Will the Worms Win? (Part 1)

Common intestinal parasites and why they are problematic .

Intestinal parasites, or "worms," have been problematic since the domestication of horses. In this first section of a two-part series, we’ll review the identification, transmission, and problems associated with internal parasites.

Dawn of a New Era

It's important that we focus on equine parasites in today's horses, as scientists recently demonstrated that despite the availability of effective antiparasitic drugs, worm burdens of some parasite species are actually at the same level as they were before the introduction of these drugs several decades ago. Eugene Lyons, PhD, and colleagues from the Department of Veterinary Science at the University of Kentucky's Gluck Equine Research Center reported this finding in 2008. Similar reports have been noted by researchers in other parts of the world.

According to Lyons, "Most current medications are not nearly as effective at controlling some parasites--such as small strongyles and ascarids--as they were when first introduced to the industry."

Since no alternate drugs against small strongyles exist and no new anthelmintics (antiparasitic) drugs are forthcoming on the market, will the worms win?

"This is a real and serious dilemma for the equine industry," emphasizes Lyons.

Equine Intestinal Parasites

Horse owners are information-savvy. They read everything they can about horses to provide the best care possible. Off the top of their heads, horse owners can likely create an impressive list of common names of several equine internal parasites. But as worm-literate as they are, can the average horse owner list the remaining 100 or so internal parasite species that are known to infect horses?

Luckily, only five equine internal parasite species are particularly common, and most horses have these parasites. Horses can harbor impressive worm counts of some of these species, excrete millions of eggs per day, and are susceptible to life-threatening medical conditions that result from parasite infection. Thus, internal parasites are not to be taken lightly just because of their ubiquitous nature.

The Big 5

Strongyles "While three species of large strongyles exist--Strongylus vulgaris, S. equinus, and S. edentatus--it is really only S. vulgaris that we are referring to because the other two species are so rare," says Andrew Peregrine, BVMS, PhD, DVM (Hons) Glasgow, Dipl. EVPC, MRCVS, of the Department of Pathobiology at the Ontario Veterinary College in Canada.

Adult female S. vulgaris worms live in the intestinal tract and lay eggs that are passed in the feces. The larvae then hatch, climb a blade of grass, and lie in wait for a grazing horse. The larvae are swallowed, and they subsequently penetrate the walls of the intestines and migrate to various arteries, leaving destruction in their wake. The larvae migrate for up to six months, primarily to the root of the cranial mesenteric artery, which is the major artery supplying blood to the gastrointestinal system.

When the larvae are ready to become adult worms, they release their hold on the inner walls of blood vessels and flow with the blood until they reach the large intestine. They then migrate through the walls of the blood vessels and into the large intestine to mature into adult blood-sucking worms.

Due to the adult worms' vampire tendencies, horses infected with S. vulgaris can suffer from anemia, lack of energy, diarrhea, and colic.

In contrast, small strongyle larvae limit their migration into the wall of the large intestine rather than traipsing throughout the abdominal blood vessels. Small strongyles either become encysted and lie dormant in the walls of the intestines or, over the course of six to 10 weeks, they mature into adult worms. The adult worms also reside in the lumen of the large intestine alongside their larger, more destructive partners in crime, S. vulgaris.

Over 50 different species of small strongyles can infect horses and cause marked damage to the wall of the large intestine during their migration.

One fact worth noting is that since the introduction of several effective dewormers against S. vulgaris, small strongyles have become the more problematic and widespread of the two strongyle types.

Roundworms "The majority of foals become infected with roundworms (ascarids, Parascaris equorum) by ingesting roundworm eggs indoors rather than on pasture," says Peregrine. "This is because roundworm eggs can build up inside barns to high numbers. The eggs are sticky and long-lived."

Once foals ingest the eggs, they hatch in the intestinal tract and the larvae burrow through the wall of the intestine to the liver and lungs, where they further develop before moving up the trachea to the pharynx (i.e., the back of the oral cavity). At that stage they are swallowed a second time. This second voyage into the intestinal tract results in the maturation of the worm over the course of the next two to three months. The adult female worms produce eggs that are shed in the feces (one female can lay up to 200,000 per day), and they become infective within approximately two weeks after excretion. They can remain infective for several years.

Foals and other young horses with impressive worm burdens (i.e., hundreds) often have a pot-bellied appearance, dull hair coats, and summer colds.

Tapeworms "Adult bisexual worms residing in the large intestine of infected horses pass (in the feces) eggs and egg-filled segments of their bodies called proglottids, which literally are a 'bag of eggs,' " relays Lyons.

Once excreted, the proglottids burst open, spewing eggs into the environment. Oribatid (soil) mites ingest the eggs. Grazing horses then ingest the miniscule tapeworm- bearing mites. The tapeworm eggs mature into adult worms in the gastrointestinal system.

The most common species is the cecal tapeworm, which lives in the cecum near the ileocecocolic junction--where the small and large intestines connect.

Signs of tapeworm infections can include recurrent bouts of colic, weight loss, or simply the catchall "ill-thrift."

Pinworms These worms (Oxyuris equi) are more of an annoyance than anything else, but infections can be problematic due to the frequency and severity of the anal itching they cause. Horses become infected by ingesting eggs containing infective larvae. Once ingested, the infective larvae are released from the eggs and mature into adult worms in the intestinal tract within three to five months. The adult female worms crawl from their home in the cecum, large intestine, or rectum to the anus, where they deposit their eggs on the skin around the anus (i.e., perineum).

"These eggs can be passed onto virtually any surface in the barn, where the larvated (infective) eggs can reportedly lie dormant for several months before being ingested by a horse," explains Lyons.

The severe itchiness associated with pinworms is due not only to the physical migration of the worm from the intestine to the outside of the anus and home again, but also to the physical presence of the eggs in an "egg-cement" that dries, cracks, and irritates the skin. The combination of these two "itch factors" drives the horse to rub its tail on anything and everything.

Bots These are the immature form of adult botflies that reside in horses' stomachs.

"Each immature bot is approximately 1 to 1.5 cm long and 0.5 cm wide, and I would estimate the average infected horse has between 10 and 40 bots in their stomachs," suggests Peregrine.

Adult female botflies, such as Gasterophilus intestinalis, lay eggs on the hairs of the forelegs and mane. The average prolific adult female bot can lay approximately 1,000 eggs over the course of a few days. The eggs hatch where they were laid and the larvae, referred to as instars, access the oral cavity when the horse chews or licks the legs. The instars then burrow into the tongue, lips, or gums for about three weeks. The horse subsequently swallows the bot larvae and they develop into third-stage larvae, which attach firmly to the lining of the stomach. After eight to 10 months, these larvae are passed in the feces, burrow into the ground, and mature into adult flies.

Who Gets Infected?

Extensive research on parasitic life cycles and risk factors associated with infection has made remarkable contributions to our current understanding of equine internal parasites. For example, we know roundworms are common only in young horses (i.e., sucklings, weanlings, early yearlings). Adult pinworms usually only infect young horses, and small strongyles and tapeworms are present in all ages of horses.

According to Tracy Norman, VMD, MA, Dipl. ACVIM, a clinical assistant professor in equine internal medicine at Texas A&M University, "Immunological differences between younger horses and adult horses are believed to exist, which explains why weanlings are commonly infected with roundworms, whereas broodmares are not."

Norman explains there are other factors that can impact this "natural resistance" to internal parasites, such as:

  • Pregnancy or nursing;
  • Advanced age;
  • Strenuous training;
  • Shipping/trailering long distances; and
  • Chronic medical conditions such as equine Cushing's syndrome.

"In addition," Lyons adds, "Drug resistance also plays a role. Roundworms are still commonly seen because ivermectin and moxidectin are now inactive against them. It may also be possible that pinworms are also more common now for the same reason. More research is necessary to delineate the current status of these parasites."

Major Associated Problems

Internal parasite infections can cause an array of detrimental and potentially life-threatening health conditions in infected horses.

Most notable are the migrating S. vulgaris larvae with a predilection for causing blood clots in the branches of the mesenteric artery. When these clots break off, they go with the blood flow into very small blood vessels, which they ultimately block.

"This results in segmental regions of infarction, which means that the section of intestine that used to be supplied by the blocked artery is no longer receiving any blood and the tissue dies," says Peregrine.

Alternatively, an aneurysm (defined as a region of the artery that bulges due to a damaged wall) can form where a larval S. vulgaris is attached to a blood vessel wall. Aneurysms can decrease the flow of blood to the intestines or even rupture, resulting in death due to massive internal bleeding. However, this is incredibly unusual.

"Individual small strongyles cause less damage than a single large strongyle larva, but as a population, small strongyles can be very destructive."--Dr. Tracy Norman

Norman adds to Peregrine's assessment with this caveat: "I agree that individual small strongyles cause less damage than a single large strongyle larva, but as a population, small strongyles can be very destructive. This is particularly true when a large population of dormant larvae emerges from the walls of the large intestine--death from a severe protein-losing colitis can and does occur."

Rare sequelae (aftereffects) associated with large roundworm infections in young horses include pneumonia secondary to larval migration, colic (from blockage of the small intestines with worms, which stop the passage of ingesta), diarrhea, and, rarely, perforation (rupture) of the small intestine resulting in a fatal peritonitis.

Pinworm and bot infections aren't linked to major medical or surgical concerns.

While tapeworms historically have been considered nonpathogenic (not disease-causing), this opinion is changing. "Tapeworms are now considered important in some colic cases, including ileal impaction, cecal impaction, and ileocecal (involving the ileum of the small intestine and cecum), cecocecal (cecum itself), and cecocolic (cecum and colon) intussusceptions, where sections of the intestine telescope upon itself to block the flow of ingesta," says Norman. "Thankfully, this is uncommon." Rarely, perforations of the small intestine can occur.

One final concern associated with internal parasites is zoonotic potential--certain infections can be transferred from horses to humans. "Horse bots have, on rare occasions, been found migrating in tissues of humans," says Lyons.

Luckily, unlike the parasites that infect dogs, most equine internal parasites do not pose any threat to humans.

Part Two of "Will the Worms Win?" addresses specific diagnostic tests, treatment methods, and management strategies, and it discusses issues surrounding the potential development of resistance to anthelmintics and the generation of the "super worm." Find Part Two here.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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