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Fall peak season for West Nile virus

BATON ROUGE, La. — It's the time of year to be even more cautious about mosquito-borne diseases, since this generally is the period when they hit their peak, an LSU AgCenter expert says.

"Louisiana residents should be aware that late summer through early fall is the period when mosquito-borne disease epidemics such as West Nile virus normally occur," cautioned LSU AgCenter entomologist Jack Baldwin.

For example, during 2002, the cumulative number of people infected with West Nile virus in Louisiana increased from 11 on July 22 to 205 on Aug. 29 — and by the end of 2002, there had been 329 human cases in Louisiana.

"Other mosquito-borne virus diseases of humans, such as St. Louis encephalitis and Eastern Equine encephalitis, also tend to increase during this same time period," Baldwin pointed out.

While the expert is cautiously optimistic about this year, he says preventing problems is the best remedy.

"Although 2003 does not appear to be another record year for West Nile virus in Louisiana, people should be aware that the potential for West Nile is still present in the state," the LSU AgCenter entomologist explained. "West Nile positive birds have been reported from at least 51 parishes this year.

"Furthermore, there have been at least 40 human cases so far in 2003, and this case count is almost certain to increase as the season progresses. Therefore, residents should continue to practice the same preventive measures that were recommended last year."

Baldwin and other experts say the best way to prevent West Nile virus and other mosquito-borne diseases is to prevent mosquito bites.

"Although mosquito abatement agencies and local governments provide areawide mosquito control in some parishes, there are still many preventive practices that the individual citizen can do to greatly reduce the chance of mosquito bites," Baldwin said.

For example, the peak biting period for most mosquito species is between dusk and dawn. So people should avoid outdoor activities during these hours, if possible.

"People who are required or choose to be outdoors during this time period should reduce their exposure to mosquitoes by wearing clothing that covers as much of the body as possible and by applying a repellent such as DEET to the exposed skin," Baldwin advised, "In some cases, it also may be necessary to apply repellents to the clothing, because mosquitoes can bite through thin fabrics."

Permethrin and certain formulations of DEET are approved for use on clothing, according to Baldwin, who says more information can be obtained from the LSU AgCenter publication 2871, "Insect Repellents." Copies can be obtained from LSU AgCenter offices in each parish or on the Web at

"Some of the mosquitoes that transmit West Nile virus often were reared around your home or work place," Baldwin said, explaining, "Since mosquitoes breed in standing water, the best way to reduce these mosquitoes is to eliminate all sources of standing water."

Among his advice is to dump water from bird baths, buckets, cans, flower pots, pet water dishes and any other water-holding container about twice a week. Also, check rain gutters and clean them out if they are clogged.

"The best way to determine the mosquito-breeding potential of your home and property is to closely inspect the premises for any form of standing water after a rain," Baldwin said. "Ditches along streets and roads and low areas in the yard often will allow water to stand for days following a substantial rain.

"This type of drainage problem can be corrected either by making physical changes that improve drainage or by filling in low areas."

Abandoned swimming pools also are a breeding source for mosquitoes if they are allowed to hold stagnant water, according to the expert, who says non-functional swimming pools should either be drained or treated with a mosquito larvicide.

In addition to those measures, the LSU AgCenter entomologist says you should deny mosquitoes the opportunity to enter your home by properly screening doors and windows. Residual insecticides also are available to the general public that can be used outdoors to treat the house perimeter, Baldwin says.

"During the hot daylight hours, mosquitoes will rest in the landscaping, under porches and eaves and other shady areas around the home," Baldwin cautioned. "Cool, damp areas that are shaded all day are often favorite resting areas for mosquitoes. These locations should be sprayed with a residual insecticide periodically."

The entomologist advises it also is a good idea to treat door frames, window frames and other locations where mosquitoes might gain entrance into the home. General-use insecticides that contain the active ingredients malathion, diazinon, permethrin, cypermethrin, cyfluthin, deltamethrin or lambda-cyhalothrin can be used outdoors to control mosquitoes around the home. Of course, all insecticides should be used in accordance with the directions on the label.

Baldwin says mosquito larvicides also are available at the retail level for home use.

"Mosquito Dunks (Bti) and Pre-Strike (methoprene) are two safe products that can be applied to standing water that can't be drained," Baldwin said, explaining, "Larvicides control mosquitoes by preventing the aquatic, immature stages from developing into adult mosquitoes."

Finally, Baldwin says you also can obtain hand-held foggers at a reasonable cost through some retail outlets.

"Foggers provide effective control of adult mosquitoes that are present at the time of fogging, but they have no residual effectiveness," he explained.

The foggers apply a 0.02 percent resmethrin solution that is formulated specifically for that use, so Baldwin warns, "Do not attempt to use other residual insecticides in this fogger."

For more details on mosquitoes and mosquito-borne diseases, visit the LSU AgCenter Web site at and see the Mosquitoes/West Nile section or such publications as "West Nile Virus — A New Mosquito Borne Disease in Louisiana."

Tom Merrill is News Editor for LSU AgCenter Communications. (225–578–5896 or [email protected])

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