Worms in Horses

Inappropriate deworming might lead to parasite resistance and potentially life-threatening problems.

The American Quarter Horse Journal
June 18, 2013

Shezaroostercat, photo courtesy of owner Kiersten Johnson

Shezaroostercat, 10 days after being rushed to the vet with the impaction, was still bloated from all her digestive upset from parasites. (Photo courtesy of Kiersten Johnson)

It didn’t make any sense. Kiersten Johnson of Bennett, Colorado, dewormed her 8-month-old American Quarter Horse filly Shezaroostercat, and the filly bloated up like a balloon the next day. After she was rushed to the veterinarian, her impaction was cleared with mineral oil and extensive testing, the filly still had a large population of parasites in her system. The deworming had cleared some of her extensive parasite load quickly, resulting in the impaction. It took two weeks and a lot of TLC to get the sick baby back to feeling well.

But Kiersten says she and co-owner Daphne Eslick were flabbergasted. They have big plans for this filly and have always been meticulous with her care.

“We had dewormed her carefully according to the veterinarian’s instructions with fenbendazole at 2 and 4 months of age, then put her on the rotation with our other four horses.”

Kiersten even gave the filly an extra dose of ivermectin at 7 months of age because she appeared to be battling worms. So how could the filly still be that wormy when given ivermectin/praziquantel at 8 months?

Well, Kiersten’s veterinarian determined that, like more and more horses are encountering, a parasite resistance problem was the issue.

Dr. Chanda Moxon, an equine veterinarian at Barnesville Animal Clinic in Barnesville, Georgia, describes these circumstances associated with parasite resistance.

“Signs of parasite resistance include weight loss, pale mucus membranes, lethargy, tail rubbing, a continuous high fecal egg count (which the filly likely had, despite repeated dewormings), and colic.”

In fact, this type of clinical parasitism, which ultimately the filly was diagnosed with, can oftentimes lead to death.

But Kiersten, like most horse owners, was doing exactly what she thought was right and had even consulted her veterinarian.
“Deworm With Caution” in the June American Quarter Horse Journal takes an in-depth look at the complex feat of proper deworming. The article by Journal special contributor Sara Gugelmeyer provides an overview of equine anthelmintics, diving into the worms they treat and the product names associated with each chemical class.

One common misconception among horse owners is that horses must be parasite-free. Learn why “parasite-free” is a misnomer, plus how grazing plays a role in parasite risk. “Deworm With Caution” starts on Page 76 of the June Journal.

The American Quarter Horse Journal focuses each month on the issues that matter most, including training, breeding, health, racing, show activities and sale calendars. Visit www.aqha.com/journal to subscribe.