While Dallas County has seen more than 200 cases of West Nile virus so far this summer, numbers in Collin County have remained significantly lower.
As of Aug. 17, 230 cases of West Nile virus have been reported in Dallas County, nearly a third of all cases nationwide. There have been 34 cases in Collin County, including 14 in Plano.
With such low numbers, Collin County Judge Keith Self said the county is not looking to make changes to its current mosquito eradication plan."We don't need to use aerial spraying yet," Self said. "We have 34 cases and [most of them] are the fever cases, which is the lesser of the two diseases. We have also had no deaths, so there is no reason for us to move to aerial spraying."
While the vast majority of Collin County will not be sprayed, the portions of Dallas and Richardson that lie within Collin County will be sprayed. The decision to ask the state for help eradicating mosquitoes was necessary, said Richardson Mayor Bob Townsend.
"The evidence is very compelling that it is time to expand on mosquito control efforts in our region," Townsend said in a press release. "Health leaders from government agencies at the federal, state and local level, and third party medical groups, all support expanded action to limit the risks to people from contracting this potentially deadly disease."
Janet McAllister, an entomologist with the Centers for Disease Control, called the transmission of West Nile virus a very complex process that is driven by local factors. As a result, large disparities in the frequency of the disease from one area to another are not uncommon.
"Even though mosquitoes don't recognize political boundaries or property boundaries, you can have epicenters of outbreaks in one county that have less affect in the next county," she said.
Once the numbers of infected mosquitoes reach the level they have in Dallas County, aerial spraying is the best way to reduce numbers, McAllister said.
"The research indicates that spraying two to three consecutive nights can drop the population to almost undetectable levels," she said. "You can knock down the population doing that, and then about a week later you will have the mosquito numbers start to come back up because the larvae are not killed by the adulticide sprays."
Mosquito larvae are best targeted using a larvicide, a technique the city of Plano has been employing for several months, said Geoffrey Heinecke, the city's environmental health manager. The larval stage lasts about a week, at which time the now-adult mosquito gains the ability to fly and becomes susceptible to the sprays.
Aerial spraying is far more effective than ground-based spraying primarily because of the delivery system, McAllister said. The ability to blanket marshes, parks and other areas that are not reachable by road is the primary advantage, but the use of airplanes also allows larger areas to be covered in a given timeframe, she added.
McAllister said the poison should not harm humans or pets, although efforts should be taken to avoid coming into contact with it.
"A lot of it has to do with the dose," she said. "Think of it like Aspirin. If you take two Aspirin it is beneficial for you, if you take the whole bottle it will kill you. The insecticides are the same way. What is put out in the environment to kill mosquitoes is a miniscule amount in micron-sized droplets that are effective in killing something small like mosquitoes, but they really don't harm larger animals."
For information regarding the city of Plano's West Nile response plan, call the city's West Nile virus hotline at 972-941-7180 or visit www.plano.gov/Health/Pages/WNV.aspx.
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