West Nile Information
Background Information
West Nile Virus
West Nile virus is transmitted by infectious mosquitoes, usually the Culex species, whose season could begin in early June. The virus is located in the salivary glands of the mosquito. During blood feeding, the virus is injected into the host. Person-to-person, animal-to-animal, or animal-to-person transmission of West Nile virus has not been documented.
According to Douglas Benevento, executive director of the Colorado Department of Public Health and Environment, the first evidences of the West Nile virus are expected to be seen in July this year. If the first evidences of the virus are found in June that will indicate that it will “hit Colorado harder this year." The West Nile virus season is expected to last from August through mid-September.
John Pape, an epidemiologist for the state Disease Control and Environmental Epidemiology Division, said that the drought will not prevent the development of West Nile virus in Colorado this year but will result in "hot spots" clustered along rivers and streams. However, if Colorado receives extensive moisture this spring and summer, then standing water where mosquitoes breed will contribute to the severity of West Nile virus and activity will be more widespread.
Mosquito Facts
Mosquitoes must have water to breed. The mosquito lifecycle is 10 to 14 days. Female mosquitoes lay an average of 200 eggs per brood in or near water. They can have 8 to 10 broods during their lifetime. These eggs hatch into worm-like larvae that mature in 4 to 21 days and enter a pupal stage. Two to eight days later, they emerge from the water as adult mosquitoes. Female mosquitoes bite only in their adult stage and may more than once. They are attracted to their prey by exhaled carbon dioxide from up to 150 feet away. Mosquitoes can fly 10 miles or more from where they hatch at an estimated speed of one to one and a half miles per hour.
Mosquito Control
An Integrated Pest Management (IPM) program for mosquito control includes surveillance, source reduction, larvaciding and adulticiding. When surveillance detects West Nile virus activity, residents are advised to reduce contact with mosquitoes and implement mosquito control measures. Source reduction can be as simple as emptying water from a container or draining a pond. Larvaciding involves applying pesticides to aquatic breeding areas. Killing mosquito larvae can reduce or eliminate the need for adulticiding, applying pesticides to adult mosquitoes. For more information see the CDC's Guidelines for Surveillance, Prevention, and Control.
Microbial larvacides are bacteria that are registered as pesticides for mosquito larvae in outdoor areas such as irrigation ditches, receding flood waters, ponds, pastures, riparian areas and storm water retention areas. Larvacides kill mosquito larvae while they mature in standing water. Duration of effectiveness depends on the mosquito species, environmental conditions, product formulation, and water quality. Few techniques compare to larvacides for speed and effectiveness.
Bacillus thuringiensis israelensis (Bti) is a naturally occurring soil bacterium registered for control of mosquito larvae. It is the most widely used mosquito larvacide in the United States. Mosquito larvae eat the Bti product, which consists of a dormant spore form of the bacterium and an associated pure toxin. Specific pH and enzymes in the gut of the mosquito larvae activate the Bti spores to produce the toxin, which binds to receptor cells in gut of the larvae. Twenty-six Bti products are registered for use in the United States. Aquabac, Teknar, Vectobac, Bactimos and LarvX are common trade names for mosquito control products.
Bti products can eliminate 95-100% of black fly and mosquito larvae, depending on the species, when applied at recommended rates. Bti products are essentially nontoxic to humans, pets and livestock, and do not pose risks to wildlife, fish, nontarget species, or the environment, when used according to label directions. Because of its specificity, Bti is well-suited for environmentally-sensitive locations.
Bti
larvacides are available as dunkers, granules, pellets, liquids or technical
powder. Dunkers are a donut-shaped larvacide used in small bodies of water, such
as stock tanks or ponds. Granules or pellets are able to penetrate vegetative
cover over mosquito breeding areas. Liquids are used for open water sites.
Biological larvacides can be applied to mosquito breeding areas by air,
hand-operated spreaders or hand-broadcast.
Preventive measures to avoid mosquito contact:
- Apply insect repellent sparingly to exposed skin. Effective repellents contain 35% DEET (N,N-diethyl-meta-toluamide). DEET concentrations higher than 35% do not increase the length of protection. Use products with 10 percent or less DEET for children. Always follow product directions for an insect repellent. Repellents may irritate the eyes and mouth.
- Spray clothing with repellents containing DEET as mosquitoes may bite through thin clothing.
- Wear long-sleeved shirts and long pants when you are outdoors.
- Stay indoors at dawn, dusk and early evening when mosquitoes are most active.
- Install or repair window and door screens so that mosquitoes cannot get indoors.
- Drain sources of standing water. At least once per week, empty water from flower pots, pet dishes, birdbaths, swimming pool covers, buckets, barrels and cans.
- Clean out rain gutters.
- Remove containers that could collect water.
- Don't over water lawns and gardens.
- Vitamin B and ultrasonic devices are NOT effective in preventing mosquito bites
General information about West Nile virus and what is being done to combat it in Colorado can be found at the Colorado West Nile Virus Web site at www.fightthebitecolorado.com.