Adverse Reaction to Fenbendazole in Horses

Fenbendazole is an anti-parasitic medication used to kill intestinal worms in horses. It is also sold under the brand names of Safe-Guard and Panacur. Its exact anti-parasitic action is unknown, but according to Drug Information Online, it is believed to disrupt the parasite's ability to metabolize energy. Fenbendazole is widely used as a dewormer due to its low level of adverse reactions, but can cause some side effects in horses.

  1. Overdose

    • Ten percent fenbendazole paste should be administered at a dose of five milligrams per kilogram of weight in adult horses. In foals and weanlings, the recommended dose is 4.6 mg/kg. Overdose of fenbendazole is unusual, but possible. According to Drug Information Online, the drug is safe up to 1000 mg/kg. In pregnant mares, the drug becomes unsafe at 25 mg/kg. In overdose, the drug causes changes to the intestines and death.

    Intestinal Inflammation

    • According to Worming Your Horse, some equines may suffer from inflammation of the intestinal wall when treated with fenbendazole, even when the recommended dose has not been exceeded. This inflammation appears to be caused by toxins excreted by dead and dying parasite larva. Owners should use caution when treating ponies and horses with fenbendazole when encysted redworm larvae are suspected, since the resulting gut irritation may cause similar damage to that from the worms themselves.

    Itching

    • When fenbendazole is administered at higher dose levels, it kills many parasites at once. As in cases of intestinal inflammation, this may cause the dying worms to release antigens – immune chemicals which cause irritation and health problems in the horse. Horses affected by antigens may suffer from local or systemic hypersensitivity, including itching and rashes.

    Collapse

    • In cases of serious infestations, antigen reactions may be more severe. Horses may breathe more quickly and appear to hyperventilate. They may also collapse suddenly. These animals require immediate veterinary treatment. As soon as a horse begins to show signs of this type of reaction, owners should contact an experienced vet.

    Considerations

    • According to My Horse, a major online publication for equine enthusiasts, fenbendazole has been linked with wormer resistance in parasites. Resistance can develop after only a small number of treatments. Worms continue to infest the animal, and must be treated using another wormer, or using steadily stronger doses, increasing the risk of an adverse reaction.