War on Worms

In the war on worms, blindly following a one-size-fits-all treatment schedule “concedes any intellectual advantage to the worms,” says Cliff Monahan, DVM, assistant professor Veterinary Preventive Medicine at Ohio State University. The indiscriminate use of dewormers is helping parasites to become resistant, he warns. This happens when a few worms survive treatment and pass the traits that helped them survive to their offspring. After a few generations—not much time, where worms are concerned—you have superworms.

To win the war, you need a strategy that fights worms on multiple fronts and is tailored to your horses’ environment and lifestyle. The person best equipped to help plan your strategy is your veterinarian, who knows what parasites are common where you live and how they’re transmitted. Here, we’ll outline what the program should include.

Know Your Enemy

Some 150 different parasites can infect horses, but the most harmful are strongyles. Horses generally ingest larvae of these parasites while grazing. Forty years ago, large strongyles were responsible for many fatal colics, but good deworming programs have made them far less of a threat than they once were.

Small strongyles now top the worm-least-wanted list. Larvae of these worms burrow into the walls of the small intestine, where they spend about six weeks encysted. When they emerge—often en masse—they can cause colic and inflammation of the intestine. Adults of both types live in the large intestine. Females shed their eggs there, and the eggs leave the horse in manure.

An effective deworming program will take dead aim at strongyles and go after other parasites as needed, Monahan says. For example, ascarids are mainly a problem for young horses; by age 2, most horses have some natural immunity to these parasites. Tapeworm risk varies—your horse is much more likely to pick up this parasite in the East or Midwest, for example, than on the West Coast. And bots are a seasonal problem in most parts of the United States. They’re the larvae of botflies, which lay their eggs on the horse in late summer. The eggs hatch, and the larvae enter the horse’s mouth and attach themselves to the stomach lining over winter. In spring, they’re passed in manure and pupate into adult flies.

Choose Your Weapons

Visit a feed store or page through a catalog, and you’ll find dozens of different dewormers. Most fall into one of three broad classes: avermectins and milbemycins (such as ivermectin and moxidectin); benzimidazoles; or pyrantel salts. Veterinarians used to recommend rotating between the classes, switching yearly or every six to eight weeks. If worms survived one drug, the idea was, they’d be knocked out by the next one.