Midwest Equine

Feature Article

Foal Owners Beware:
The Danger of Ascarids

By Bryant Craig, D.V.M., Merck Animal Health Equine Technical Services

ArticleFebruary 2016
Foal Owners Beware: The Danger of Ascarids

Dangers of Ascarids

While all intestinal parasites are concerning, one of the most lethal threatens horses during their first 18 months of life.

The Suspect
Ascarids, also known as roundworms, travel through the bloodstream to the horse's liver and lungs during their juvenile stage before returning to the small intestine to develop into a mature parasite. Because of this migration, clinical signs of an ascarid infection can vary from respiratory disease to impaction colic, both of which can threaten your foal's health and potentially his life

To Complicate Matters
Not only are ascarids extremely prevalent in foals, but they also have developed widespread resistance to ivermectin and moxidectin1, two deworming compounds commonly used to control them.

In a recent study led by parasitologist Craig Reinemeyer, D.V.M., Ph.D., foals with confirmed ivermectin- resistant ascarid infections were split into two groups and either treated with a Panacur® Powerpac (fenbendazole 10 mg/kg daily for 5 days) or with an oral ivermectin dewormer at the labeled dose. The group treated with the Panacur Powerpac had an average egg per gram fecal count of 1.35 posttreatment. The group treated with ivermectin had an average egg per gram count of 281.03 post-treatment.2

"Five consecutive days of treatment with fenbendazole at 10 mg/kg (Panacur Powerpac) decreased the number of adult ascarids by 96.3 percent and associated egg counts by 99.5 percent," Dr. Reinemeyer says. "Given the high prevalence of macrocyclic lactone (ivermectin/moxidectin) resistance in ascarid populations, a five-day regimen of fenbendazole is the only anthelmintic consistently effective against immature ascarid infections."

Sounds Good, but is it Safe?
Not all dewormers on the shelf at your tack and feed store are safe for use in foals, but the Panacur brand is one option that is extremely safe at all of its labeled doses.

"Overall, benzimidazole anthelmintics (which include Panacur) might be the best therapeutic choice for ascarid infections in all foals," Dr. Reinemeyer says. "The benzimidazole drug class affects worm metabolism and kills them slowly. The other anthelmintic classes approved for use in foals (pyrantel salts and macrocyclic lactones) affect the nervous system of worms, which kills them rather quickly. The latter situation seems to result in small intestinal impactions with dead ascarids more frequently than after treatment with the benzimidazole class."

What About My Other Horses?
Due to widespread ascarid resistance to ivermectin and moxidectin, ascarids can also threaten horses older than 18 months that were not effectively dewormed as foals and have harbored an ascarid infection into their adulthood.3

A single dose of Panacur (5 mg/kg for adult horses and 10 mg/kg for young horses less than 18 months of age) is labeled and recommended for the control of large and small strongyles, pinworms and ascarids. Additionally, Panacur Powerpac is the only dewormer labeled for the treatment of all stages of encysted small strongyles.4 Panacur is safe and recommended as part of a strategic deworming regimen for:

  • Foals
  • Weanlings
  • Yearlings
  • Broodmares
  • Performance horses
  • Debilitated or thin horses
  • Horses with recurrent colic or chronic diarrhea
  • Horses with chronic weight loss
  • Senior horses

Take-Home Message
Ascarid infections are not something you should take lightly. Contact us today to protect your foals from this potentially fatal parasite and to create a deworming regimen based on fecal egg count tests for every horse in your barn.

Do not use in horses intended for human consumption. When using Panacur® (fenbendazole) Paste 10% concomitantly with trichlorfon, refer to the manufacturer's labels for use and cautions for trichlorfon. Consult your veterinarian for assistance in the diagnosis, treatment and control of parasitism.

1Reinemeyer CR. Diagnosis and control of anthelmintic-resistant Parascaris equorum. Parasites & Vectors 2009, 2(Suppl 2):S8.
2Reinemeyer CR, Vaala, WE. Larvicidal efficacy of fenbendazole against a macrocyclic lactone-resistant isolate of Parascaris equorum in foals. 2010 AAEP Proceedings.
3AAEP Parasite Control Guidelines. Revised 2013.
4Panacur® (fenbendazole) Power Pac Equine Dewormer product label.