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Parasite Control — Summer 2009 | Out Here Magazine

Keeping horses healthy requires strategic and aggressive action

By Carol Davis
Photography by Greg Latza

Deworming horses every two months has been standard practice for decades. However, veterinarians suggest that new research is showing signs of resistance issues, and recommend taking a proactive approach to protect horses against parasite infections.

Controlling parasites is critical because they deplete essential nutrients, inhibit growth, and cause colic. Parasites such as pinworms, tapeworms, stomach worms, and large and small strongyles — perhaps the most deadly — injure or kill a horse by damaging organs, blocking circulation, and affecting digestion.

Monitoring a horse's parasites begins with a fecal egg count, says Dr. Bobby Cowles, a veterinarian for Pfizer Animal Health.

"Every horse presents a different situation," Cowles says. "Administering routine fecal egg counts and the expertise of a veterinarian is the best way to set up a good deworming program."

So is careful, strategic, and timely rotation of deworming medicine that will protect your horse from harmful parasites.

Pfizer Animal Health's "Rotation is Key" program suggests that properly rotating between three chemical classes up to six times a year is the most effective protection against all kinds of parasites, Cowles says.

Those classes are:

  • Benzimidazole class — includes the oxibendazoles and fenbendazole. Pfizer's brand is ANTHELCIDE EQ(r), which controls strongyles, roundworms, pinworms, and threadworms.
  • Pyrantels class — includes pyrantel pamoate and pyrantal tartrate. Pfizer's product is STRONGID(r) C/C2X, which battles strongyles, pinworms, and ascarids.
  • Macrocylclic Lactones class — includes the popular ivermectin and moxidectin dewormers. Pfizer offers EQUIMAX(r) in this classification, which controls bots and tapeworms.

Rotating among the classes reduces the chances of developing resistance to parasites that can take hold at different stages. It also maximizes the best attributes of each chemical compound, Cowles says.

Pfizer's "Rotation is Key" program offers a calendar that breaks down which chemical class dewormer should be given and at what time of the year. These calendars are distributed at your local Tractor Supply store or can be downloaded at

Carol Davis is editor of Out Here.


Natural Parasite Control

Medications are only part of the fight against parasites. It's also essential to keep your horse's environment as parasite-free as possible:

  • Clean stalls and paddocks frequently.
  • Dispose of manure properly. Keep it away from feed and water.
  • Rotate pastures regularly.
  • Deworm horses housed together at the same time.