While small strongyles are the principal parasitic pathogen of adult horses, roundworms (ascarids) are the primary concern for foals until they mature at about 2 years of age. Since mares transmit ascarids to their foals and the stress of foaling can trigger the development of encysted worms, it is a good idea to treat the mare one month prior to the anticipated foaling date, and retreat the mare again within 2 weeks of foaling.

The pyrantel pamoate label recommends treating foals every 4 weeks from 2 to 8 months of age.¹ The Association of Equine Practitioners recommends treating foals for the first time at 4 to 6 weeks of age or even earlier in certain situations.² Veterinarians may vary the dose amount and frequency depending on the level of infestation. If there is an indication of very heavy infestation, then some vets recommend treating initially with a half dose at 2-3 weeks of age followed by a full dose at one month of age and monthly dosing thereafter until 6 months of age.² This strategy helps avoid side effects that occasionally occur when a foal with a heavy infestation is treated with a horse dewormer and, as a result, has a heavy parasite die-off. On the other hand, if there is average to below average risk, then deworming at the label dose at one month, 2 months, 4 months, and 6 months of age has been advised.²

For a variety of reasons, some veterinarians in certain situations prefer to treat foals with an ivermectin horse dewormer. When treating foals with an ivermectin horse dewormer it is important to be aware of the weight marking on the syringe, use the lock ring provided, and be careful when pressing the plunger. Since the syringe has enough ivermectin to treat a 1250 lb horse, it is possible to administer a greater than 9X overdose if the entire syringe is given to a foal weighing approximately 140 lb or less. Overdoses of that magnitude may result in signs such as visual impairment, depression, and stumbling (ataxia).³ As a result, many vets prefer treating foals, especially young ones, with pyrantel pamoate instead of ivermectin. Pyrantel pamoate may be more effective against roundworms² and has a wide margin of safety with doses of up to 20X the recommended dose resulting in no adverse effects.³ Regardless, when treating foals it is important to be aware of the dosing chart body weight range listed on the product insert or syringe barrel. The bottom line is that it is essential to get advice from your local veterinarian regarding the horse deworming program that will be best for your situation.

1. Bimeda. Exodus Paste Product Insert; 2005.
2. Chamberlain L. Ask the vet: Deworming April 07.  2007;  Available at: http://www.aaep.org/ask_the_vet_questions.php?category_id=81; Accessed February 15, 2008.
3. Plumb DC. Veterinary Drug Handbook. 3rd ed. Ames, Iowa: Iowa State University Press; 1999.