- Jul 1, 2007
Horses can be host to three types of tapeworms, says Thomas Craig, DVM, MS, PhD, professor of Veterinary Parasitology at Texas A&M University, but the only one of consequence today in horses in the United States is Anoplocephala perfoliata.
"It grows to a length of 1.5 to three inches and is found near the junction between the small intestine and cecum," says Craig.
Tapeworms are resistant to many dewormers. They are not affected at all by ivermectin or moxidectin. Prior to 2003, there were no products labeled by the Food and Drug Administration (FDA) for tapeworm control, and most horse owners didn't worry about these worms. Today there are two compounds available for combating tapeworms, and many veterinarians feel horses should be treated once or twice a year.
Tapeworm infections are difficult to diagnose; you can't always tell if horses have tapeworms by fecal or other checks. Many horse owners add a product for tapeworms to their rotational deworming schedule just to be safe.
There might be a new diagnostic test available soon that can show whether or not a horse actually has these worms.
Products for Tapeworms
Craig Reinemeyer, DVM, PhD, formerly on the faculty at the University of Tennessee College of Veterinary Medicine and now president of East Tennessee Clinical Research, has been performing studies on equine internal parasites--and drugs to control them--for many years. Today horse owners have several options for control of tapeworms. These include combinations of moxidectin and praziquantel (the latter kills tapeworms), marketed as Quest Plus and ComboCare; combinations of ivermectin and praziquantel marketed as Zimecterin Gold or Equimax Paste; and pyrantel pamoate paste.
"All approved anthelmintics are more than 95% effective against tapeworms, and the FDA only requires 90% efficacy for a label claim," says Reinemeyer.
If you're trying to control tapeworms, occasionally alternate the products to prevent future resistance problems. The products we've been using for decades to control strongyles and ascarids are all experiencing resistance issues, and we don't want that to happen with tapeworms.
"Five years ago we had no resistance problems in ascarids, the most important worms in juvenile horses," says Reinemeyer. "Since then, some failures of ivermectin and moxidectin have been reported in the scientific literature, and I've observed them in the field. The diversity of new products for tapeworms offers a chance to get ahead of the game in preventing resistance. We should seriously consider rotating these products, just because we can.
"There hasn't been much research on equine tapeworms recently--now that we have effective products to control them," Reinemeyer says. "We still see clinical problems due to tapeworms, but there's very little new information about their clinical importance or practical control. This is a recurring pattern with parasites of livestock; once we have effective chemicals available to treat them, basic research slows or stops. The urgency is gone. In the long run this is a short-sighted way to look at things; none of the deworming products works forever. When they finally fail, then what?"
Strategic recommendations for controlling tapeworm infections would be useful for horse owners and practitioners, yet this hasn't been investigated, he says. "Right now veterinarians are just recommending seasonal treatments. The most common pattern would be deworming in spring and fall, but I can't really say how well that's working," says Reinemeyer.
There are few signs an owner can observe that might indicate a horse has tapeworms. Upon close inspection, you might on rare occasion find shed tapeworm segments on the skin or hair around the base of the tail or in the manure, says Craig, but these segments are much smaller than those seen in cats and dogs and rarely pass through the horse intact.
"If you could see it in the horse where it is still attached, you might recognize it as a tapeworm, but by the time the segments have gone through the fermentation vat of the large intestine, you won't find them in the manure," says Craig.
"The only way to tell if a horse has tapeworms is to find eggs in feces," he adds. "One problem in diagnosing them in the living horse is that unless there are lots of tapeworms present, we don't find the eggs."
Your veterinarian might have to take repeated manure samples to check, using a centrifuge and flotation method with a saturated sugar solution to separate eggs from feces (see www.TheHorse.com/ViewArticle.aspx?ID=5193). "If we suspect a horse may have tapeworms, this is the only test we run; the other common tests are unlikely to reveal presence of tapeworms," notes Craig. "I occasionally find some tapeworm eggs when running other worm tests on fecal samples, but if I am actually looking for tapeworms, I want to increase my chances by using the sugar flotation test.
"There are ways to detect specific antibodies in blood serum (which only tells you the horse has been exposed, and not whether it presently has tapeworms) or check for antigen in the feces, but not many vets have access to these methods," says Craig. "A vet in Great Britain developed a coproantigen test to check feces for tapeworms, which would give more evidence of present infection, but as far as I know, this is still just an experimental procedure."
Horse owners and veterinarians hope there will eventually be better tests, and there is promising research in this direction. The hard part in developing an accurate diagnostic test is difficulty in finding known negatives and known positives (horses that have definitive diagnoses--negative or positive--for tapeworm infection) to check the validity of the test.
"In the present research, fecal samples were taken from horses known to have tapeworms," says Reinemeyer. "Some of these we proved by finding tapeworm eggs repeatedly in their feces. Other specimens came from horses at slaughterhouses or horses necropsied for various reasons and tapeworms were seen in their intestinal tract. Samples from those horses were classified as known positives.
"It was harder to come up with known negative samples," he adds. "Since tapeworm eggs are so hard to find, absence of eggs proves nothing about a horse's infection status. A negative sample does not necessarily mean the horse does not have tapeworms. The only way to get a confirmed negative sample from a horse is post-mortem. You have to look within the intestinal tract of a horse."
Reinemeyer explains that a diagnostic test needs to evaluate sensitivity and specificity. Sensitivity is the ability of a test to identify an infected animal correctly. For example, if 10 horses in a pasture all had tapeworms and a test indicated five of them were positive, then the sensitivity of that test would be only 50%. Specificity is the ability of a test to identify an uninfected animal correctly. If you test 10 horses that were not infected with tapeworms and tests indicated that nine were negative and one was positive, the specificity of that diagnostic test would be 90% because it incorrectly identified one of the 10 as being infected.
"Any test for tapeworms with a high specificity would be very good since the consequences of incorrectly calling a horse infected are no big deal," says Reinemeyer. "You might spend $15 for a dewormer a horse might not need. Sensitivity is more important (for tapeworms), because that criterion would identify an infected animal, herd, farm, or premise, to know whether or not you actually have a tapeworm problem. Once we finally have good diagnostic tests, we would then be able to eradicate this parasite on a lot of farms. The products are available now to do this. The efficacy of praziquantel, for instance, is literally 100%."
Another potential benefit of an accurate diagnostic test is it could foster more applied research. "The frustration of trying to do any research now is that you really can't determine tapeworm infection status of every member of a herd," states Reinemeyer. "Consequently, we can't answer important questions such as: How soon do foals first get infected? Do horses stay infected all through the year? How long can pastures remain infective for new horses brought onto the farm? Right now we don't really know when to treat, or how often to treat, to eradicate the problem.
"If we can eradicate tapeworms on a farm, we may begin to recognize health problems in horses we don't presently associate with tapeworm infection," predicts Reinemeyer. "We do know they can cause colic and some severe gastrointestinal upsets, but those are extreme circumstances. We don't know what the average infection does to the average horse."
Horses with tapeworms might have diminished performance, increased susceptibility to certain disease conditions, or other problems we're not aware of.
One of the things we don't know is how tapeworm infections cycle through the calendar year. Most other parasites reproduce on a seasonal basis, so eggs are passed at a time of year when there is enough warmth for them to hatch, and forage plants for the hatching larvae to crawl onto, so grazing horses can ingest them.
"We don't know if this seasonal pattern occurs with tapeworms," says Reinemeyer. "The present diagnostic test relies on seeing eggs in manure, which means the worm is reproducing. If there is a period of several months when tapeworms stop producing eggs, our test is fruitless during that time."
A DNA test would be effective, since DNA is always leeching out of worms, whether they are reproducing adults or immature juveniles. Horse owners and veterinarians are hoping current research will result in a DNA test for tapeworms.
Without a positive test to say if, for certain, a horse has tapeworms or not, it's difficult to do anything except deworm your horses with a product that is effective against tapeworms. Consult with your veterinarian to determine if once-a-year or twice-a-year deworming with such a product is recommended.
PROBLEMS CAUSED BY TAPEWORMS
"During the past decade we found more tapeworm infections in horses that had been routinely wormed with ivermectin or moxidectin, which don't kill tapeworms," says Thomas Craig, DVM, PhD, a parasitologist at Texas A&M University. Some of the earlier drugs might have had more effect.
"There are reports of tapeworms associated with mild colics, and with ileocecal intussusception--in which the end of the small intestine prolapses into the large intestine," says Craig. "This telescoping type of prolapse is very serious, requiring immediate and heroic surgery to save the horse, and is almost exclusively associated with tapeworm infection. But until you open the horse up, you usually don't know what you are dealing with."
Tapeworms attach at the valve between small intestine and cecum and cause mild inflammation; if large numbers are clustered there, the intestinal lining becomes swollen and ulcerated. The swelling can interfere with passage of food through the intestine, and this can lead to colic due to partial blockage.
Degenerative changes can occur; the valve into the cecum might become thickened. Polyps occasionally develop, and, on occasion, rupture of the cecum can occur. Heavy infections interfere with gut motility and increase the risk for colic or for torsion of the cecum or colon.
In one study of colic cases, 22% of spasmodic colics examined and 81% of small intestine impactions (with blockage near the cecum) were possibly caused by tapeworms.
"The most common problem, however, is mild colic," says Craig. "Because of the constricted food flow, contents of the small intestine get backed up. Some of the bacteria normally found in the large intestine (for fermenting and digesting forage) will proliferate in the upper small intestine and produce gas, which can lead to colic. It's not severe or life-threatening; these horses are mainly just uncomfortable. They tend to stretch out a little, look at their sides, etc., but their clinical signs are all within normal range." --Heather Smith Thomas
About the Author
Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog, www.heathersmiththomas.blogspot.com, she writes a biweekly blog at http://insidestorey.blogspot.com that comes out on Tuesdays.
POLL: University Equine Hospitals