The History of Ivermectin for Horses

Entering the drug market in 1974, ivermectin combats ectoparasites like lice, ticks, fleas and roundworms in humans and animals. The compound was extracted from a screening of naturally occurring soil organisms at a golf course in Japan. According to Topaz Pharmaceuticals, the medicine blocks a person's or animal's muscle activity, which paralyzes and kills parasites.

  1. Origins

    • Introduced to the global veterinary market in 1981, ivermectin treats parasitic conditions in domesticated animal species, including dogs, cats and horses. Horses generally take the medicine in oral form. Since 1981, Ivermectin has been approved in horses for the control of parasite invasions such as pinworms, hairworms, large-mouth stomach worms, lungworms and summer sores.

    Testing Results

    • In the 1980s, veterinarians and veterinary pharmaceutical companies tested to ensure the drug's dosages were correct and would not harm horse breeds. In horses, an oral dose of 1.8 mg/kg--nine times the recommended dose--did not produce signs of toxicity in horse breeds.

      However, doses of 2 mg/kg, 10 times the recommended dose, caused signs of visual impairment, depression and ataxia. Horse veterinarians and owners feared overdosing their horses because the discrepancy was so small.

    Mixed with Silverleaf Nightshade

    • In the early 1990s, a syndrome affected horses taking ivermectin. Even if horse owners gave their horses the correct dosage, horses who imbibed the ivermectin and grazed in grasses with silverleaf nightshade--a branched, deep-rooted, noxious weed--developed depression, ataxia, liver problems, hepatic encephalopathy, weight loss or occasionally went into comas.

      If a horse owner removed the noxious plant from the horse's diet, or anywhere near its grazing field, the horse often recovered from the poisonous mix, according to Texas A&M University. Silverleaf nightshade appears to have the ability to alter the characteristics of the blood brain barrier and allow more ivermectin into the brain, where high levels can be fatal.

      Not all horses are affected, and many can and do recover. Some do not, however, says Texas A&M University.

    Toxicity

    • According to the Department of Primary Industries, Parks, Water and Environment (DPIW), silverleaf nightshade is toxic to horses. When a horse eats the plant while on ivermectin medication, the combination can reportedly create a potent poison for horses.

      According to the Texas Veterinary Medical Diagnostic Laboratory, in the 1990s veterinarians and horse owners contacted the principal supplier of ivermectin to report and discuss the poisonous effect. However, the firm did not issue or see the need to add a warning label or start an investigative study.

    Considerations

    • Since 2008, the Food &Drug Administration Center for Veterinary Medicine has sought funding for a horse study, but no money has been awarded, as of 2010.

      In the U.S., ivermectin for horses is marketed as a veterinary product and parasite medicine used in brands such as Zimecterin or HeartGard, says Topaz Pharmaceuticals.