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West Nile virus remains a threat, even in droughts
By Keryn B. Page
MISSISSIPPI STATE -- Dry summer weather may make some people think they don't need to worry about mosquito-borne diseases, but experts say to keep up the defenses against West Nile virus.
Entomologist Blake Layton with the Mississippi State University Extension Service said the Southern house mosquito is the main carrier of West Nile virus. He said this mosquito breeds in stagnant water and containers and doesn't require much water to quickly increase in number.
“During dry conditions, you've still got standing water that will support mosquitoes. Where you have standing water, you'll have mosquito breeding. The organic matter is actually more concentrated when it is dry, and this is what Southern house mosquitoes like,” Layton said. “The mosquitoes may have fewer places to breed, but they still can reach high numbers. So you don't need to say, 'I don't have to worry about mosquitoes.'”
Mississippi Department of Health officials have reported 12 human and nine bird cases of West Nile virus as of July 25 in Mississippi. One human victim died as a result of the virus.
One human case has been reported in each of the following counties: Bolivar, Copiah, Lamar, Monroe, Pike and Scott. Six human cases have been reported in Forrest County.
This time last year, Mississippi reported five human cases. At the end of 2005, 70 people had contracted West Nile virus, and six people died from the disease.
Health department officials said Mississippi's West Nile virus season usually peaks in July, August and September, and they encouraged Mississippians to practice personal protective measures. These include removing sources of standing water, avoiding mosquito prone areas, wearing protective clothing and using insect repellents that contain DEET, the chemical picaridin or oil of lemon eucalyptus.
Sharon Sims, health programs specialist with the Mississippi Department of Health, said senior citizens and those with compromised immune systems are most at risk from this virus. Healthy people are at little risk, as most people who contract the virus recover. As with any virus, if diagnosed, only the symptoms can be treated.
“Most people who get sick don't even know they're sick,” Sims said. “Some get a headache or flu-like symptoms for two or three days. Others develop encephalitis or meningitis. A physician will decide if the West Nile virus test needs to be run.”
Layton said the most effective method of managing mosquitoes is to control them at the immature stage, either by eliminating breeding sites or by applying larvicides to those sites.
“Any container, low area or other area where standing water accumulates for more than a few days can serve as a mosquito breeding site,” Layton said. “Eliminating such breeding sites can greatly reduce mosquito populations around home lawns.”
Here are some steps to eliminate mosquito breeding sites:
* Discard old cans, bottles, tires and other unused items that can hold water.
* Where possible, install drain holes in items that tend to catch and hold water.
* Repair leaks in outdoor plumbing, sewage lines and livestock waters.
* Clean and repair gutters to prevent them from holding water.
* Empty and refill birdbaths, pet water bowls and other containers at least weekly.
* Avoid excessive lawn irrigation that may result in standing water.
* Where possible, drain or fill small, low areas to keep puddles from forming.
* Where drainage is not possible, deepen seasonal pools into permanent ponds.
* Control aquatic vegetation in ditches and permanent ponds.
* Stock ornamental ponds and livestock ponds with fish, especially mosquito fish.
* Treat standing water with appropriate mosquito larvicides.
The larvicidal products most commonly available to homeowners contain one of two active ingredients: Bacillus thuringiensis israelensis (or Bt israelensis) and methoprene. Both of these products are labeled for use in areas like abandoned swimming pools, ornamental fountains or ponds, rain barrels, storm drains, ditches, fish ponds, cesspools, seasonal pools and other small bodies of water where mosquitoes may breed.
“Successful use of these products requires that they be reapplied at frequent intervals throughout the mosquito breeding season. Re-treatment may also be necessary following heavy rainfall, which washes out or dilutes the treatment,” Layton said.
These products are formulated as granules or briquettes, allowing for easy application. The briquettes often are designed to provide slow release of the product in order to extend the control time.
Adult mosquitoes like to rest in heavy vegetation, so keeping shrubs trimmed can help reduce their numbers in an area. Occasionally mowing or bush-hogging any nearby pasture areas also can reduce resting sites for adult mosquitoes.
“If large numbers of adult mosquitoes are migrating into your lawn from areas outside your control, fogging with resmethrin can provide limited short-term control,” Layton said. “Also, commercial area repellents containing citronella or other active ingredients can provide some degree of relief.”