Defined
Internal parasites are a primary cause of colic in horses and can contribute to many respiratory, digestive and performance problems.
Although different parasites have different life cycles, the basic pattern involves egg hatching, developing into larvae, migrating throughout the body, and maturing into adults, which in turn, lay eggs.
Adults lay from hundreds of thousands to millions of eggs per day. Adults are often found in groups of hundreds or thousands. Horses eat grass, grain, or hay contaminated with manure and become infected with the eggs or larvae. Some parasites physically invade the horse via the mouth or skin, but the fecal-oral route is the most common.
In the wild, parasites serve a very important population control function. When horse populations get too large for their ecosystem to support, worm populations increase and horses die. On the other hand, several factors serve to control the damage worms cause. Wild horses tend to graze on large tracts of rangeland, roaming as much as 25 miles daily. Their rangeland is also typically dry and arid, with sparse vegetation – conditions which tend to be very hard on parasites.
In a controlled environment, our methods of housing horses favor and perpetuate high parasite loads by:
These conditions make parasite control of paramount importance. In fact, a pamphlet published by the American Association of Equine Practitioners states: “In terms of management priorities, establishing an effective parasite control program is probably second only to supplying the horse with clean plentiful water and high quality feed. It’s that important!”
Some parasites can be detected by observing the adult in manure, others by microscopic examination of feces, and others remain undetectable by these methods even though they are doing their damage.
Types
Four common types of internal parasites affect horses: bots, strongyles, ascarids and pinworms.
Large strongyles (bloodworms), small strongyles, ascarids and tapeworms present the greatest health risks.
Large Strongyles (Bloodworms): The term strongyle refers to a large group of closely related species of internal parasite. Strongyles are also called bloodworms. They are very dangerous because the immature worms migrate through blood vessels of the intestine, and produce intestinal inflammation, which may result in “fatal” colic. Horses of all ages are infected.
Large strongyles, the most common and dangerous intestinal parasite of the horse, are ingested by the horse in the larval stage. Large strongyles attach themselves to the intestinal wall and cause the horse to become anemic. The horse can have colic from poor circulation, an aneurysm, ruptured vessel, or transient lameness – all related to blood circulation problems.
Small Strongyles attach to the wall of the small intestine and can cause diarrhea and colic. The eggs of the strongyles are shed in the manure, the larvae then hatch outside in the grass, and the horse ingests the larvae along with forage. Unlike large strongyles, small strongyle larvae may become encysted – laying dormant, waiting for the proper conditions to emerge. Other signs of small strongyle infestation include loss of condition, loss of appetite, weight loss, poor coat condition, and slowed growth.
Ascarids (large roundworms) affect young horses more often, as mature horses will build up an immunity to the parasite. The 6 to 12-inch long worms can number in the hundreds in the horse’s small intestine and can adversely affect its nutrition. Foals acquire infested ascarid eggs from feces that other horses have passed. Infested eggs, swallowed in contaminated hay or water, hatch in the intestinal tract. The migration of these parasites through the lungs leads to coughing, fever and pneumonia. From there, the young worms travel up the trachea to the mouth, to be swallowed a second time. Colic, coughing, and diarrhea are common signs associated with an infection.
Bots are the larvae (or immature flies) of the botfly. Since these flies are common in the horse’s environment, it is likely that most horses will become infested. While bots are very persistent, in low numbers they are not normally a problem. Some horses however, are particularly bothered by the adults (large flies) and have been known to go to extreme measures, such as running through fences, to avoid them. The larval form live in the stomach and migration can cause ulcers in the mouth. These ulcers result in pain and can lead to all sorts of problems associated with decreased appetites, such as colic.
Pinworms: Although less dangerous than other internal parasites, pinworms are annoying to the horse because they cause severe anal itching, and are evidence that the horse has other intestinal parasites. Horses acquire the parasite by consuming contaminated water, grain, hay, or grass. As young worms mature, they crawl part way out of the anus to deposit their eggs on the adjacent surface. The eggs hatch outside of the horse’s body and become infective in a few days. A characteristic of pinworm infestation is rubbing of the tail and the anal region, causing broken tail hairs and bare patches around the tail. While problems are usually limited to the horses’ appearance, some horses do become irritable.
Tapeworms: Mites living in a horse pasture may consume tapeworm eggs from the feces of infested horses. Grazing horses may then swallow the mites and become infested with tapeworms. Tapeworm infestation in horses may lead to varying degrees of colic.
Other less common worms, which typically cause less severe problems, include threadworms, stomach worms (summer sores), lungworms, and hairworms.
Treatment
A common misconception is that deworming, through the use of medication, will result in the elimination of worms. The reality is that any situation in which horses are grouped together usually results in a more persistent infestation. The issue is not one of eliminating intestinal parasites, but of decreasing their numbers.
Parasitism of horses is best controlled when thought of and treated as a herd or farm infection, not an individual horse problem. The most successful parasite control programs include regular deworming with appropriate dewormers, manure disposal, and semi-annual fecal exams to monitor the effectiveness of the controls.
Parasite control is best achieved by a three-pronged attack:
Decreasing the number of adult parasites laying eggs: This first stage, basically killing the adults, is accomplished by regular deworming. Methods include purge deworming through the use of feed additives, medication through a stomach tube, or paste. Continuous deworming, through pellets of dewormer in the feed every day, is ideal when the horse is exposed to recontamination daily, and when the owner wants maximum control. All are considered good methods.
Internal parasite medication is a poison to the parasite and should not be used at a level higher than necessary. Regardless of the application method, correct dosage and application are important. For example, many owners paste worm horses themselves rather than pay the veterinary service call. However, they must be sure that the correct dosage is used, that it all gets swallowed, that the appropriate drug is used, and that other aspects of horse health are observed.
Parasites can develop immunities to specific drugs, so active ingredients should be rotated to maximize control and reduce resistance. Also, some parasites may not be affected by dewormers, so other treatment methods may be required.
A veterinarian should be consulted to help establish a deworming program that will be practical for your management conditions.
Decreasing the number of larvae and eggs eaten by the horse: This second stage is accomplished by minimizing the fecal contamination of food and water. An excellent way to decrease fecal contamination of food is to use feeders versus ground feeding.
Clean stalls also decrease fecal contamination of food and water. Manure should be removed from the stall at least daily. Since most of the parasite eggs are in the manure, avoiding application to pastures used by horses will help minimize infection.
Another method is cleaning pastures through the pickup and disposal of pasture manure. Manure should be removed weekly in pastures.
Other valuable control measures include:
Frequent harrowing or mowing of pastures will also help to spread out the dry manure, thus killing the parasites. And pastures and paddocks should be well drained.
Evaluating the efficacy of the first two controls: This third phase of parasite control involves monitoring the effectiveness of the other control procedures. Measuring the effect of all the control measures is critical in decreasing parasite numbers and damage. Once the damage is seen, it will have already been going on for a long time, and is often be irreversible.
The best method of measuring success of parasite control is via microscopic evaluation of manure. A recommended protocol is to examine the manure of 20% of the herd twice yearly. Feces should be examined just before a deworming and repeated in 1 to 2 weeks.
Summary
Follow good management practices to control the frequency and spread of internal parasites.
Sources:
American Veterinary Medical Association: “What you should know about internal parasites in horses”
Ron Friedman - Friedman Veterinary Service: “Internal Parasites in the Horse: Some New Thoughts on Damage Control”
Petcaretips.net: “Parasites In Horses”
www.wisc.edu: “Internal Parasites of Equines and Their Control”
Written by Ron Pocs
January 12, 2007