Parasite Control Recommendations for Mares and Foals

Times are changing when it comes to equine parasite control: Anthelmintic-resistant parasites have prompted new, more targeted deworming recommendations. Two important classes of horses that fall under this deworming protocol are broodmares and foals. At the 2013 Society for Theriogenology Conference, held Aug. 7-10 in Louisville, Ky., Wendy Vaala, VMD, Dipl. ACVIM, presented a lecture on current parasite control recommendations for mares and foals.

Vaala, the senior equine technical services veterinarian at Merck Animal Health, began by reviewing the current challenges of effective deworming, specifically anthelmintic resistance. She described anthelmintic resistance as "the ability of worms to survive treatments that are considered effective against that species and/or stage of infections." Essentially, previously reliable treatments no longer eliminate worms from a horse's body successfully.

"In order for anthelmintic resistance to develop on a farm, the gene mutations that confer drug resistance must already be present within the resident worm population," she explained. Once that occurs, frequent drug use allows resistant parasites to survive and multiply. "The history of drug use on a farm plays a major role in how quickly resistance develops and to which drugs."

Veterinarians can use fecal egg count reduction tests to determine if certain worm populations have become resistant to different anthelmintic classes, Vaala said, and can be used to infer drug efficacy against different types of parasites (such as ascarids or cyathostomes) on individual farms. Additionally, she said, veterinarians can use fecal egg count tests to determine whether horses are high or low strongyle egg shedders and, thus, which animals require more aggressive deworming programs targeting cyathostomes.

Finally, Vaala said, the egg reappearance period (ERP)—the time between the last effective deworming treatment and the resumption of significant egg shedding—can help alert veterinarians and horse owners to impending anthelmintic resistance: "A shortening of the ERP is a precursor to the development of full drug resistance," Vaala said. ERPs vary between different anthelmintics, which Vaala reviewed with the veterinary attendees.

Parasite Control for Broodmares

There are several important points to consider when formulating a deworming plan for broodmares.

First, focus on controlling cyathostomins, or small strongyles, she said. Most healthy adult horses need two to three treatments per year to manage these successfully, while high shedders might require four to five treatments. Additionally, she said, a minimum of two treatments per year will help prevent re-emergence of large strongyles, in particular Strongylus vulgaris. "All drug classes are still effective against adult stages of large strongyles, while ivermectin, moxidectin, and larvacidal doses of fenbendazole are effective against larval stages," she said.

Vaala also suggested including a cyathostomin treatment (typically a larvacidal dose of fenbendazole or moxidectin) and a tapeworm treatment (if necessary) at the end of the grazing season.

Finally, Vaala offered two important tips:

  • She prefers to avoid administering anthelmintics during the first 60 days of pregnancy which is the period of early fetal development. "The first 60 days of pregnancy is when all major organ systems are developing (e.g., the period of organogenesis)," she explained. "Teratogenicity (or evaluating the capability of a substance to produce fetal malformation) studies have not been performed for many equine drugs and I prefer to avoid the possibility of drugs interfering with normal fetal development."
  • Strategically schedule one of a broodmare’s treatments during the immediate peripartum period (in the last month of gestation or the first few months after delivery), preferably post-foaling; ivermectin is commonly used for this purpose. "This anthelmintic treatment not only reduces the risk of transmission of Strongyloides westeri (threadworms) in the mare’s milk, but also decreases the number of parasite eggs and larvae passed in the dam’s manure—which is a frequent 'meal' for most newborn foals," Vaala explained.

Parasite Control for Foals

"Foals and weanlings are susceptible to a wide range of parasites including ascarids, cyathostomes, large strongyles, tapeworms, pinworms, and threadworms," Vaala said, noting that the most pathogenic—or disease-causing—worm for young horses is Parascaris equorum (ascarids). While mature horses have developed immunity to these dangerous worms, they can cause respiratory problems, poor growth, colic, and death in foals and young horses.

"During the first year of life foals should receive a minimum of four to five deworming treatments," Vaala said. She recommended the following schedule:

  • Ideally, the first deworming should take place when the foal is no less than 2 months old, unless signs of parasite-related disease are noted. She recommended using a benzimidazole drug (such as fenbendazole, oxfendazole, or oxibendazole) or pyrantel to target ascarids. When administering fenbendazole to young horses, Vaala cautioned, "The label dose for fenbendazole for juvenile horses less than 18 months of age is 10 milligrams per kilogram of bodyweight. This is double the label dose of 5 mg/kg recommended for mature horses. The higher dose in young animals is critical to ensure efficacy against ascarids."
  • After a foal'' first deworming, Vaala said, he should be dewormed once or twice more prior to weaning. If you used pyrantel initially, then fenbendazole would be a good choice for the second treatment. "The average interval between these early anthelmintic treatments should range between eight and 12 weeks with the goal of reducing the number of patent ascarid infections among the foal population while minimizing the drug selection for young foals less than six months of age," she said.
  • At weaning veterinarians should perform a fecal egg count to determine the target parasites for the next deworming treatments. Most foals will probably benefit from two additional deworming treatments before they reach 12 months of age. "Unless the fecal reveals that ascarid infection remains a persistent problem, these later drug treatments should target cyathostomins," she said. She also recommended including treatment for tapeworms and a larvacidal treatment for encysted small strongyles (those cyathostomes hibernating in your horse's intestinal wall). Vaala recommended fenbendazole for this first larvicidal treatment.

Environmental Control Strategies

Finally, Vaala touched on strategies owners can employ to help control parasites in the environment. In this scenario, she suggested owners look at how stallions are managed for some tips.

"A testimony to the impact of a clean environment is the observation that most stallions, usually confined to individual, well-groomed paddocks and pastures, typically have low to negligible egg counts," she relayed. "It is unlikely that all stallions are genetically more resistant to strongyle parasites and serves as a reminder that good husbandry practices are among the best parasite control measures."

Some environmental parasite control tips include:

  • Remove manure from pastures at least twice weekly.
  • Maintain good biosecurity protocols for new mares and foals and those returning from other farms; include fecal egg counts and prophylactic treatment targeted at cyathostomins and ascarids picked up at other farms.
  • Reduce pasture parasite egg and larvae concentrations by cross-grazing pastures with other ruminants, especially sheep that graze close to the ground.
  • Keep pastures properly mowed.
  • Harrow and then rest pastures for at least a month or longer when ambient temperatures remain above 85°F to reduce pasture parasite egg and larvae concentrations;
  • Avoid overcrowding pastures.

Take-Home Message

Parasite control is an important part of mare and foal care. Consider employing targeted deworming and fecal egg counts to help reduce and control resistant parasite concentrations on your farm, and seek veterinary advice if questions arise.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.