Because worms have developed (and are continuing to build) resistance to deworming drugs, horse owners need to investigate other feasible options for parasite control—for example, pasture management.
For several decades horse owner didn’t think we needed to do any pasture management. We have assumed we could depend on deworming drugs. We now know that managing pastures can help control the small strongyles (cyathostomes) that plague mature horses. “On any horse farm, most of these worms are out on the pasture and not in the horses,” says Andrew S. Peregrine, BVMS, PhD, DVM, Dipl. EVPC, ACVM, associate professor in the Department of Pathobiology at the University of Guelph’s Ontario Veterinary College. “How you manage the pasture is more important than deworming when trying to keep parasites down to reasonable levels.”
Strongyles live inside the horse, embedded in the gut lining, then mature and lay eggs that are passed out with feces. The eggs hatch and larvae develop through two stages within the manure pile over a period of 3 days. Third-stage larvae, which are capable of infecting a horse, leave the manure and migrate onto forage plants to be swallowed. Removal of manure within 3 days can help decrease the number of mature larvae capable of infecting horses on the pasture. Successful establishment in a new host represents the beginning of a new parasitic cycle.
When removing manure from pasture for composting, using windrows helps kill the parasites in the manure. Windrow manure composting involves piling organic matter or biodegradable waste into long rows. Mary Rossano, PhD, an assistant professor in the University of Kentucky’s Department of Animal and Food Science, in Lexington, has been researching the effect of windrowing manure on parasite populations in horse pastures. In her recently published study, she showed that no strongyle or ascarid eggs survived more than a few days with windrowing.
Winter pasture management recommendations include moving the field before the snow arrives. Several studies during the 1920s and 1930s (before the advent of deworming drugs) looked at pasture management. “One study was done in the northeastern U.S. where winters are cold,” Nielsen recalls. “It showed that mowing pastures at the end of the grazing season, after horses were removed but before snow cover arrives, reduced winter survival of parasites. Tall forage, and then snow cover, helps insulate and protect the parasites. If the tall grass is gone, however, undulating temperatures, freezing and thawing, before and after snow cover, is detrimental to the strongyles.” Implementing this strategy in the winter could assist in parasite management.
Some parasitism is natural and low worm burdens likely won’t harm the horse. After all, horses must come into contact with a few parasites to develop immunity. “It’s impossible to eliminate the worms completely,” Nielsen says. “We tried that, and the end result is drug resistance.” This is why performing a quantitative fecal egg count regularly to determine if deworming is necessary is also an important part of routine health care. Ideally, by managing our pastures we can ensure horses don’t pick up many worms in the first place, which reduces the need for dewormers.