Texas Tech University

Recent Rainfall Increases Risk of Pests and Diseases

Glenys Young

June 3, 2015

TIEHH expert expects mosquitos to be a big problem this year.

Mosquito Spray
Mosquito-transmitted diseases will become more of a concern as the season progresses.

As spring transitions into the warmer days of summer, many people may be planning to camp out, fire up the grill and enjoy the outdoors. But the recent rains mean something not so pleasant is on the horizon: a plethora of pests and the threats they carry.

“The mosquitos are horrible right now,” said Steve Presley, a professor in the Texas Tech University Institute of Environmental and Human Health. “With the floodwaters in Houston, Dallas and off the Caprock, mosquito numbers are going to be high this year. Coming out of the drought, they haven't been a big issue for the last several years. They're more just pests right now, but as the season progresses West Nile virus and other mosquito-transmitted diseases will become more of a concern.”

Among the other prevalent mosquito-transmitted diseases are St. Louis encephalitis virus, dengue fever and chikungunya virus. Another threat, although not prevalent in the U.S., is malaria, which kills nearly 1 million people around the world each year – more than half are children. And mosquitos are not the only pests to worry about: ticks and fleas carry their own troublesome viruses and bacteria.

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“In West Texas, it's No. 1, West Nile and No. 2, St. Louis encephalitis. Dengue fever is a nasty disease,” said Ronald Warner, who retired from the Texas Tech University Health Sciences Center in November 2012 after a post-military career in veterinary medicine and epidemiology. “Typically it's been reported from Central and South America and many other tropical and subtropical countries in both hemispheres. The nickname for dengue fever is ‘break bone,' because it feels so bad, the joints hurt so bad and it hurts so much to move that it feels like you've got a broken bone. Chikungunya is very similar to dengue fever, but some clinicians feel chikungunya is worse.”

West Nile was first found in West Texas in fall 2002 when there was a severe equine epidemic but not many human cases. The following year, because the virus had become established in the environment, health officials were overwhelmed with human cases, said Warner, who was a consulting epidemiologist for the Lubbock Health Department at the time.

Steve Presley

“We've seen varying degrees of West Nile since then, largely dependent on the weather,” he added. “When there's more rain, there's more of the virus.”

Texas' first case of West Nile virus for 2015 was confirmed in Houston on May 21.

“We're in for a big mosquito year, but in terms of whether that will translate to a big year for mosquito-transmitted disease, you can't really predict that,” Presley said.

So why does more rain mean more mosquitos? There are several reasons. First, mosquitos lay eggs in or near water, where their larvae grow and develop. More water means more breeding grounds. Another reason is rain leads to more vegetation. Plant growth often harbors vermin, which are a food source for mosquitos and ticks. The latter can carry diseases like tularemia and Rocky Mountain spotted fever, a big threat to campers.

“One disease a lot of people talk about is Lyme disease, but honestly, Lyme disease is more of an East Texas, Arkansas, Louisiana, Eastern Oklahoma problem,” Warner said. “It isn't here because we don't have the deer tick up here. Classically, this is not Lyme disease country.”

But it is prairie dog country. The rodents are often covered in fleas, which can carry the plague.

“It is truly a disease that is scary because we've all read the history books about what happened in Europe in 1400s and 1500s and before that,” Warner said. “Occasionally there are die-offs of prairie dogs from fleas carrying plague.”

Culex tarsalis mosquito
Culex tarsalis mosquito

New Mexico routinely reports a few human cases of plague, as well as in rock squirrels and prairie dogs. Colorado has reported a few human cases in the past several years, but Warner hasn't heard of any cases in Texas in years.

“I'm sure it's out there,” he said. “If you have a really wet year and grass really grows well, there will be a prairie dog population explosion, then a flea explosion and we might see plague step in to rebalance the situation. It could be this year.”

The way to prevent getting the disease is to avoid camping near a prairie dog colony and letting fleas get in the sleeping bag, Warner said. For people living on the outskirts of town, make sure outdoor pets have flea collars or have been treated with systemic products to prevent fleas and ticks from being brought inside the house.

“Most tick species need a blood meal, so they follow the rodents and the pets,” Presley said. “I've seen a lot more cottontail rabbits this year and a lot more squirrels. When there's plenty for them to eat, there are more blood-feeding arthropods. I'm not complaining about the rain, because we needed it – but there is a cost for everything.”

The diseases presented by mosquitos and ticks are nothing new in this area, and they have been costly.

“Dengue fever was prevalent throughout Texas in the early 1920s with 41,000 cases,” Presley said, “and there were 43,000 cases of malaria in Texas. Yellow fever devastated Galveston and Corpus Christi in the mid-1800s. Texans have dealt with these diseases before, but now that we have air conditioning and window screens, we're less vulnerable to such diseases in Texas. We have forgotten how bad those diseases can be if they get established.”

Aedes mosquito
Aedes mosquito

Presley said his fear is that Lubbock will repeat Dallas' history from 2012.

“They had been dry for a few years and then had excessive rains, and they had a boom of West Nile virus,” he said. “That can happen.”

Of the 27 or 28 mosquito species found in this area, about five can transmit West Nile virus and St. Louis encephalitis.

Presley's lab collects mosquitos from all over the city each week, and studies others that the city's Vector Control workers collect. Lab assistants then screen the insects for those two viruses.

Jordan Hunter, a graduate researcher pursuing his doctorate in environmental toxicology, said traps are set out at sundown and picked up at sunrise, and they collect hundreds of mosquitos each night, which they refrigerate to kill while preserving the body for observation.

“We identify the types of mosquitos; we take a handful at a time and determine the species, then screen for West Nile virus and extract genetic information,” he said, indicating a mosquito under his microscope. “When you do enough, you learn a lot are the same few species. You get to the point that you know what it is when it lands on you.”

The lab will begin screening mosquitos for chikungunya virus for the first time this year.

“Usually diseases show up in the mosquitos 2-3 weeks before we see human cases,” Presley said. “That's not 100 percent accurate because people come and go, but we'll monitor it and it gives us a heads up.”

According to Jeff McKito, public information officer for the city, Lubbock does not have a routine mosquito-spraying program, but Vector Control is currently blanketing the entire city because of the amount of rain and mosquito activity. Vector Control has traps around the city that it checks for signs of mosquitos and larvae, especially after periods of rain. If a trap shows mosquito activity, Vector Control sprays the area. Anyone who wants an area sprayed can request it by calling the Mosquito Hotline, (806) 775-3110. Typically, the wait time between request and spray is two to three days, but it can take up to a week.