Health & Science

Mosquito Hunters Set Traps Across Harris County, Searching for Signs of Zika

Harris County operates one of the largest mosquito control operations in the country, with more than 50 scientists and technicians who trap, freeze, and grind up mosquitoes to test for viral threats such as West Nile and dengue.

Roberts retrieves a mosquito trap from a backyard in southwest Harris County. Homeowners give permission to have traps placed on their property.
Roberts retrieves a mosquito trap from a backyard in southwest Harris County. Homeowners give permission to have traps placed on their property.

Local health officials are preparing for the possibility that the Zika virus could soon find its way into mosquito populations along the Gulf Coast.

Harris County operates one of the largest mosquito control operations in the U.S., with a permanent staff of more than 50 scientists and technicians. In the summer, two dozen extra seasonal workers are hired to help with the work of setting traps, sorting mosquitoes by species, and conducting lab tests for West Nile, St. Louis encephalitis, dengue, and chikungunya. Zika was added to the testing panel this year.

Watch: Mosquito Processing

The division has an annual budget of $4 million, and operates its own virology lab. In the insectary, an entomologist breeds a steady supply of Culex mosquitoes, used as a control group for periodic tests of pesticide resistance. Spraying is part of the overall strategy, but pesticides are applied selectively, only in neighborhoods where mosquitoes have tested positive for harmful viruses.

The surveillance cycle begins outdoors, with three types of traps. Technicians set and retrieve traps every weekday, hiding them beneath manhole covers, in medians, and in yards (with homeowners’ permission.) Harris County, the third most-populous in the country, covers 1, 777 square miles. For mosquito surveillance, the county is divided into 268 sectors. During mosquito season, from May to October, mosquitoes are collected from each sector at least every other week. 

Mosquitoes are kept alive during the drive to a laboratory, where they are killed through freezing. Freezing also helps preserve viruses the mosquitoes might be carrying.
Mosquitoes are kept alive during the drive to a laboratory, where they are killed through freezing. Freezing also helps preserve viruses the mosquitoes might be carrying.

Christy Roberts, a Texas A&M-trained entomologist, makes her rounds in a white, county-issued pickup truck. At each stop, even on quiet residential streets, she sets out orange traffic cones before crossing into yards to retrieve traps. If the trap was set inside a storm sewer, she uses a pick-axe to pop open the manhole cover, then pulls up the trap.

One type of trap, a “gravid,” sits on top of a plastic tub of water. The stagnant, smelly water is irresistible to female mosquitoes. Roberts pulls the tub out from underneath some bushes along the side of a house. She peers inside the trap’s cylindrical net, spotting individuals  in the blur of  flickering wings.

“Lots of females, some of them are blood-fed, and we have males in there too,” she said.

“The female will fly over the water and then land on top of the water to lay their eggs,” Roberts explains. “And as they’re floating on top of the water, the fan will suck them up into the net.”

The focus of surveillance is females: they need at least one blood meal in order to reproduce. 

Roberts makes a circuit around southwest Harris County, pulling traps from Westbury, Missouri City, and stops in between. The empty traps and batteries go in the truck bed, but the nets of trapped mosquitoes are hung inside, from the roof of the cab.

“We hang the nets, so that the mosquitoes will be able to continue to fly,” Roberts said. “They won’t rub their scales off, which is what we need to identify them. And it also helps keep them alive until we can freeze them.”

Traps stay out overnight, about 12 hours. In the mornings, technicians fan across the county to retrieve traps. In the afternoons, they set out fresh ones in different locations.

Back at the lab, the mosquitoes are flash frozen, which helps preserves any viruses they might harbor. The cold mosquitoes are boxed up and labeled by location, and then must be sorted by sex and species.

Roberts sits down at a lab bench and uses green tweezers to pick through the fragile, feathery bodies. To the untrained eye, they just look…grey. But Roberts is searching for banding, wing color, and variations in the shape and size of the insect’s proboscis and other organs.

“I am sorting for Culex quinquefasciatus, which is our primary vector for West Nile. And I’m just placing them directly in to the vial.” (As the Latin name indicates, that mosquito has five bands across its abdomen.)

Vials are sent to virology, where they are tested for West Nile, St. Louis encephalitis, dengue, and chikungunya. For Zika testing, samples are currently sent to the University of Texas Medical Branch in Galveston. But soon, the lab will add a PCR assay machine, allowing genetic analysis of Zika and other viruses on site. The expansion will cost $300,000.

Harris County is home to 56 different species of mosquitoes, says Dr. Mustapha Debboun, director of mosquito control. But only three species are relevant, because they carry viruses harmful to humans.

“We have the Aedes aegypti, which is the yellow fever mosquito. And also the Aedes albopictus, which is the ‘Asian tiger mosquito,’ and the Culex mosquito,” Debboun explains. “These are the three that we are after. And thank God, we’re only dealing with three.”

Zika has not yet been found in Harris County mosquitoes.

Most people assume “mosquito control” simply means spraying chemicals, Debboun says. And in many municipalities that’s true.  But in Harris County, spraying is done strategically, only in places where a mosquito has tested positive for a virus.

“We go hit the area where we know the mosquitoes have the disease in them,” Debboun explains. “So we don’t just go randomly and just spray, and not only waste the pesticide, but also put a pesticide in the environment when you don’t need to.”

Selective spraying also keeps the insects from developing resistance, Debboun said. Occasionally, aerial spraying is also used, but only twice in the past eight years. In 2008, large areas of the county were sprayed after Hurricane Ike. Some areas were sprayed in 2014, when hundreds of samples turned up positive for West Nile.

Debboun says his team is prepared for Zika. In 2013, he diversified the traps, purchasing  models that are especially attractive to Aedes aegypti and Aedes albopictus. At that time, Debboun was anticipating a possible outbreak of dengue or chikungunya. But those two species also transmit Zika. His employees have already had a few years to learn to use those traps. He’s planning to ask the county commissioners for 74 more of those traps this year, at a cost of $300 each.

Entomologist Christy Roberts retrieves a mosquito trap from a storm drain. The trap uses a light and dry ice to attract mosquitoes.
Entomologist Christy Roberts retrieves a mosquito trap from a storm drain. The trap uses a light and dry ice to attract mosquitoes.

Ruthy Muñoz contributed reporting to this story.

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