Mosquitoes are more than a nuisance, they are a public and equine health hazard. In addition to spreading West Nile virus (WNV), mosquitoes can carry malaria, yellow fever, dengue, filariasus (e.g., dog heartworm), and several encephalitis viruses including St. Louis, Eastern, Western, Venezuelan, and La Crosse. In a backhanded way, this is a benefit to the horse industry--mosquito control receives more attention than it would if mosquitoes only spread equine diseases. If you live in an area where mosquitoes are a particular problem, chances are some form of public control is already in place. The American Mosquito Control Association web site (www.mosquito.org) has links to 32 mosquito control districts, 15 U.S. mosquito control associations, 12 university and medical lab sites, six governmental sites, and eight military entomological sites, among others. A lot of people out there are concerned about the same thing horse owners are, which is to keep mosquitoes from biting.
What can horse owners do to control mosquitoes and prevent WNV and other diseases? An integrated approach includes keeping mosquitoes away from horses, stopping them from biting, and preventing disease in case a horse is bitten by an infected mosquito.
Know Thy Enemy
While all mosquitoes might look alike to those of us who swat them, Joe Conlon, technical advisor for the American Mosquito Control Association, explains that there are about 176 species of mosquitoes. How many of these species can transmit WNV is a matter of debate. Of the 79 species that have been tested, 26 have tested positive for WNV and six are considered primary vectors.
Some of these species feed only on birds--they keep the disease circulating, but do not affect horses and humans. Unfortunately, some of the species that have tested positive breed near humans and are known to bite viciously.
The likelihood of any one mosquito carrying WNV is very low. Speaking epidemiologically, an infection rate of 1% among mosquitoes in a particular population is considered a serious problem. This is because the low rate is countered by the vast number of mosquitoes. That is why, Conlon says, that the most important step is to lower the bite rate. The fewer bites, the more the odds are in your favor.
Putting Up Barriers
Every horse owner is familiar with the plethora of products available to keep biting insects from landing on a horse. Insect repellents come in spray bottles, creams, and roll-ons. Repellent-laden tags can be braided into forelocks, manes, and tails. Automatic systems can release measured doses of spray into stalls and barn aisles.
Says Randall L. Crom, DVM, senior staff veterinarian for emergency programs at the United States Department of Agriculture's (USDA) Animal Plant Health Inspection Service (APHIS), Veterinary Services, fly repellent is not necessarily mosquito repellent. However, "Generally, compounds like synthetic pyrethroids applied topically will have decent effectiveness against mosquitoes," he says. "Nothing is 100%, however, especially after perspiration."
Some people feel that over-reliance on chemicals can damage the environment, build up chemicals in insect-eating creatures, and ultimately lead to increased chemical tolerance in mosquitoes. Others argue equally strongly against this position. Natural alternatives include sprays made with oil of citronella and/or oil of eucalyptus; individual results vary.
Another way to keep flies and mosquitoes away from horses is with physical barriers--face masks, ear covers, sheets, and leg covers. Many of these articles can be impregnated with fly repellent. On the down side, no clothing covers a horse completely. A sheet, for example, leaves the sensitive belly exposed. Furthermore, some horses object to having their faces covered.
Stable management practices can also help repel mosquitoes and reduce their numbers. If there is not an automated system in the barn, consider manually spraying stalls and aisle walls. Also consider spraying the lower limbs of shade trees and any place where large congregations of mosquitoes are seen. Adult mosquitoes rest in weeds, so trim weeds around barns and houses and keep lawns mowed.
Barn fans can also help. Steven Halstead, DVM, equine programs veterinarian for the state of Michigan, explains that mosquitoes are "fair-weather flyers that don't do well in air currents." Screens might work on barns, as long as they do not trap flies and mosquitoes inside.
"Overall," says Crom, "screens are probably useful as long as there are no mosquito breeding sites inside the screened area. Screens might not be practical, however."
Research is still being done on which mosquitoes transmit WNV. Some mosquito species feed at dawn and dusk, some during the day, and others at any time. Until the exact vector of disease in an area is known, horse owners can't know which time of day to avoid.
The USDA/APHIS mosquito control web site says that a recent study suggests keeping horses in at night. Halstead recommends to his clients to keep their horses in at dawn and dusk. Conlon adds that some of the mosquitoes associated with WNV are day feeders. Since you can't keep your horse indoors all the time, you should avoid having your horse outside at times of the day when the most mosquitoes are seen in your area, and you must make use of other management techniques that prevent mosquito bites.
Exterminating the Enemy
Population control of mosquitoes centers on interrupting the mosquito life cycle. Water is essential to this life cycle--mosquito eggs are laid in water or damp areas. A floating raft of 200 to 300 eggs looks like a speck of soot, about one-quarter inch by one-eighth inch. Eggs hatch into larvae, then molt into pupae, both of which live in water. Even adult mosquitoes rest on top of water to dry out after emerging from the pupae.
To stop mosquito breeding, one must eliminate or treat all sources of standing water. Cleaning these breeding sites will get rid of eggs, larvae, and pupae. Mosquito larvae feed on organic matter, so cleaning the water will leave the next generation of larvae with nothing to eat.
Potential mosquito breeding sites exist anywhere water stands for days--cans, barrels, tires, toys, buckets, potted plant trays, horse troughs, water collecting under or near water troughs, wheelbarrows, clogged rain gutters, puddles on flat roofs, puddles near faucets or air conditioners, seepage from cisterns/cesspools/septic tanks, ornamental ponds, tree stumps, trash that has collected along fence lines, swimming pool covers that have collected water, other plastic covers or tarps, puddles, creeks, ditches, and marshes.
These water collectors should be emptied, turned over, removed, or filled in. Clean and unclog gutters. If an object can't be moved, drill a hole in the bottom to drain the water. When watering lawns and gardens, look for puddles that remain for days, and landscape to eliminate them.
Bigger areas like ditches or swampy areas might require a community effort. If a roadside ditch needs to be left open for drainage but doesn't always drain completely, Conlon recommends keeping vegetation to a minimum. Remove dead leaves, plants, and grasses on the banks. This will give the larvae fewer places to hide and mosquito predators a clear shot. Also consider that there might be environmental concerns and regulations preventing the filling in of wetland areas.
A mosquito takes at least five days to grow from egg to adult. Therefore, water sources that cannot be removed need to be cleaned and/or treated at least once a week with a chemical or biological larvicide such as mosquito dunks. Keep water troughs clean and swimming pools treated and circulating. Ornamental ponds can be stocked with mosquito-eating fish called gambusia.
Some species of mosquitoes can lay eggs in as little as a few tablespoons of water. Therefore, you should dump all standing water and keep your eyes open for new puddles that collect after a rain.
Comprehensive mosquito control includes killing adult mosquitoes and keeping new ones from growing up. Space spraying is not as effective outdoors as it is indoors, since it only kills the adults that are present with no long-term effect against future mosquitoes. Various traps and bug zappers can attract adult mosquitoes, so they are not recommended. If you use them to kill other insects, keep them away from horses.
"I would encourage people to throw their bug lights away," says Halstead. "They destroy more helpful insects than harmful." For example, dragonflies that eat mosquitoes can be zapped. Halstead also finds that traps work for mosquito surveillance, but do not catch enough mosquitoes to effectively reduce the population.
Don't Worry About...
There is some good news. As far as WNV virus and other mosquito-borne disease are concerned, horse owners do not need to worry about other biting flies or ticks. While these need to be controlled for other reasons, they do not figure into the transmission of WNV. In addition, manure piles are not breeding sites for mosquitoes unless water is involved, perhaps collected on the tarp over a manure pile.
"Other hemovores (blood-feeding organisms) are not vectors," Halstead says. Ticks will bite wild birds, but do not transmit WNV, possibly because the virus needs to replicate inside a mosquito.
Halstead explains that flies are probably not vectors due to their different feeding mechanisms. A mosquito is a "flying syringe" that taps into a blood vessel to feed. While biting, the mosquito releases saliva as an anticoagulant and lubricant; the virus crosses over in the saliva.
Flies, on the other hand, have "scissor-like" mouthparts. They slash open skin and mop up the welling blood. Since this is immediately painful, the horse dislodges them. The fly then moves to the next horse to finish its interrupted meal, taking with it fresh blood from the first horse. Other infections are spread this way, but not WNV.
The genus of mosquito associated with WNV in New York (where WNV first made its appearance in the United States) prefers wild birds over horses or humans. So, they would prefer not to bite us if birds are available. Also, these species are relatively weak as mosquitoes go, so they do not travel very far from breeding sites. However, salt marsh mosquitoes can travel up to 40 kilometers (about 25 miles) to feed. They prefer to bite what is near, but have been known to travel that far for a blood meal. Since some mosquitoes prefer mammals, this can endanger horses and humans.
Humans do not, in the normal course of events, get encephalitis viruses such as WNV from horses. Both humans and horses are infected by way of a mosquito that has bitten an infected wild bird. Infected horses do not have enough of the virus to retransmit it to a mosquito to carry away.
A horse owner will never eliminate mosquitoes from a barn; the best you can hope for is to reduce them as much as possible. No one method or single application will achieve this--you need to plan an integrated, ongoing approach that incorporates public and private means, population control, and repellents. The exact balance of these depends on your area of the country, your land configuration, and your horse arrangements.
Crom recommends, "Talk to your local mosquito control authority (usually at a county level) or an agricultural extension agent." Halstead recommends using all approaches: habitat elimination, topical products, and fly masks, which he uses on his horses. Mostly, he says, "The (WNV) vaccine is going to be the solution."
In 2001, the USDA-APHIS Center for Veterinary Biologics conditionally granted a license to Fort Dodge for a WNV vaccine. The vaccine must be approved by each state veterinary authority for use in that state and administered by a veterinarian. Crom says, "The WNV vaccine met full safety requirements prior to licensing." State health authorities or individual veterinarians will know if a particular area is considered at risk for WNV and whether or not the vaccine is currently recommended. Given the rapid spread of the disease since its discovery in 1999, the risk factor for a given area can change each year.
According to John H. Tuttle, DVM, manager of Equine Professional Services at Fort Dodge Animal Health, a mosquito infected with WNV can carry a very large number of virus particles. When an infected mosquito bites a horse, it deposits some of these particles into the horse's bloodstream. Due to the naïvete (previous lack of exposure) of our equine population to WNV, it might not take many of these particles to infect the horse because he has no residual immunity from previous exposure. In horses which contract the disease, the incubation period is thought to be between five and 15 days.
The Fort Dodge WNV vaccine is a killed vaccine. This means the horse does not get a mild case and thereby develop antibodies, as with modified live viral (MLV) vaccines. Instead, the vaccine causes the horse's immune system to produce neutralizing antibodies, as well as sensitizes the horse's immune system to produce a type of "memory" in regard to future recognition of the virus. When a mosquito's WNV particles invade the bloodstream, the circulating antibodies work to neutralize the virus. Lymphocytes are activated and their "memory" of the virus from the vaccine allows them to produce the proper antibodies, as well as initiate a cascade of immune events to help neutralize the virus.
Halstead recommends that every horse owner consider having his or her horses vaccinated against WNV. He says that field experience has confirmed the safety claims of the manufacturer. He also expects the vaccine to be as protective as claimed if given properly, since the WNV vaccine is a killed virus vaccine like the Eastern equine encephalitis (EEE) vaccine that has proven to be safe and effective (see "West Nile Virus Vaccine: Where It's Been, Where It's Going" on page 16).
Vaccinations, physical barriers such as screens, insecticide sprays, mosquito repellents, and standing water removal should all be part of your barn's mosquito control program. Proper tailoring of all these tools to your barn can help keep your horse's chances of getting WNV to an absolute minimum.
American Mosquito Control Association: www.mosquito.org/mosquito.html.
U.S. Department of Agriculture, Animal and Plant Health Inspection Service: www.aphis.usda.gov/oa/wnv/prv.html.
About the Author
Katherine Walcott is a freelance writer living in the countryside near Birmingham, Al. She writes for anyone she can talk into paying her and rides whatever disciplines she can talk her horses into doing.
POLL: University Equine Hospitals