While grazing, horses can ingest mites containing tapeworm larvae. |
If you had to live as a primitive animal, being a tapeworm wouldn't be so bad. Imagine a perpetual vacation at a Caribbean resort, where you have nothing to do but bask languidly in the sun, while an unending stream of wait staff carries to you all the food and drink you desire; you don't have to move a muscle as you dreamily contemplate the poor blokes struggling to meet their deadlines back at the office.
That's not far from a tapeworm's existence. Unlike most animals, who spend their lives evading predators while battling for food, mates and territory, tapeworms spend their adult lives in warm environments safe from predation, bathed in a continuous stream of nutrition.
OK, the scenery may be nicer on the beach than inside a horse's intestine, but tapeworms don't have eyes, anyway. The point is that tapeworms live a pretty stress-free life, and it's really in their best interest to avoid damaging their host-injuring the horse would mean jeopardizing their only source of survival. In fact, tapeworms are among the least dangerous internal parasites your horse can have. "There have been some studies that have been able to correlate the presence of tapeworms with certain disease signs but most horses probably don't suffer anything remarkable," says Craig Reinemeyer, DVM, PhD, a parasitologist with East Tennessee Clinical Research. "If you were leaning over the fence looking at a horse with tapeworms, you probably wouldn't notice anything at all wrong with him."
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When they do appear, the outward signs of tapeworms-frequent mild colics, unthriftiness, mild diarrhea-are easy to miss or to mistake for other conditions. But if a horse is consistently "off"-dull coated, not gaining weight as fast as he should, colicking frequently-and all other physical maladies have been ruled out, tapeworms may be the culprit.
The generally benign nature of tapeworm infections is good news because many horses have them. "Surveys here have shown that 50 to 60 percent of horses that died from various causes and were necropsied here also had tapeworm infections," says Eugene T. Lyons, PhD, a parasitologist with the University of Kentucky's Gluck Equine Research Center. That infection rate has been consistent over the past few decades. In separate surveys published in 1983 and 2000, Lyons and his colleagues found the tapeworm Anoplocephala perfoliata in 54 percent and 52 percent of the horses they examined in Kentucky. Infection rates vary from region to region, however. "We find tapeworms in as few as 5 percent to as many as 25 percent of the different populations of horses we study," says Reinemeyer. "But we have found individual farms where the rates can go to 60 percent or higher."
A Tapeworm's Life
Because tapeworms pose a relatively small threat to horses, they have received much less research attention than more dangerous worms, such as strongyles. "We know so much about other parasites and so little about this one because it usually doesn't cause serious problems," says Lyons. "It's also difficult to reproduce the tapeworm life cycle in study conditions."
Researchers do know how the tapeworm's life cycle works. An adult tapeworm consists of a head-that attaches to the intestinal wall with a set of suckers-and a segmented body; each segment contains within it a complete set of reproductive organs that can produce eggs independently. As the worm grows, the lower segments separate and their eggs are carried off in the passing stream of digesting food on their way out of the horse's body. Once on the ground, the manure is broken down with the help of oribatid mites; the mites ingest the eggs, which develop into larvae inside their bodies. If the larvae-carrying mites crawl up onto the grass and are eaten by a grazing horse, the tapeworm larvae will settle into a new host.
But many questions remain. "In a site where tapeworms are common, some in a herd will have them and some will not. No one knows why," says Reinemeyer. "Acquired immunity probably plays a big role, and like any type of immunity, some will develop it better than others." Age doesn't seem to matter; tapeworms have been found in horses young and old. "But we've never found any in a horse younger than nine months, so we don't even bother to look anymore," he adds. "We don't really know why."
What Harm Do Tapeworms Do?
No one is quite sure how much--if any--harm these tapeworms inflict on a horse's gut. A. perfoliata, which is by far the most common of the three tapeworm species known to infect equines in the United States, is too small to physically block a horse's intestine, even in relatively high numbers. Its adult size is only 5 to 8 cm long and 1.2 cm wide. And a worm that size isn't likely to "rob" your horse of enough nutrition to seriously affect his health, either. But they can do some damage. "They cause inflammation of the intestinal wall at the site of attachment," says Reinemeyer. "Can it be fatal? In rare cases, yes." A. perfoliata is most likely to attach to the horse's intestine near the ileocecal valve, the point where the small intestine empties into the cecum. "The supposition is that the worms favor that location because the material from the small intestine is very nutritionally rich with sugars and proteins that have already been broken down into forms that may be better absorbed," says Reinemeyer.
Concentrations of worms at that small opening-at about 5 cm in diameter, the ileocecal valve is one of the narrowest points of the horse's gastrointestinal tract-are associated with several health problems. A British study published in 1998 showed that horses with tapeworms were 22 percent more likely to experience spasmodic colic and 81 percent more likely to experience an impaction colic at the ileocecal valve. In rare cases, the inflammation can also cause ulcerations of the intestine, leading to peritonitis, an infection of the abdominal lining. Tapeworms are also believed to contribute to a thickening of the intestinal wall, as well as ileocecal intussusception, a condition where the end of the small intestine "telescopes" through the valve and into the cecum. "The gut basically crawls inside itself, pulling itself inside out, like when you pull off a stocking," says Reinemeyer. "When that happens, the inside layer is squeezed by the outside layer, and it can cause painful colics."
Although researchers have been able to connect the presence of tapeworms with increased prevalence of these conditions, they don't know exactly how the worms cause the problems, if indeed they do. After all, horses can develop these types of colics without worms, and many who carry tapeworms all their lives show none of these signs. "We see many older horses who have a high wormload and yet never developed any pathological changes," says Lyons. "Just because the worms are there doesn't mean they are causing problems. But people should be aware that they sometimes have been associated with serious detrimental effects, especially in younger horses."
While grazing, horses can ingest mites containing tapeworm larvae. |
A complication of tapeworm research is the fact that most of the data about tapeworms are derived from counting them in dead horses. In living horses, it's difficult to know whether tapeworms are present because their eggs are notoriously hard to find in standard fecal float tests, which analyze the number of parasite eggs that come to the surface when a manure sample is mixed with a dense salt solution. "Their eggs don't float very well," says Lyons. In addition, tapeworms release eggs only intermittently, so the fecal exam would have to be repeated every day for several days. All of which means that the presence of tapeworms can easily go undetected unless the horse is carrying a particularly heavy load.
"The number of worms in a single horse can reach the high hundreds-800 or 900," says Reinemeyer. "We commonly find as many as 150, but the average is probably less than 100. But no one has been able to prove an association between the number of tapeworms and the onset of disease."
Horses get tapeworms by ingesting oribatid mites that carry tapeworm larvae. Oribatids are a superfamily of mites that live in different ecosystems all over the Earth, including Antarctica, and they play a vital role in recycling organic wastes. "Their job is to help improve soil fertility by eating organic matter, excreting it, and mixing it up within the soil," says Merijo Jordan, DVM, a graduate student at the University of Florida who has studied the tapeworm life cycle. "They are little decomposers." About 7,000 species of oribatids are thought to live in the United States, says Jordan, but only 14 genera are known to act as intermediate carriers of A. perfoliata eggs.
Oribatid mites are present in every grassland in the country. "Usually, when we are doing a study we can find 30 to 50 species on a pasture in a temperate zone," says Jordan. In her studies in Florida, Jordan has counted a range of different species in the upper teens and lower 20s. "Generally, about half are suspect carriers." But the mites themselves pose no threat for horses. Oribatids are free-living animals, not parasites; they live on every pasture, whether or not the tapeworm eggs are present. Nevertheless, tapeworm infections are likely to be more prevalent under climate conditions that favor larger populations of the mites, so researchers are working to understand the living conditions oribatid mites like best.
"Temperature and humidity are thought to rule where the mites live," Jordan says. "In the dawn and dusk, they seem to like to move up onto the grass. In the heat of the day, they will be down in the top layer of soil, and when it's really hot or really cold, they'll go down deeper into the soil." But Jordan cautions that these behaviors are not absolutes, and pulling a horse off pasture at certain times of the day is not likely to have any effect on whether he will ingest the mites. As of yet, there is still no absolute way to predict what time of the day or what season of the year the mites are most likely to be active-and the most likely to cross paths with grazing horses.
"We're also still trying to figure out how humidity affects oribatid populations," says Jordan. "Most of the work seems to be in the temperate regions. The mites are probably not as prevalent in the arid climates, but no one has proven it." Reinemeyer suspects that horses in the West and Southwest, where larger ranges are more common, are less likely to graze over areas tapeworm eggs have been deposited. "In my studies, we've never seen tapeworms in horses from those regions," he says. "That's not scientific, but my general impression is that they are more likely to occur in horses from the Eastern pastures and on the West Coast."
Because the mites live in green grass, it seems likely that horses on pasture are more at risk of encountering tapeworms than are horses kept stabled and fed only hay and grain. "But some confined horses could still be at risk, especially if they are fed green chop recently cut from outside," says Reinemeyer. "There is also some evidence that round bales, because they sit outside, may still provide a reasonable habitat for mites, especially if the bales sit on the ground, where the hay may still harbor moisture."
The first equine dewormer formulated specifically to control tapeworms was recently approved by the Food and Drug Administration and should be available to the public by the end of the summer. Zimecterin Gold, manufactured by Merial, controls 61 species of equine parasites, including tapeworms. The new product combines two active ingredients, ivermectin, an anthelmintic agent common in many equine dewormers, and praziquantel, a drug used to control tapeworms in dogs and cats. According to Merial, Zimecterin Gold can be included in any deworming rotation program. "We recommend treating for tapeworms at least twice a year," says Duane Maye, DVM, product manager for Merial. "The fall and spring are good times to treat for tapeworms, But you could use it at any point in the year." Zimecterin Gold will be available through veterinarians, tack and equine supply catalogs.
Tapeworms are rarely a problem for horses, so they haven't received a lot of attention, but they may occasionally cause serious trouble. Tapeworms are sometimes the culprit in horses who mysteriously fail to thrive or develop frequent digestive problems, and it may be wise to consider taking preventive action against these worms, even if you're not sure whether they're present. "Unlike other parasites, which can really cause serious problems, we can't predict what tapeworms will cause," says Lyons. "But there's always a potential for trouble. We don't want people to overreact, but this is a problem we want people to be aware of."
This article first appeared in the April 2001 issue of EQUUS magazine.