Devising a parasite control program for your newest arrivals can be a confusing task, especially when recommendations advise alternating the de-wormer used. "Rotating your de-wormer–not by brand but among classes of drugs–can prevent parasites from developing resistance, thus preventing development of a "superworm," explains Dr. Doug Hutchens, veterinary parasitologist formerly at the University of Illinois College of Veterinary Medicine in Urbana. Interval control programs (see table) for horses less than two years of age, programs need to control Strongyloides westeri, Parascaris equorum (roundworms), Oxyuris equi (pinworms), and strongyles.

Strongyloides westeri may be a problem on farms with a high concentration of horses. Dosing the mare with ivermectin within 24 hours after foaling best controls it. "Milk-borne infections may occur, leading to enteritis and foal heat diarrhea in foals," says Dr. Hutchens.

Roundworms (Parascaris equorum), also known as ascarids, are problems in horses less than 2 years of age. Roundworm larvae migrate through the lungs and liver and mature in the intestine. Larvae cause a summer cold syndrome, nasal discharge, dull hair coat, "pot-belly" appearance, colic, or pneumonia that may get better with antibiotics but return when antibiotics are discontinued. Treatment with an effective anthelmintic (such as ivermectin, pyrantel pamoate, moxidectin, or daily feeding of pyrantel tartrate) will prevent this problem. Younger horses, because they are exposed to roundworm eggs year-round, need a daily or interval de-worming program.

If a heavy or severe infection is suspected, your veterinarian may suggest a de-wormer that kills the parasite slowly and safely to give the foal’s body a chance to eliminate the worms. Fenbendazole at double the normal dose (consult with your veterinarian) is a safe and slow treatment. Piperazine, a fast ascarid kill product, should not be used in animals with a high ascarid load; it could result in an ascarid-impaction colic or even perforation of the small intestine. Moxidectin (Quest(c)), although effective against roundworms, should never be used in foals under 4 months.

Yearlings and two-year-olds often suffer from pinworms, which cause intense itching around the tail and lead to tail rubbing and development of a "rat tail"–loss of hair around the tail head. Most de-wormers are effective against this parasite. Don’t worry; these pinworms are not transmissible to humans. Most tail rubbing in regularly de-wormed horses is a compulsive vice caused by boredom rather than by parasites.

Environmental control will aid the de-worming program in controlling parasites:

  1. Remove feces routinely to decrease transmission of eggs and larvae; do not dispose of feces on pastures or near water or feed supply.
  2. You may harrow pastures to break up fecal pats and kill larvae only in hot, dry weather or below-freezing weather.
  3. Quarantine all new additions to your herd and have your veterinarian perform fecal examinations. De-worm with appropriate anthelmintics if indicated.
  4. De-worm all horses at the same time.
  5. Evaluate parasite control via fecal examination of 10 percent of horses two or three times per year, 14 days after de-worming. "If environmental control is adequate and de-worming is effective, less than 10 percent of samples obtained should contain eggs," says Dr. Hutchens. If samples show significant egg counts, re-evaluate drugs used and method of administration.

"Most do-it-yourself programs breakdown when the wrong drug is used or the drug is improperly administered because the dose is too small or the horse spits it out," says Dr. R. Dean Scoggins, retired equine Extension veterinarian at the College. The best way to develop an effective parasite control program is to contact your local equine veterinarian, who will know the prevalent parasites and transmission patterns in your area and can supervise an effective parasite control program.

Table. Possible interval control program for foals*

Age in Months
Efficacy Desired
Example of Anthelmintic

Roundworms, strongyles,

Roundworms, strongyles,
pinworms, Stongyloides

Roundworms, strongyles,

Roundworms, strongyles,
pinworms, and tapeworms
Pyrantel Pamoate

Roundworms, strongyles,

Roundworms, strongyles,

*Dose mare with Ivermectin 24 hours after foaling to prevent milk-borne infections of Strongyloides westeri.
**Moxidectin should never be used in foals under 4 months of age.

Contributed By: Sarah Probst (Information Specialist)
University of Illinois / College of Veterinary Medicine

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