Tapeworms: An Underrated Threat

EDITOR'S NOTE: This is part two of a 12-part series on internal parasites of horses.

Of all the types of internal parasites that plague our horses, the one that was long considered fairly inconsequential was the tapeworm--an intestinal innocent bystander, if you will. Parasitologists knew that horses, like most mammals, harbored a few species of these cestodes (flat, segmented worms; in contrast to most equine worms, which are nematodes, or roundworms). However, parasitologists felt tapeworms were uncommon and did little damage in horses. As a result, there were few chemical means of addressing tapeworm infestations; the vast majority of the drugs available, including ivermectin and moxidectin, were developed to deal with the nematodes that were considered the real threat to your horse's health.


Tapeworms appear to be more of a problem than we once realized.

Recent research, however, has given us startling news: Not only are tapeworms far more prevalent in the equine gut than was previously suspected, but they might well be responsible for some serious health issues in horses.

Here's an update on what we now know about the secret lives of tapeworms.

Tapeworms at Work and Play

Tapeworms are so named because the body of an adult specimen typically is flattened and segmented at regular intervals, like a measuring tape. Each body segment (proglottid) is a separate unit, like the box cars that form a train. Single proglottids, or several linked ones, can break off from the worm's body without killing the worm, which remains attached to the horse's intestinal wall. Because the segments of most equine tapeworms disintegrate within the large intestine, it's rare to see intact proglottids passed in the feces of horses. That fact gave us the illusion for many years that tapeworm infections in horses were uncommon.

The tapeworm's mouth parts are contained in a scolex, which has four suckers enabling it to attach quite fiercely to the horse's gut wall. Below these suckers are lappets‚ tiny ear-shaped flaps. As we'll see, tapeworms can cause severe inflammation at their attachment sites. They have no digestive system to speak of, absorbing the nutrients they need into each proglottid through their integuments (skins).

All tapeworms are hermaphroditic, meaning that there are no separate males or females; each individual worm contains the reproductive organs of both sexes. Tapeworms produce eggs, but they usually don't deposit them singly or continuously like most common internal parasites of horses. Rather, tapeworm eggs come in packets that break away from the end of the worm when they've matured, then are carried out of the horse with the manure.

All tapeworms have indirect life cycles, meaning that they must develop initially within a different animal before they can be transmitted to their final host. The first animal infected in this cycle is known as an intermediate host. Apparently, some critical, biological change occurs in the intermediate host that renders the parasite capable of infecting its final host. Familiar examples of intermediate hosts and their respective parasites include mosquitoes for heartworms and fleas or rodents for the tapeworms in your barn cat.

The intermediate hosts for equine tapeworms are oribatid mites, which exist as free-living forms on pasture and often can be found in very high numbers. Oribatids are more common on permanent pastures than on cultivated or newly sown grasslands. The mites apparently swallow tapeworm eggs while feeding on organic material in equine feces. The eggs then hatch and develop to an infective stage (cysticercoid) within the body cavity of the oribatid in about two to four months. Horses get tapeworms by swallowing infective mites while grazing, and the worms mature within the horse in six to 10 weeks.

Tapeworm Species

Three species of tapeworms are known to infect horses in North America, and each has staked out its own little area of the equine intestine. Anoplocephala perfoliata, the most common tapeworm of U.S. horses, is commonly found at the junction of the small and large intestine (the ileocecal valve). A. perfoliata are usually just over one inch in length and can accumulate in large numbers.

The smallest of the tapeworms, Paranoplocephala mamillana, is usually in the range of a half-inch long, and it inhabits the small intestine and stomach.

The monster of the trio is Anoplocephala magna, which is also found in the small intestine or stomach of infected horses. This impressive (some might say horrific) tapeworm can range anywhere from four to 30 inches in length.

Tapeworm infections were first described in North American horses during the late 1800s. Curiously, A. magna was the most common tapeworm in those early descriptions, but this species is uncommon today. In contrast, A. perfoliata was a rare finding a century ago, but today it is the most prevalent species worldwide.

Population Patterns of Infection

Historically, it has been difficult to study the population patterns of tapeworm infections because no highly sensitive diagnostic methods were available. Fecal examination, which is excellent for detecting the presence of ascarids and strongyles, frequently doesn't reveal the presence of tapeworm segments or eggs. Parasitologists speculate that the disintegration of tapeworm segments within the bowel results in uneven distribution of eggs within feces. It's also suspected that tapeworms might not release ripe segments on a regular basis. So it is difficult for a veterinarian to prove that a given horse is infected with tapeworms.

The one definitive way to demonstrate tapeworm infection is direct examination of the gut, either post-mortem or during surgery. Numerous post mortem surveys have been conducted in various countries, and several have reported that 50-60% of horses examined were infected with tapeworms. The most thorough investigations in the United States were done in Kentucky, where three surveys found the prevalence of tapeworm infection in adult horses to be 53% (1983), 54% (1984), and 64% (1992).

In 1995, researchers in the United Kingdom developed a test that could detect antibodies to A. perfoliata in the blood of horses. A positive result indicated exposure to tapeworms, but not necessarily a current infection with adult worms. This technique was adapted by Stephen Kania, PhD, of the University of Tennessee's College of Veterinary Medicine, and it was used in a nationwide survey in 2002 and 2003. Veterinary diagnostic labs in 19 states were asked to submit excess serum from blood samples submitted for Coggins (equine infectious anemia) testing from horses which were at least one year of age. In addition to age, the labs also provided information about the donor horse's sex, breed, and state of residence. Serum samples were frozen and shipped to a lab in Tennessee, where they were examined for antibodies to tapeworm infection.

The survey found that more than 54% of all horses sampled had antibodies to A. perfoliata infestations. Prevalence estimates ranged from below 2% in California to higher than 98% in Minnesota; in all, the prevalence of tapeworm antibodies was greater than 30% in 15 out of 19 states--a startling finding.

Analysis of the results found that horses older than 15 years were significantly more likely to have antibodies than younger horses. (Researchers already knew that foals rarely harbor significant numbers of tapeworms before weaning, but once forage becomes a significant part of the diet, exposure to tapeworm infection becomes a lifelong possibility.) The study also demonstrated that stallions were less likely to have been exposed than geldings or mares. Both of these patterns might be explained by relative amounts of pasture exposure: A lot of older horses are turned out permanently, rather than kept in stalls, and stallions are more likely to be confined indoors.

Also, according to the analysis, mustangs, Arabians, and Quarter Horses were significantly less likely to have antibodies to tapeworms. The pattern in mustangs was attributed to their arid residence (Nevada), but no explanations were found for the differences in Arabians and Quarter Horses.

Tapeworms Cause Problems

We know very little about the horse's reaction to tapeworms. Several authors have expressed the opinion that horses do not develop acquired immunity to these parasites, based on the observation that horses of all ages appear to be infected, with some of the most severe infestations found in older horses. Here's some of what we do know:

  • High tapeworm burdens are more likely to be associated with serious clinical manifestations, such as spasmodic colic or severe colic due to an ileal impaction or ileocecal intussusception.
  • The pathologic findings associated with tapeworms include severe local inflammation of the ileocecal valve (or other attachment sites), ulceration of the mucosa, increased fluid retention and thickening of the gut wall, and local scarring. Sometimes the lesions associated with the attachment sites can partially obstruct the horse's bowel.
  • The typical tapeworm infection is not associated with any subtle or chronic clinical signs that would be obvious to the owner or to an examining veterinarian. Most of the clinical events, then, are acute and severe.
  • Tapeworm infection is an important cause of various types of colic.

For years, tapeworm infection has been associated with ileocecal intussusceptions in young horses. In this condition, the last foot of the small intestine (ileum) telescopes into the first section of the large intestine (cecum), and swells, blocking passage of intestinal contents. The condition is a result of tapeworms setting up shop at the ileocecal junction, where their pointy mouth parts (which they use to adhere to the gut wall) cause intense inflammation and irritation in a small area of the gut. That stimulates exaggerated peristalsis (the waves of muscle contraction that push food through the gut), resulting in the small intestine itself getting pushed through the opening.

Tapeworms are now considered the primary cause of ileocecal intussusceptions, which can only be corrected surgically (even then, the prognosis is guarded).

Tapeworms also contribute to ileal impaction colic, in which the wall of the ileum becomes thickened and inelastic, slowing or blocking the passage of food materials. Like intussusceptions, these can only be addressed surgically. In severe cases, they can even trigger ruptures of the small intestine. Chris Proudman, MA, VetMB, PhD, CertEO, FRCVS, of the University of Liverpool, estimated in a 1998 study that some 81% of ileocecal impaction colics were tapeworm-related, and he demonstrated that horses with tapeworms were 28 times more likely to experience this condition than uninfected horses.

Tapeworms also have been shown to cause spasmodic (gas) colic. The presence of tapeworms made horses eight times more likely to experience this than uninfected animals. Spasmodic colic is one of the most common types and can be treated medically.

Although we don't yet fully understand the relationship between tapeworms and some types of colic, researchers speculate that the parasites somehow interfere with normal nerve transmission in the gut. A. perfoliata has been shown to contain large quantities of a chemical called acetylcholinesterase, which could interfere with normal peristalsis in the gastrointestinal tract. Any disruption of normal gut motility could certainly be a plausible explanation for signs of colic.

Tackling the Tapeworm

Until recently, no equine anthelmintics were approved for use against tapeworms. In the absence of labeled products, many practitioners recommended doubled or tripled dosages (13.2 or 19.8 mg/kg) of pyrantel pamoate paste or suspension because these regimens were reportedly effective against A. perfoliata. Limited studies also have indicated that pyrantel tartrate (2.64 mg/kg) fed daily for 30 days is effective against tapeworms in horses. But at this writing, none of the pyrantel formulations are approved in the United States for use against tapeworm infections.

Gene Lyons, PhD, at the University of Kentucky's Gluck Equine Research Center in Lexington, Ky., demonstrated several years ago that praziquantel was highly effective against A. perfoliata infections at all dosages greater than 1 mg/kg. Praziquantel was already in common usage to treat tapeworms in cats and dogs, but its use in horses had not been explored. When praziquantel subsequently came off patent, several veterinary pharmaceutical companies simultaneously began to develop it for equine applications. These efforts culminated in 2003 with the approval of three new deworming products that offer a complete anti-parasitic spectrum, including tapeworms, bots, and most types of nematodes.

All of these products are combinations of a macrocyclic lactone anthelmintic (ivermectin or moxidectin) plus praziquantel (PRZ) at various dosages (see "What Works Against Tapeworms?" above right). The concentrations of praziquantel vary among the three competing products, but this is strictly a formulation issue and it has absolutely no impact on efficacy or safety as long as an appropriate amount of drug is delivered to a horse for his respective body weight. The label dosages of praziquantel also vary among the combinations. Again, any dosage of 1 mg/kg or greater is virtually 100% effective, so there should be no differences among these products in their efficacy against tapeworms. Finally, praziquantel is an extremely safe compound, so toxicity is unlikely within the range of dosages currently indicated by the labels of available products.

An Ounce of Prevention

The arrival of deworming products approved for, and effective against, tapeworms just as we begin to understand the true impact of these parasites is one of veterinary science's little serendipities. But the best time to use a product with praziquantel is still something of a question.

A practical approach is to see your objective not as eradication of tapeworms, but as prevention of the accumulation of large numbers of worms that are associated with severe conditions. So how do we do that?

Since horses infect themselves with tapeworms while grazing, one of the simplest methods of preventing tapeworm infection would be to deny them access to pasture. However, for most of us, this is an impractical control recommendation.

At present, we really don't know enough about tapeworm biology to make definitive recommendations for various climatic regions. The most critical piece of missing information is the length of time that infective oribatid mites can survive in the environment. If tapeworms are like most other equine parasites, they probably persist much better in cool or freezing conditions than in hot, dry weather. Logical, evidence-based control recommendations will only be possible after the regional survival patterns of tapeworms and oribatid mites are known.

In the meantime, let's theorize that oribatid mites are likely active only during the warmer months of the year. There are two schools of thought on the best time to treat horses against tapeworms--late winter/
spring or autumn/early winter.

Deworming horses in late winter or early spring, before the start of the annual grazing cycle, could help eliminate egg passage as a potential source of infection for mites. A subsequent treatment two to four months later would go a long way toward reducing the numbers of tapeworms that could accumulate in grazing horses.

On the other hand, treating in late autumn or early winter can get horses "cleaned out" soon after transmission ends from the mites. By waiting until spring to deworm horses, tapeworms might have more time to cause damage and seed pastures with eggs. Also, with the combo products available, a late autumn treatment serves dual purpose of bot and tapeworm cleanout.

One bit of good news is that because praziquantel is a completely new class of anthelmintic to horses, there are currently no problems with drug resistance. Coupled with its extremely high margin of safety and almost 100% efficacy, it's a treatment that is bound to be embraced by North American horse owners as part of a comprehensive deworming program.

We'll be looking at recommendations for that deworming program, and other important parasite issues, over the next 10 months in our continuing 2004 series on parasites.


ComboCare (Farnam) Gel Moxidectin (0.4 mg/kg) 2.5 mg/kg No 6 months
Equimax (Pfizer) Paste Ivermectin (0.2 mg/kg) 1.5 mg/kg No 1 month
Quest Plus (Fort Dodge) Gel Moxidectin (0.4 mg/kg) 2.5 mg/kg No 6 months
Zimecterin Gold (Merial) Paste Ivermectin (0.2 mg/kg) 1.0 mg/kg No 2 months

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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