With West Nile Virus Confirmed in Ky., Mosquito Control Measures Outlined

West Nile virus (WNV) is spreading. Horse owners are asking, "What can I do to protect my horses?" The good news is that a vaccine for horses has been developed and approved by the USDA. Experts say the WNV vaccine should work like other encephalitis vaccines, which are protective. However, responsible horse owners must take steps to prevent the disease by wisely managing property and horses.

At this time, WNV has been detected in eight states that had not been previously affected, plus Canada. After biting infected birds, mosquitoes spread the virus to horses, humans, and other birds. Horses and humans cannot spread the disease; they are dead-end hosts. Only mosquitoes infected from biting birds with the virus can spread the disease to horses and humans.

Therefore, horse owners need to target mosquitoes for eradication.

"There are three steps: source reduction, larval control, and if that fails, adulticides," explains Don Barnard, PhD, Research Leader of the Mosquito and Fly Research unit at USDA's Center for Medical, Agricultural, and Veterinary Entomology.

Reduce the source of mosquitoes by eliminating breeding sites. "Many mosquitoes develop in small bodies of water or containers," explains Barnard. Drain or eliminate anything on your property that traps water, such as old buckets, tires, or birdbaths. Water troughs for horses that are used infrequently can be drained and cleaned weekly to discourage mosquito breeding.

Horse owners should also eliminate standing water in pastures. However, Barnard says it is not always environmentally acceptable or feasible to fix these wet spots.

Some mosquito breeding sites are on adjacent property, such as tire dumps. According to Barnard, many states have statutes that control the disposal of tires. Your local government can tell you if cleanup of these areas is in order.

Mosquitoes begin life as larvae in standing water (More on Life Cycle of Misquitoes). Microbial larvacides and insect growth regulators are acceptable and safe ways to eliminate larvae without endangering the horse. The products are more expensive than some adulticides.

Microbial Larvacides

Bacillus thuringiensis israelensis (BTI) is one type of microbial larvacide commonly sold under the trade name Vectobac. This product's one-time application is better for water not loaded with manure or organic matter. Bacillus sphericus, often sold under the name Vectolex, is better for more organically "enriched" wet areas. It has some residual activity, with a slight ability to continue growing in the environment. BTI is also sold in a solid "donut" forms in hardware stores under names such as "skeeter dunks" for smaller areas of standing water. These products are not harmful to horses or other animals

Insect Growth Regulators

These are sold as briquettes, liquids, pellets, granules, and concentrates. They contain synthetic chemicals that prevent larvae from emerging as adults. Altosid is a common trade name for the insect growth regulator agent methoprene.

Some people have questioned using exotic fish to eat the mosquito larvae in ponds. Barnard warns that larvae-eating fish have trouble getting into grassy, shallow areas where larvae live. Additionally, many of these fish aren't native to the United States, and there is significant concern about the introduction of foreign species.

Natural mosquito predators such as dragonflies and bats are an option, but they don't feed only on mosquitoes. Barnard says that introduced mosquito predators also destroy beneficial insects. Bug zappers kill good and bad insects indiscriminately, and Barnard discourages their use.


Adulticides kill adult mosquitoes and can be applied from the ground or by aircraft via mists, fogs, or granuals. All have instructions on their use from the Environmental Protection Agency.

"The timing, knowledge, and skill are very critical to proper application," Barnard explains. "If not applied properly, they could do a lot of damage to other animals or beneficial insects."

Mosquitoes can develop a resistance to pesticides. Therefore, it is important to have a reputable pest control operator help plan and administer the application of pesticides to your property.

Stable Management

There is debate over the best turnout arrangement to minimize horses' exposure to mosquitoes, mainly because the mosquitoes will go wherever the horses go.

"One problem with keeping horses inside is that mosquitoes rest on the walls during the day and are active at night," explains Barnard. "Look in the cracks and crevices of the barn-typically in the corners up near the ceiling-for resting mosquitoes. One cure is spraying an insecticide to kill adults in the barn. Large screens over doors and windows are effective, and of course, good barn-keeping goes a long way in keeping the mosquitoes at bay."

More on West Nile Virus, The Horse

Other information links:

Kentucky Department of Agriculture

Department for Public Health

More, EPA

More, CDC