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Equine Internal Parasites
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ImageLearn How to Prevent the “Inner Pests”... and How to Eliminate Them if Infestation Occurs

Equine internal parasites pose serious health risks to your horse, including weight loss, anemia, tissue damage and colic. Control of these parasites is an important part of your horse’s regular health care program.


ImageThe Bad Guys

A “parasite” is defined as any organism that grows, feeds, and is sheltered on or in a different organism (called the host) while contributing nothing to the survival of the host. And in many cases, the parasite is detrimental to the health of the host.

External parasites are those found on the skin and hair. These include such things as lice, ticks, and fleas (on small animals). Internal parasites live within the host’s body for at least one stage of their life cycle. As you will see, the subject of parasites is a bit complicated. Explanations and advice from your veterinarian will answer your questions and help alleviate any confusion.

Large strongyles are also known as blood worms and red worms. Under favorable environmental conditions, the larvae develop to the infective stage within 1-2 weeks after the eggs have passed in the manure of the host horse. Infection of the next horse is by ingestion of the infective larvae. The larvae develop to maturity in various parts of the horse’s intestinal tract. Adult large strongyles then migrate to other parts of the body, including the liver, pancreas and major intestinal arteries. Damage to the arteries from migrating strongyles can lead to inflammation, thrombosis (blockage) and aneurisms (a weakened area that ruptures).

Adult large strongyles are actively blood feeding as they move about the intestine. The associated blood loss may lead to anemia. Weakness, emaciation, diarrhea, and colic are also common clinical findings with large strongyle infestation.

There are over 40 different species of small strongyles found in the cecum and colon of horses, each with its own site of preference. Unlike large strongyles, small strongyles do not migrate out of the intestine. Their development is confined to the wall of the intestine. When they emerge from the wall, they feed superficially on the intestinal lining. They may rupture capillaries but are less pathogenic than large strongyles as their “mouth parts” are much smaller. In heavier infections, however, the tissue disruption may be extensive enough to disturb digestive and absorptive function in the large intestine. This results in poor body condition of the horse.

Ascarids (roundworms) are stout, whitish worms up to 30 cm long. Large numbers of infective eggs can remain viable for years in contaminated soil. Adult horses usually harbor very few worms but foals may be seriously infected. The principal sources of foal infection are pastures, paddocks and stalls contaminated with eggs from foals of the previous year.

In heavy ascarid infections, the migrating larvae may produce respiratory signs (“summer colds”). In heavy intestinal infections, foals show a “failure to thrive,” loss of energy and occasional colic. Intestinal obstruction and perforation have been reported. After administration of an appropriate deworming medication, large numbers of immature worms may be seen in the feces.

Adult pinworms (Oxyuris) are more common in young horses and are found primarily in the terminal portions of the large intestine. Females pass toward the rectum to lay their eggs, “cementing” them to the perineum around the anus. Masses of eggs and cement around the anus appear as a white-yellow, crusty mass. The eggs become embryos in a few hours and are infective in 4-5 days.

Adult pinworms are of little significance in the intestine but cause perineal irritation after egg-laying (as does the egg mass itself). Rubbing of the tail and anal region, with resulting broken hairs and bare patches around the tail and buttocks, is characteristic and suggests the presence of pinworms. Fecal examinations in the lab may or may not reveal pinworm infection.

Stomach worms (Habronema) may be found in tumor-like swellings in the stomach wall or free on the mucosa (lining of the stomach wall). The eggs or larvae are ingested by larvae of house or stable flies, which serve as intermediate hosts. Horses are infected by ingesting flies that contain infective larvae or by free larvae that emerge from flies as they feed around the horse’s lips.

Gastritis may result from heavy infections with adult stomach worms. Clinical signs are usually absent unless stomach damage eventually results in mechanical obstruction or rupture.

If the Habronema larvae emerge from flies that are feeding on pre-existing wounds or on moisture of the genitalia or eyes, they migrate into and irritate the tissue. The resulting chronic, reddish-brown, greasy lesions are called summer sores.

The hairworm of horses is also found in ruminants. Consequently, it is generally a clinical problem only in horses commingled or rotated on pasture with ruminants. Horse bots, which are found in the stomach, are the larvae of botflies. The adult flies are not parasitic and cannot feed; they exist long enough to mate and lay eggs, dying as soon as their nutrients remaining from the larval stage are used up. When the eggs are laid, they are glued to the hairs of almost any part of the horse’s body, but are most likely found on the forelimbs and shoulders.

The bot larvae hatch from the eggs in about a week when stimulated, usually by the horse licking the eggs, with the eggs often becoming stuck onto the lip hairs. The larvae crawl into the horse’s mouth, where they become embedded in the tongue or tissue lining the walls of the mouth.

After about a month, the larvae pass to the stomach and attach themselves to its lining for further development. This causes erosions and ulcerations at the sites of attachment, as well as inflammatory reactions around them. Fully-developed larvae pass out in the manure 8-10 months later. After pupating in the soil for a few weeks, the adult botfly emerges.

Three species of tapeworms are found in horses, ranging from 8 to 25 cm long. Diagnosis is by characteristic eggs in the manure, but because the discharge of eggs is sporadic, a single fecal examination may not be diagnostic (i.e. it could be a false negative). In light infections, no signs of disease are present. With heavy tapeworm infestations, GI disturbances, unthriftiness and/or anemia may be present. Ulceration of the intestinal lining at the site of tapeworm attachment is common. These areas may also become secondarily infected (with other organisms) or abscessed.

The Arsenal of Weapons

The first step in parasite control is manure management because in most cases, manure is the medium for spreading internal parasites from one horse to another. Although life cycles of each worm species vary a little, most of them have egg and/or larval stages that are shed in the manure. These end up on pasture grasses, in the soil or mixed into the paddock sand. When the appropriate infective stage of the parasite is ingested by the grazing horse (either from the grass or the hay on the ground), it is able to continue its life cycle in the new equine host.

Therefore, minimizing your horse’s exposure to parasite eggs/larvae is your first defense. Remove the manure from turnout areas as often as is practical. Some parasites eventually die when horses are not present in the field (their life cycle is interrupted and they can’t wait it out), so if you can rotate the horses in and out of several different pastures, that will help.

Cold doesn’t deter parasites; manure that is frozen solid can be a safe haven for eggs waiting for more favorable weather. Don’t allow manure to accumulate in protected areas, such as the corners of run-in sheds, as these piles may retain enough heat and moisture for eggs to hatch.

Even with the most careful farm management practices, horses will still be exposed to, and possibly infected by, worms. We all hand-graze our horses at shows, and the stabling areas and parking fields are full of “foreign” manure. Thus, every horse is at risk for internal parasites and needs to be on a regular deworming program. Modern deworming medicines (anthelmintics) are effective and easy to use. Most horse owners should be able to do their own deworming after consultation with their veterinarian to determine which products to use at which times of the year.

Pyrantel tartrate is the active ingredient in daily dewormer pelleted products. Dosage is based on the horse’s weight and is given once a day to provide constant protection from parasites via the horse’s diet. Daily dewormer paralyzes parasites in the digestive tract and moves them harmlessly out of the horse’s system.

Paste dewormers come in single-dose syringes and are designed to be given every few months. There are three basic types based upon the main active ingredient in the paste. Most deworming programs using these pastes follow a rotational system, with successive treatments switching to a different active ingredient (not switching to a different product or brand with the same ingredient).

Pyrantel pamoate kills large and small strongyles, ascarids and pinworms. This drug kills the parasites slowly via paralysis. Administering double the standard dose of pyrantel pamoate also kills tapeworms.

Benzimidazoles kill parasites quickly and are effective against large and small strongyles, ascarids and pinworms. Fenbendazole and oxibendazole are part of this chemical class.

Macrolytic lactones are anthelmintic products containing either ivermectin or moxidectin. They cause paralysis in the following parasites: large strongyles, small strongyles, ascarids, pinworms, stomach worms and bots.

Combined macrolytic lactones affect the parasites listed above via paralysis but are also effective against tapeworms by breaking down their membranes. The additional ingredient in these products that kills tapeworms is praziquantel.


Planning the Attack

As previously mentioned, the first method of “worm attack” is manure management. Daily stall cleaning and removal of manure from turnout areas is the key. The smaller or more densely populated your paddocks/pastures are, the more important it becomes to clean up the manure frequently.

There are basically two different chemical deworming options—daily or rotational. In busy boarding stables with shared turnout areas and a changing equine population, daily deworming is a popular option. A palatable, pelleted anthelmintic containing pyrantel tartrate is put into each horse’s grain ration on a daily basis (in one meal per day). Proponents of this system like it because there is no build-up of worms in the horse’s system and infection of other horses via contaminated manure is reduced.

Manufacturers of daily dewormer recommend dosing a horse with a broad-spectrum larvacidal dewormer such as ivermectin or moxidectin before beginning administration of a daily dewormer. This is to help destroy existing larvae and adult parasites. Subsequently, horses on a daily dewormer should also receive ivermectin or moxidectin once or twice a year to control bots. Include praziquantel with that at least once a year to control tapeworms.

Rotational deworming is the most common approach, with the owner, barn manager or veterinarian giving an anthelmintic paste every 60-90 days, using a rotating schedule among the major drug families (pyrantel, benzimidazole, macrolytic lactone). Once a year, substitute a combined macrolytic lactone (containing praziquantel also) for the simple one to kill tapeworms as well.

The idea behind rotational deworming is that the few resistant parasites that manage to survive drug A, and then continue to grow and reproduce, will probably be killed off by drug B.

Most equine dewormers are safe for use throughout pregnancy, but precautions and contraindications on the package inserts should be heeded. Foals should be treated at 6-8 weeks of age on the same day as the dam and again at weaning.  

Generally in parasite control programs, all horses on a farm should be treated, and those commingled on the same pasture or paddock should be treated at the same time. Otherwise, you have the risk of one untreated horse re-infecting all the others. New horses and horses returning after having been off the premises for an extended period of time should be quarantined and de-wormed before being admitted to the herd or allowed to share the same turnout areas.

Whatever program is used (daily or rotational), fecal samples should be examined periodically to monitor the effectiveness of the program. For about $18.00, your veterinarian can do a fecal egg count. Use a clean plastic bag to pick up several fresh fecal balls. Store it in the refrigerator or on ice until you can drop it off at the veterinarian’s office or the laboratory within 24 hours. (Parasite eggs can hatch at room temperature, thus distorting the egg count.)

The veterinary lab technician will be able to identify the egg types (i.e. which worm species) and give you a “count” of the number of eggs per gram of manure. These test results will give you and your veterinarian an objective assessment of how effective your current deworming program is. Based on the results, you may decide to make changes in the deworming frequency or the drug families used so as to make the deworming more effective.

There is, however, a cloud hovering over these types of deworming programs. Just as long-term, extended use (as well as over-use) of antibiotics in humans has resulted in some pathogens developing resistance to certain drugs, we are seeing that trend occur with equine internal parasites and anthelmintic medications. Recent findings question the current efficacy of some deworming drugs and their regimens. In other words, are some anthelmintics becoming less effective because of the development of resistant “super worms” due to the overzealous use of these deworming drugs?

To be safe, check with your veterinarian on the latest medical developments concerning internal parasites and deworming protocols. Do a fecal egg count and review the program that you have been using regarding what drugs have been given and when they were given.

Keeping your horse healthy requires a lot of attention, and parasite control is one of the most important aspects of good care.


Sue Perry is a Certified Veterinary Technician and equine massage therapist. She lives in Upton, Mass., with three event horses and runs “Muscle Magic,” an equine massage service.

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