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

BY KAREN BRIGGS, WITH CRAIG REINEMEYER, DVM, PHD; DENNIS FRENCH, DVM, MS, DIPL. ABVP; AND RAY KAPLAN, DVM, PHD

For the past 10 months in this series, we’ve examined a host of issues relating to equine parasites and their control. Now it’s time to get down to the nitty-gritty: How to ensure that parasites have the minimum impact on your horse’s health and well-being. Let’s start with young horses, because parasite populations in immature animals are often quite different than those found in adults. Some kinds of worms, in fact, prefer young horses so strongly that they’re almost never found in equines past a certain age. Infections with threadworms, roundworms, and pinworms, for example, are found almost exclusively in horses less than six months, 18 months, and 24 months old, respectively. Because of the age distribution of these worms, parasite control recommendations for immature animals are necessarily different than those followed for adults.

Susceptibility of Youth

Most of the common nematode (roundworm) parasites of horses are transmitted by ingestion, so the risk to young foals is diminished somewhat while they’re nursing rather than grazing. But as curious colts and fillies investigate their environments, they inevitably come into contact with, and swallow, infective stages of various parasites, including large and small strongyles, roundworms, pinworms, tapeworms, and possibly threadworms (see “Parasites Affecting Juvenile Horses” below).

Weanlings and yearlings on pasture are particularly susceptible to parasitic disease because unlike their dams, they don’t yet have the advantage of acquired immunity or resistance. And foals that are under stress from shipping, weaning, or environmental changes, to name only a few,