Remove standing water to keep mosquitoes, West Nile virus at bay

August 23, 2012

UNIVERSITY PARK, Pa. -- The itch of a mosquito bite is one of the common nuisances of summer.

But with mosquito populations seemingly exploding this year -- and cases of mosquito-borne West Nile virus reaching unprecedented numbers nationally -- it's a good idea to take a few simple precautions to reduce the chances of being bitten, says an urban entomologist in Penn State's College of Agricultural Sciences.

"Ordinarily, mosquitoes are little more than a mild irritant," said Steven Jacobs, senior extension associate in entomology. "But because they can transmit diseases to humans and pets -- such as West Nile encephalitis, eastern equine encephalitis and canine heartworm -- you should take steps to avoid being bitten and to eliminate mosquito breeding areas."

As of Aug. 23, the Pennsylvania Department of Health reports that West Nile virus has been found in 47 counties. Testing has returned positive results from more than 2,200 mosquito samples and from 74 dead birds. Eight human and seven veterinary cases have been reported in the state so far this year.

According to the Centers for Disease Control, approximately 80 percent of people who are infected with West Nile virus will not show any symptoms. Up to 20 percent of infected people will have symptoms such as fever, head and body aches, nausea, vomiting, and sometimes swollen lymph glands or skin rash on the chest, stomach and back. Symptoms can last from a few days to several weeks.

Only one in 150 people infected with West Nile virus will develop severe illness. Associated symptoms can include blindness, disorientation, coma, convulsions, headache, high fever, muscle weakness, neck stiffness, numbness, paralysis, stupor and tremors. These symptoms may last for several weeks, and the neurological effects may be permanent.

Mosquitoes breed in standing water, and they typically will remain active at least until the first frost, according to Jacobs.

He explained that only female mosquitoes bite. "The female must have a blood meal before laying eggs," he said. "And the females' persistent search for blood brings them into houses and yards."

Many mosquito problems can be traced to containers of water around the yard, such as children's toys, pots, cans, tire swings, animal tracks and clogged rain gutters. Neighborhood breeding areas can include construction sites, trash dumps and cemetery urns or planters. Most mosquitoes remain within a half-mile of where they hatched, but some can fly many miles.

During warm weather, mosquitoes can breed in any puddle that stands for more than four days. "The most effective way to control mosquitoes is to eliminate standing water," says Jacobs. He offers the following tips to homeowners:

--Remove old tires, tin cans, buckets, glass jars, toys and other water-catching objects.
--Tightly cover rain barrels to prevent egg-laying.
--Change water in bird baths by flushing with a hose at least once a week.
--Fill tree holes with sand or cement or drill holes to allow drainage.
--Keep rain gutters clean and free of obstructions.
--Drain excess water from flower pots.
--Keep swimming pools covered when not in use.
--Turn over wading pools and wheelbarrows when not in use.
--Empty accumulated water from boats and cargo trailers.
--Clear aquatic vegetation from around the edges of ponds to allow fish to feed on mosquito larvae and pupae.

Jacobs recommends excluding mosquitoes from buildings by keeping windows, doors and porches tightly screened. "For mosquitoes inside the house, use a fly swatter instead of an aerosol spray," he said.

When going outdoors for an extended period of time, insect repellents can provide protection from mosquito bites. "Repellents can protect for up to five hours," said Jacobs. "But because people vary in their attractiveness to mosquitoes, the effectiveness of the repellent may depend on the individual."

Before using a repellent or insecticide, be sure you thoroughly read and understand all directions and cautions on the product label, Jacobs warned.

A Penn State fact sheet on mosquitoes can be found online at

  • Culex pipiens, the northern house mosquito, is thought to be the leading vector of West Nile virus in Pennsylvania.

    IMAGE: Centers for Disease Control
Last Updated August 28, 2012