Foal Owners Beware: The Danger of Ascarids
Ascarids can damage various body systems during the first 18 months of a horse’s life. | Photo: Wendy Vaala
One of the most lethal equine internal parasites—the ascarid, or roundworm—shows no mercy to the young. It infects and damages various body systems of horses during their first 18 months of life.

The Suspect

Ascarids migrate through the bloodstream to a horse’s liver and lungs during their juvenile stage before returning to the small intestine to mature. Because of this migration path, clinical signs of ascarid infection can vary from respiratory disease to impaction colic, both of which can threaten your foal’s health and, potentially, his life.

“Ascarid infections are extremely prevalent in foals and are virtually ubiquitous at breeding facilities where new foals are raised annually,” said Craig R. Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research Inc., in Rockville, an independent business that conducts research for animal health companies.

To Complicate Matters

Ascarids have developed widespread resistance to ivermectin and moxidectin, two deworming compounds veterinarians and horse owners have commonly used to control them

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