Insects and wildlife that affect horses are expected to burgeon from warm winter and early spring

by Robin Stanback

A WARMER than usual winter and an early spring throughout the United States mean insects and wildlife will have a jump-start in making the summer of 2006 one during which horsemen will have to be even more vigilant. Mild winter weather may have a gentle impact on horses and their caretakers, but experts predict that a larger than normal crop of insects and other wild creatures will wreak havoc. Making efforts now to reduce wildlife and insect pests from your horses' environment will help make the summer months safer and healthier.

Tim Gibb, Ph.D., an entomologist at Purdue University's Cooperative Extension Service, warned horsemen of an early emergence of insects linked to equine disease.

"Insects develop in direct relation to the amount of heat they get over a year's time," Gibb said. "Eastern tent caterpillars, mosquitoes, and horseflies certainly can come out earlier in years like this one."

He cautioned, "It might be a bit early to predict this year. However, winter temperatures do not seem to have as strong an effect on the population as spring weather. Insects just seem to be able to figure out a way to get through terribly cold weather, which is why they continue to exist. It is really the temperature and moisture fluctuations in the spring that often determine the onset of insect emergence.

"Wet weather is much more detrimental to them. If you have a mild, wet spring, fungal diseases can get into their population and seriously affect the number and health of the insects that hatch."

According to Gibb, an early spring can mean more generations of such insects as flies and mosquitoes, while others such as Japanese beetles and Eastern tent caterpillars are genetically limited to their season rather than temperature or conditions.

"If our weather trends continue as they have this year," Gibb concluded, "I predict a very early onset for Eastern tent caterpillars. They can emerge even before the leaves come out on the trees. You would think they would die out without their food source, but they are amazingly resilient."

Eastern tent caterpillars

The earliest insect-related problems could come from Eastern tent caterpillars, which Gibb said might begin to hatch as early as March. Hatching occurs concurrently with forsythia blooms, so watch for the festive yellow flowers.

The six-legged insects are believed to have been responsible for thousands of abortions, foal deaths, and incidences of pericarditis, uveitis, and encephalitis during the 2001 outbreak of mare reproductive loss syndrome (MRLS). After an exhaustive study by one of the largest collaborations of equine specialists ever brought to focus on one issue, those experts determined that the outbreak was caused when grazing horses ingested the caterpillars.

In 2001 and '02, large numbers of Eastern tent caterpillars were found in a wide area that encompassed parts of Indiana, Kentucky, Ohio, and West Virginia.

According to Bruce Webb, Ph.D., associate professor of entomology at the University of Kentucky, a natural factor in controlling the population of Eastern tent caterpillars from year to year is a baculovirus that infects the insects. This virus may be the primary reason for a decline in Eastern tent caterpillars in Central Kentucky over the past two years. Researchers at the University of Kentucky are studying the possibility of using a laboratory-grown version of baculovirus to artificially infect caterpillar nests each season to suppress their population. Because no vaccine to prevent MRLS exists, eliminating Eastern tent caterpillars appears to be one of the few options for horse owners.

Grubs are here already

Roger Allman, vice president and co-owner of the Farm Clinic, an independent research firm in West Lafayette, Indiana, with a branch office in Central Kentucky, was one of the agronomist consultants summoned to examine pastures during the MRLS crisis. This year, one of his concerns is grubs, the larvae of scarab beetles, including May beetles and Japanese beetles.

"I have noticed a slightly above-average number of white grubs in Central Kentucky pastures," Allman said. "While these creatures in and of themselves don't seem to be dangerous to the horses, they do affect the pastures the horses graze."

Grubs gnaw away at the roots of pasture grass, leaving large, irregular bare spots.

"Since the only insecticides that are successful in eliminating these creatures are not pasture-safe, the best we can do is to identify those areas and seed them with bluegrass to keep weeds from coming up in the dead patches of grass," Allman said. "It might sound a bit funny that we are, in essence, feeding the grubs, but if we don't, something less desirable than grass will grow in its place."

Mosquitoes and encephalomyelitis

Mosquitoes spread two of the most deadly equine neurologic diseases, West Nile virus and Eastern equine encephalomyelitis. They feed upon infected birds and then pass the virus to other birds, animals, and humans.

The United States Department of Agriculture reported 1,075 confirmed cases of West Nile virus in horses from 37 states in 2005. Affected horses show signs of ataxia (lack of coordination), depression, weakness of limbs, partial paralysis, and muscle twitching.

No cure exists for West Nile virus, so horses are treated with supportive care, including administration of anti-inflammatory drugs, while their bodies fight the disease. Yet, about one-third of horses that show clinical signs of West Nile virus die.

Two fully approved West Nile virus vaccines are available for horses. Each requires an initial series of at least two vaccinations, followed by periodic boosters.

Eastern equine encephalomyelitis killed 150 horses in Florida alone in 2005. The disease, which causes neurological symptoms similar to those of West Nile virus, is fatal in 90% of cases. A highly effective vaccine is commercially available.

Fairfield T. Bain, D.V.M., an internal medicine specialist at Hagyard Equine Medical Institute in Lexington, recommends that horsemen review their vaccination schedules and begin vaccinating early this year.

"The mosquito is the important issue," Bain said. "Vaccination is the first, best defense, but eliminating pooling and stagnant water where mosquitoes reproduce is the next."

Wildlife dangers

Opossums, raccoons, skunks, and coyotes have shown an affinity for the warmer weather, too. Anthony Wilcox, head trapper of Central Kentucky's Wilderness Company, has been capturing wildlife for Thoroughbred farmers for more than 15 years.

"The very mild weather in January brought an earlier start to the wild-animal breeding season," Wilcox said. "If the end of March continues with the warming trend, we won't be seeing den-site deaths usually attributed to colder temperatures. So far this year, I have captured over 30% more coyotes than last year. We had an unusually large crop of raccoons, opossums, and skunks last year. Mild weather will only increase that number this spring."

At least 50% of all horses in the United States have been exposed to Sarcocystis neurona, the causative agent for equine protozoal myeloencephalitis (EPM). The opossum is the definitive host of S. neurona. After ingesting the protozoa, the opossum sheds sporocysts, which are the infective stage for horses, in its urine and feces. The horse can consume the sporocysts as it eats grass, hay, or grains that have come in contact with opossum feces. Once ingested, the sporocysts migrate to the central nervous system where they cause damage.

Signs of infection easily can be confused with other problems, such as lameness or back soreness. Other manifestations are airway problems, including laryngeal hemiplegia (paralyzed flappers) and dorsal displacement of the soft palate, which result from protozoa infecting the nerves in the throat. Most commonly, affected horses show ataxia, seizures, weight loss, blindness, loss of balance, head shaking, and weakness.

Treatments for EPM vary according to the severity of the symptoms. Aggressive treatments with anti-protozoal drugs to suppress the parasite, anti-inflammatories to limit inflammation and swelling, and vitamin E are recommended. Marquis, a ponazuril oral paste manufactured by Bayer, is the only approved treatment.

As Wilcox anticipates a bumper crop of opossums, raccoons, and skunks this year, he strongly urges farm managers to examine their feeding practices.

"Avoid leaving out any excess feed that might attract wildlife," Wilcox said. "If you notice feces from one of these creatures in or around any area where you store feeds or hay, disinfect the area thoroughly and try to block the animals' access to the area."

Leptospirosis

Raccoons, skunks, opossums, rats, deer, and foxes carry a highly invasive, spiral-shaped bacteria that causes leptospirosis. The bacteria are shed in bodily secretions from animals that have had the disease but recovered. The organism enters the horse's body through a cut in the skin or by direct contact with the mucous membranes of the eyes, nose, and mouth.

As wild creatures walk through or drink from infected sources of water, they can carry the leptospires to healthy animals and humans. In equines, the disease has caused abortions, moon blindness, fever, jaundice, depression, and a lack of appetite.

Carol Maddox, Ph.D., an associate professor at the College of Veterinary Medicine at the University of Illinois, has been studying a leptospirosis "abortion storm" that occurred last year in Illinois.

"There are vaccines to protect cattle and swine from abortions caused by the leptospires, but, to date, no vaccine exists to protect horses," Maddox said. "Some horsemen have used the cattle vaccine in an effort to protect their mares, though most veterinarians do not recommend this practice, as the vaccine has not been sufficiently tested in horses to be considered safe.

"We are working toward that goal, but until we reach it, horsemen should carefully monitor their horses' water sources and do everything possible to limit wildlife from contact with horses or their surroundings."

Coyotes

"The coyotes have been out much sooner this year," Wilcox said, "and if the weather holds as it has been, we are unlikely to see the usual number of den deaths we associate with bad weather. This will mean even more coyotes this summer."

While the animals pose a minimal threat to horses as carriers of disease, they do cause problems for young horses.

"You see the most field accidents in the yearling population," Wilcox said. "Older broodmares don't seem to be spooked by a few coyotes running across their pastures and paddocks, but young horses do. The coyote doesn't chase the horses, but just darting across the field causes alarm in the colts and fillies and can make them run."

Wilcox recommended replacing plank fencing with wire mesh fencing whenever possible, as it makes access more difficult for the coyotes.

"A young horse will bounce off a wire fence," he said, "but they can plow through a plank fence and cause a great deal of damage to themselves and leave a hole in the fence that allows others the opportunity to escape."

To discourage coyotes and other wildlife from encroaching into paddocks and barns, Wilcox suggests:

•Clean and disinfect fence and ground feeders regularly;

•Keep all feeds in tightly closed containers;

•Remove excess feed and dispose of it in a closed container;

•Do not leave feed for barn cats and dogs where wildlife can get it;

•Store hay in enclosed buildings; and

•If any type of feces is found on hay, discard that bale and surrounding bales to assure that none of it gets fed to livestock.

Robin Stanback is a freelance writer based in Versailles, Kentucky.