Wet Spring Brings Mosquitoes, West Nile Virus

Wet Spring Brings Mosquitoes, West Nile Virus

May 25, 2007

Spring flooding in Nebraska left standing water in low areas across the state, providing excellent breeding areas for mosquitoes. People who want to stay safe this summer should start by draining standing water, said Jack Campbell, entomologist at UNL's West Central Research and Extension Center at North Platte.

Breeding sites include wading pools, bird baths, leaf-clogged gutters, standing water in yards or alleys, potted plants, poorly drained curbs leading to storm sewers, old tires or any other water containers. Farmers should check irrigation re-use pits and areas where irrigation or heavy rains drain to roadside ditches.

When mosquitoes are present, outdoor activities should be curtailed, if possible, said David Boxler, UNL entomology research technician.

"Light-colored clothing is less attractive to mosquitoes and should cover most of the skin," he said. "Although some people, especially small children, may be sensitive to them, repellents containing DEET should be used on exposed skin whenever possible."

Generally, the higher the percentage of active ingredient in the repellent, the longer the residual activity, Boxler said. DEET products containing more than 30% active ingredient will remain potent five to six hours.

Insecticides can be used as residual sprays for mosquitoes that rest in shrubs, flowers or trees, but be sure to check the label for rates and toxicity to lawn and garden plants, he said.

"A major mosquito outbreak requires city, county or state mosquito abatement programs," Campbell said.

Mosquitoes have always been and continue to be a major concern to people across the globe because of the serious diseases they carry, such as malaria, filariosis, yellow fever and dengue fever. In Nebraska, they also carry serious diseases, including encephalitis viruses, particularly West Nile virus, Campbell said.

In 2006, Nebraska ranked fifth in the nation with 264 confirmed cases of West Nile virus. Buffalo, Douglas, Lincoln, Scottsbluff and Lancaster counties led the state in the incidence of the disease, although most counties had at least one confirmed case, he said.

Nebraska has 50 species of mosquitoes. The two most abundant are Aedes vexans and Culex tarsalis, Boxler said.

Epidemiologists classify C. tarsalis as a very efficient and A. vexans as a moderately efficient carrier of the West Nile virus. The latter carries it from birds to mammals. It may be the most dangerous to Nebraskans because it is a daytime feeder, while C. tarsalis feeds primarily in the evening or early morning, he said.

"Disease organisms multiply in mosquitoes until they can be transmitted at the next feeding," Campbell said. "Although the West Nile virus has been identified in over 30 mosquito species, many cannot transmit it.

Unfortunately, Nebraska's two most numerous species can. A. vexans is an early-appearing mosquito. In fact, it is already present, and C. tarsalis appears somewhat later in midsummer."

West Nile virus is similar to St. Louis encephalitis in symptoms and transmission cycle. Although most people will have no symptoms, 10%may develop a fever, headaches or other body aches, occasionally with a rash on the trunk, swollen lymph glands or other symptoms. Less than 1% will develop severe symptoms of encephalitis or meningitis, including intense headache, high fever, stiff neck, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and possible death.

The elderly and people with compromised immune systems are most likely to experience severe symptoms. The incubation period in humans is about three to 14 days. The disease can be treated with antibiotics. People who survive an infection are immune for life.

For more information, consult Nebraska Extension NebGuide G154, Mosquitoes, available online and at local UNL Extension offices. More information about mosquitoes also can be found on UNL's Department of Entomology Web site.

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