Healthy Defenses

With Spring around the corner, so too is the disease-carrying insect population. Here's how to keep these potentially deadly bugs at bay.

EPM, West Nile virus, Potomac horse fever, Lyme disease—these diseases can hit horses with devastating effects. Their patterns seem random, even mysterious. One horse contracts EPM while his stablemates stay healthy; another is struck by West Nile while horses down the road are untouched. Is it just the luck of the draw?

Luck may play a part, but fortunately, you can stack the deck in your favor. First, you can vaccinate against several of these diseases (see the box on page 26 for more about this). Second, all four diseases are spread by vectors—insects or other animals—and you can take steps to protect the horses in your barn from these unwelcome critters. Read on to learn what works and what doesn’t.


Equine protozoal myeloencephalitis (EPM) is caused by a microscopic parasite, the protozoan Sarcocystis neurona, that invades the horse’s central nervous system. Symptoms can be subtle or severe and range from lack of coordination to seizures to paralysis. EPM is notoriously difficult to diagnose, and treatment is expensive, lengthy, and not always successful. Almost every part of North America has seen cases of EPM, although the West has the fewest.

Horses pick up S. neurona while eating grass, hay, or grain or drinking water contaminated by opossum feces. The opossums get the protozoan by eating dead birds or other animals, including raccoons, that are its intermediate hosts. In these animals, the protozoan lodges in muscle tissue, but in the horse it can migrate to the brain and spinal cord.

“Many horses that are infected don’t get sick,” notes Michigan State University researcher Linda S. Mansfield, VMD, “but those with weak immune systems are vulnerable.” Horses don’t pass the disease to other horses, nor do they get it from raccoons or the other intermediate hosts—unlike opossums, these animals don’t pass the protozoan in feces.

To cut the risk of EPM, start by controlling opossums:

• If these animals are on your property, Mansfield advises trapping them. “There’s no need to rid the world of opossums, but it makes sense to keep them away from horses,” she says.

• Measures that keep rodents out of your barn—such as tight covers on grain bins—will also discourage opossums. Opossums love pet food, so don’t leave dog or cat food out. They eat carrion, so dispose of dead birds and other animals.

• Mesh wire fencing, with a “hot wire” strung low, around the outside, will help keep opossums out of pastures and paddocks.

• Guard against contamination of commercial grain mixes by using processed feeds—steam-crimped oats, pellets or extruded feeds. The heat used in the processing kills the parasites.

Top-flight care can also protect your horses, Mansfield says. An alarming 50 percent of horses carry antibodies to EPM, indicating that they’ve been exposed to the protozoan; yet only a small percentage develop the disease. “If you maintain your horses in high health, you’ll enhance their immune response and help them fight it off,” says Mansfield. That means a balanced diet, regular exercise, and good vaccination and deworming programs.

Mansfield is investigating a possible preventive: Research in her lab has shown that the daily dewormer Strongid C (pyrantel tartrate) is lethal to S. neurona. She’s now running field tests to see if the medication is equally effective against the protozoan in living horses.

West Nile Virus

Since the first U.S. cases of West Nile virus were reported in 1999, this disease has spread rapidly. In 2002 the U.S. Department of Agriculture (USDA) reported more than 14,000 equine cases in 40 states. The virus attacks the horse’s central nervous system and can produce signs ranging from fever, muscle weakness and stumbling to partial paralysis, inability to stand and convulsions. While most horses recover, about 30 percent die. People can also get West Nile, but infection and fatality rates are much lower than in horses.

West Nile is primarily a disease of birds; mosquitoes that feed on infected birds pick up the virus and can pass it on when they bite horses—although not every horse that’s bitten gets sick. The virus isn’t transmitted between horses or from horses to people.

Reduce your horses’ exposure to mosquitoes, USDA and state officials say, and you’ll cut the risk of West Nile virus. The do’s and don’ts:

• Don’t turn horses out at dusk and dawn, peak periods of mosquito activity.

• Use fly repellents labeled for use against mosquitoes, applied (and reapplied) according to label directions.

• Put screens on the barn, or use a premises spray to reduce the number of mosquitoes in the barn. Choose a product that’s labeled for use around horses, and follow the directions carefully.

• Keep incandescent lights switched off as much as possible—these lights attract mosquitoes.

• Reduce mosquito breeding grounds. The insects lay their eggs in standing water, so get rid of old tires and anything else that catches rainwater. Turn over wheelbarrows and other containers when not in use. Empty and refill water troughs weekly. Keep wash-stall drains, storm drains, and roof gutters clean, so that water doesn’t back up in them, and cover cisterns and rain barrels. Drain puddles and ditches.

• In areas that can’t be drained, use mosquito dunks—blocks that dissolve in water, releasing microbes that kill mosquito larvae. Stock ponds with fish that eat the larvae (such as the mosquito fish, Gambusia affinis).

Don’t rely on ultraviolet and electric “bug zappers”—mosquitoes aren’t attracted to them. And there’s no evidence that ultrasonic mosquito repellers work, says the Federal Trade Commission, which has hauled at least one maker of these products to court for false advertising.

Potomac Horse Fever

Fever and severe diarrhea are the hallmarks of Potomac horse fever (PHF), known on the West Coast as “Shasta River crud” and to veterinarians as equine monocytic ehrlichiosis. Laminitis is a common and serious complication. Bacteria, Neorickettsia risticii, cause this disease, which can be fatal if not treated. PHF often occurs on farms located along rivers and freshwater streams.

Researchers at the University of California at Davis only recently discovered how N. risticii infects the horse. The bacteria’s route sounds a bit like the song about the old woman who swallowed a fly. It begins in water, where N. risticii infects microscopic parasites called cercariae. These parasites in turn infect snails and the larvae of mayflies and caddis flies, which live in fresh water. The larvae mature into adult flies and leave the water; when they die, they fall into the grass. Then grazing horses accidentally swallow the flies that hold the parasites that hold N. risticci, and the bacteria are in.

John Madigan, DVM, who headed the UC-Davis research, says that his team hasn’t yet determined the most effective ways of reducing risk. Still, some common-sense approaches are worth trying:

• Turn out lights to avoid attracting flying insects. Use electronic bug zappers to reduce their numbers.

• Drain ditches and other water traps to reduce numbers of insect larvae.

• Reducing the number of freshwater snails may also help, as snails are the main host of the parasites. Copper sulfate is sometimes used to control snails, but the chemical can be harmful to fish and other animals. Check local regulations before taking this route.

It’s probably not necessary to fence your horses away from streams and other natural water sources, Madigan says. N. risticci doesn’t seem to be transmitted directly from water or from snails.

Lyme Disease

Lyme disease has been called the “great pretender” for its variable symptoms, which range from vague lethargy and stiffness to weight loss, lameness and even laminitis. The disease is caused by bacteria (Borrelia burgdorferi, or Bb) and has turned up in most parts of the country. It’s most common in the East, Midwest and Pacific Northwest.

Blood-feeding ticks, mainly pinhead-sized deer ticks and western black-legged ticks, transmit Lyme disease. They pick up the bacteria by feeding on infected mice and pass it on when they bite horses—or people, dogs, deer or other animals. So, minimize your horses’ exposure to ticks:

• Use fly repellents that are effective against ticks, such as those that contain permethrins. They’ll discourage ticks from latching onto your horses.

• Give horses a thorough daily grooming, with special attention to often-overlooked areas like the base of the mane and tail. Ticks crawl around for a while before settling in for a meal, so daily grooming can help prevent bites.

• Make your property unattractive to ticks by mowing tall grass, which these pests love. Clear brush and trim overhanging branches—both prime tick hangouts.

While nothing is fool-proof, a few simple steps around your property and with your horses can greatly diminish the odds of these potentially deadly diseases and viruses hitting your barn.






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