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The buzz on mosquito surveillance

Posted Monday, August 25, 2008

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What is that buzzing you hear?

If you've lived along the Texas Gulf Coast very long, you will almost certainly recognize it as the sound of a swarm of mosquitoes, something you're apt to hear from March or April till November, and sometimes much longer.

(Left) Nicole Corona, an employee of the Brazoria County Mosquito Control Department, tests mosquito sample for West Nile Virus.

Surveillance of mosquito populations is a key part of the job done by the Brazoria County Mosquito Control Department.

Employees study the numbers and types of both freshwater and saltwater mosquitoes during warm weather.

For the saltwater varieties, employees go through marsh areas and other places where saltwater stands. They don't need to look for the pesky insects, they say, as the mosquitoes are more than willing to come to them.

Freshwater mosquitoes – particularly the night-biting varieties such as the culex, which are the most likely to carry disease such as West Nile Virus – are caught in small traps.

In years past, after the culex mosquitoes were collected, they were sent to a state laboratory to be tested to determine whether they carried West Nile Virus and other diseases.

The tests took two weeks, however, translating to a significant delay before local authorities could determine whether the mosquitoes presented a health hazard and institute a stepped-up spray schedule.

That situation changed this spring, when the county's mosquito control department employees began a program of in-house testing of trapped mosquitoes to determine whether they carried West Nile virus.

"We're trying to be more pro-active," Brazoria County Mosquito Control Director Jim Ryan said. "By testing them here, the mosquitoes collected in a trap one night can be tested the next day, and if we find a positive pool, we can dispatch trucks immediately, in an effort to get rid of them before they can infect someone with the disease."

Once the department finds a positive pool, an accelerated spray schedule goes into effect, with the area where the mosquitoes were located sprayed twice each night – early evening and late evening – for three consecutive nights.

The speedy determination of contaminated pools is of prime importance to give the county a two-week jump on the previous notification and treatment schedule, Ryan said.

"Testing in house is not difficult, but it takes a couple of hours," he said. "We trap the mosquitoes and log where they were found. They are brought back here, and we sort out only the female culex mosquitoes, which we process with buffer solution, run through a centrifuge and filtering process, and put the resulting liquid on a test strip."

After a specified period the test strip goes into an automatic reader and the result is printed automatically.

The equipment now used in this process was originally developed to allow hospital emergency rooms to determine its patients' cause of illness more quickly, so that treatment could begin sooner, Ryan said.

"That procedure is neither complicated nor time consuming," he said. "The equipment cost about $4,000, which we consider very cheap in terms of our department's goal of protecting human life."

The speed of the process is also important considering the mosquito's life cycle, Ryan said, explaining that these insects live only eight to ten days, so the time saved in determining whether they carry West Nile Virus is extremely important.

"A good Mosquito Control Department was built here 25 years ago," Ryan said, referring to the work done by his predecessor, Charles Wilhite, who retired several months ago.

"The department is very well situated as to both equipment and people. We're just looking for ways to make it better. My focus previously was the aerial department, the planes we use for aerial spraying, and building that into what it is now."

Fran Henderson, one of the department's supervisors, has been involved in the spray operation for 18 years, and has had charge of the spray trucks for the past four years, he said.

As to what people should do to avoid getting bitten during mosquito season, Ryan suggested that they use insect repellant, and wear long sleeves and long pants.

Since the high point of activity for mosquitoes is shortly after sunset, he recommended that people stay indoors at that time.

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