Common Equine Parasites Involved in Colic

Jeff Dulany
Jeremy Hubert, BVSc, MS, DACVS

Parasites have many adverse effects on horses including respiratory, skin and especially the digestive tract. The presence of parasites in the gut may cause by irritation to the surfaces of the gut; however, the migrating larvae of the parasite often pose a greater threat than does the adult parasite. There are a multitude of parasites that can adversely affect the horses digestive system, three major parasites are noted here. They are large strongyles (Strongylus vulgaris), roundworms (Parascaris equorum), and tapeworms (Anoplocephala perfoliata).
The large strongyle is a small parasite that, as an adult, lives in the large intestine and cecum of the horse. The adult lays eggs in the large intestine that are passed into the feces and on to the pasture. The egg will mature, depending on the weather, after a period of time. Larvae hatch and are ingested as the horse grazes. The larvae will migrate into the wall of the gut, and make their way to the blood vessels that surround the intestine. This is where the problems may start for the horse. The larvae must enter into the blood stream to mature before entering the intestine as adults to start laying eggs. A favorite place for these migrating larvae to lodge is in the arteries supplying the intestine with freshly oxygenated blood. This artery is called the cranial mesenteric artery. The larvae migrate into the arterial wall, encyst and causes the body to mount an immune response to their presence. This response results in inflammation of the arteries, and can cause blood clot formation. These clots can lodge in the small blood vessels that supply the large intestine with oxygen. This decreases the blood flow to the intestine, which results in damage or death to the intestinal tissues. This is where strongyle prarasites may inflict the most damage.
The prevention of strongylus infections resulting in colic is relatively simple as recent development of new dewormers like Ivermectin are available. These more modern dewormers kill the migrating larval stages before they cause problems. An aggressive deworming program that can be molded to your personal needs can be made with the help of your veterinarian.
The roundworm is another important parasite of young horses. The large adult worms reside in the small intestine of the horse. The adult lays eggs that pass in the feces onto the ground. The eggs are ingested by the animal, and hatch in the small intestine. The larvae bury themselves in the wall of the intestine and make their way to the blood vessels. Once in the blood stream, they go to the liver and lungs. The larvae cross the blood vessels in the lungs to the airway passages, are coughed up. Upon arriving in the small intestine, the cycle starts over by larvae maturing and laying eggs.
Most bouts of colic from roundworms worms happen in foals twelve weeks to one year of age. These young horses may not have built up an adequate immunity to keep infections under control. Colic usually occurs after administering a deworming medication that results in an obstruction of dead worms in the lumen of the intestine. When a heavy worm burden is suspected, such as a weanling that has never been dewormed, you must be careful in the method by which you deworm the animal. Some veterinarians prefer administering one-half the dose of dewormer in order to only kill part of the worm infestation. The dead worms are removed via the feces, and a week later the other half of the of dewormer is given to kill the rest of the worm burden. This is much safer than giving the full dose at one time and potentially risking a small intestinal impaction of roundworms.
Prevention and control of roundworm infestation can be achieved using a strategic deworming program, and controlling the parasite burdens in the pasture. Young horses should be de-wormed at two month intervals starting at six weeks of age until they are one year old. It is accepted practice to alternate between different medications. Good practice also includes keeping clean water sources, not overgrazing pastures, removing manure from stalls and not spreading the manure on the pastures. The diagnosis of a roundworm infection is made by identifying the adult worms or eggs in the feces.
The tapeworm is another parasite that can cause colic in horses. The tapeworm passes eggs while residing in the cecum or between the small intestine and the cecum. The eggs are in small packet-like structure called a proglottid. The proglottid is passed on the pasture and eaten by a mite. The eggs develop into the infective state while in the mite. The mite is eaten by the grazing horse, which initiates the cycle all over again.
Damage to the cecum is incurred by the adult worm attaching to the inner surface. This can cause erosions, ulcers, and inflammation that leads to severe pain. The presence of large numbers of tapeworms can block the lumen of the gut and cause pain and discomfort. The pain is thought to result of a spasm type colic, intestinal impaction or intussusception. Intussusception is where one segment of intestine telescopes into another which is a serious condition that commonly needs immediate surgical intervention. In this case the cecum is inverted into the colon.
A study conducted in 1998 in the United Kingdom showed that 81% of small intestinal ileal impactions were tapeworm related. When the tapeworms attach to the intestinal wall at high numbers, the lumen can be narrowed so it increases the chance of impaction. Another study in 2000 at a training yard in the United Kingdom revealed that all ileal impactions that required surgery were heavily parasitized with tapeworms. This is thought to have resulted from the sole use of Ivermectin for several years by owners. Ivermectin does not treat for tapeworms but has dramatically reduced the incidence of colic due to large strongyles.
Infections of tapeworms can be treated by using Pyrantel Pamoate at high doses or Pyrantel Tartrate daily dewormer. A few tapeworms may not cause problems and may not warrant treatment. Infection is diagnosed by demonstrating the presence of proglottids in the feces. .
An emerging parasite problem are cyathostome infestions. Most dewormers effectively remove all parasite stages except encysted cyathostome larva. As such these small strongyles- or blood worms- which affect the cecum and colon appear to becoming a clinically recognized entity. Again, again diagnosis of cyathostome infection is warranted before developing a treatment strategy. It is very important to keep a good deworming protocol in place to prevent health problems from parasite damage. A thorough parasite control program should be discussed with your veterinarian to choose which is most effective and economically feasible in your situation. This program is tailored by fecal egg counts from your animals that determine the type of parasites present, gives you a strategic therapeutic plan, and evaluates the efficacy of the treatment. The fecal egg survey will also catch other parasite infections such as small strongyles which may be an emerging problem in the future.
A quick review of the three most common internal parasites involved in colic and dewormers that they are susceptible to are:

A. Round worms(Parascaris equorum)
Most dewormers will remove the parasite burden including Pyrantel Pamoate, at Pyrantel tartrate, Ivermectin, moxidectin, piperazine, and Fenbendazole.
B. Large Strongyles (Strogylus vulgaris)
Effective dewormers include moxidectin, ivemectin, pyrantel, and fenbendazole.
C. Tapeworms (Anoplocephala perfoliata)
Effective dewormers are Pyrantel Pamoate at two to three times the regular dose, and Pyrantel tartrate.



References


1: Proudman CJ, Holdstock NB. Investigation of an outbreak of tapeworm associated colic in a training yard. Equine Vet J. Suppl. 2000 Jun;(32):37-41
2: Proudman CJ, French NP, Trees AJ. Tapeworm infection is a significant risk factor for spasmotic colic and ileal impaction colicin the horse. Equine Vet J. 1998 May;30(3):194-9.
3: Bearsden, Glascow. The role of equine strongyles in the pathogenesis of colic and current options for prophylaxis. Equine Vet J. Suppl. 13, 5-9;88 ref.
4: Monahan CM, Chapman MR, Taylor HW, French DD, Klei TR. Foals raised on a pasture with or without daily pyrantel tartrate feed additive: comparison of parasite burdens and host responses following experimental challenge with large and small strongyle larvae. Veterinary-Parasitology. 1997, 73: 3-4, 277-289;15 ref.