Editor’s Note: This is part 3 in a 12-part series on internal parasites of horses.

In the world of internal parasites, ascarids get no respect. Unlike strongyles, they aren’t a high-drama threat to your horse’s health, and they aren’t a “hot topic” parasite like the tapeworm. But that doesn’t mean they should be overlooked or discounted in your war on worms. Ascarids, or roundworms, wreak their havoc largely on young horses with naïve immune systems, and that can set your youngster up for depression, stunted growth, and potentially fatal colic.

What Are They?

Ascarid is a general term referring to a large family of closely related parasites that infect a variety of vertebrates, including dogs, cats, horses, cattle, swine, birds, skunks, raccoons, and even humans. Most species of ascarids are host-specific, meaning they will grow to adults and reproduce in only a single type of host animal. So the ascarid of equids, Parascaris equorum, occurs in horses, donkeys, and zebras, but is not capable of infecting pigs or dogs.

Most ascarids are comparatively large parasites, ranging in width from one to two millimeters (imagine a pencil lead) to three-eighths of an inch, and in length from one to 14 inches. Their large size meant ascarids were some of the first internal parasites to be recognized by man. Adult specimens of P. equorum, by far the largest of the common species to infect horses, are approximately the dimensions of a pencil. Ascarids are easily observed in the manure of infected horses, especially after treatment with an effective anthelmintic.

Unlike the tapeworms discussed last month, ascarids have a relatively conventional sex life, developing as separate sexes, males and females. Like most other parasitic nematodes (a phylum of elongated cylindrical worms), the females are much larger than the males because they are the egg fa