for Disease Control and Prevention. Of those, 1,599 have caused neuroinvasive disease, which means it can cause brain and spinal cord infections such as meningitis, encephalitis or acute flaccid paralysis, per the CDC. However, that data is a little complicated (more on that in a moment).

2023 also saw 2,556 dengue cases across 52 jurisdictions. The majority of cases were travel-related, though locally acquired cases, most notably in Florida, which had 168 such cases compared to two in California and one in Texas, did garner attention. (It's worth noting that Puerto Rico, where dengue is endemic, had 933 locally acquired dengue cases.)

There are typically around 2,000 cases of malaria diagnosed in the U.S. each year, per the CDC, but the disease is usually detected in people who travel to the states from areas outside the country where malaria is common. Between May and October 2023, however, the U.S. had 10 cases of locally acquired malaria reported in Florida, Texas, Maryland and Arkansas.

“It’s summertime and mosquito populations are up there,” Dr. William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine, told Yahoo Life last summer. “In recent decades, there as been a slight increase in West Nile infections during this point in time, but the locally acquired malaria is something new.”

Speaking to Yahoo Life last summer, doctors said it depends. “We’ve been hearing a lot about malaria because it’s fairly exotic,” Dr. Thomas Russo, chief of infectious diseases at the University at Buffalo in New York, said. “Locally acquired transmission ... [is] unusual, but not unprecedented — but it’s very uncommon and treatable."

Malaria can cause symptoms such as fever and flu-like illness, along with anemia and jaundice, per the CDC. Russo noted that locally acquired cases of malaria would "likely burn out at some point,” given that the mosquitoes that can carry it only live for eight or so days.

With dengue, Russo said that "the level of concern should be low, but not zero." Dengue can cause high fever, headache, body aches, nausea and rash, per the World Health Organization. In severe cases, it can be fatal.

"There have been cases over the past few years of dengue in Florida and Texas that appear to be locally acquired in the state," Russo said. "We have the vector in this country and that mosquito is fairly widespread."

But West Nile is a different concern. “West Nile is a completely different beast,” Russo said. While many people can get the disease and have no symptoms, he pointed out that others can develop flu-like illness and even serious brain infections. “A big concern with West Nile is that a minority of individuals could develop neurological complications,” Russo says.

He stressed that, while the latest data showed that the majority of people in the U.S. with West Nile virus had neurological conditions, the “vast majority of cases of West Nile virus go undetected.” Doctors often only think to test for West Nile virus when someone develops serious neurological complications, he said. That means there are a lot of people who get West Nile virus who don’t even know they have it.

Doctors say that the average person should not worry about the risk of contracting malaria, dengue or West Nile virus. “The concern should not be anything to panic about,” Schaffner said. “But there are things you can do to reduce your risk.”

The most important thing you can do is to wear insect repellent if you’re going outdoors in areas where mosquitoes are prevalent, “particularly if you’re living in areas where these cases have occurred,” Schaffner said.

The CDC recommends picking an insect repellent that’s registered with the Environmental Protection Agency and contains one of the following ingredients:

  • DEET

  • Picaridin

  • IR3535

  • Oil of lemon eucalyptus (OLE)

  • para-menthane-3,8-diol (PMD)

  • 2-Undecanone

It’s also a good idea to wear loose-fitting, long-sleeved shirts and long pants to protect yourself, use screens on windows and doors to keep out mosquitos, and run your air conditioner if you own one, the CDC says.

“Additionally, people need to minimize standing water, which mosquitoes use to lay eggs, by cleaning up trash and other receptacles in their yard,” infectious disease expert Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told Yahoo Life.

Experts say that you shouldn’t be overly alarmed about the risk of malaria, dengue and West Nile virus in the U.S., but that it’s important to know that the diseases are in the country.

West Nile cases in particular may also become more common with time. “There is an increasing concern of more of these infections happening as the country gets warmer,” Russo said. “The secret is mosquito control and to not get bitten by mosquitoes, if you can help it.”

This article was originally published on Aug. 17, 2023, and has been updated.