by Sally Colby
It’s barn policy: on the first day of an even-numbered month, every horse in the barn will be dewormed.
“We give dewormer every two months year round and it’s taken care of,” said veterinarian Dr. Harold McKenzie of the DuPont Equine Medical Center. “It’s been great that we’ve been able to get away with that for so long, but the reality is that it isn’t that simple when you’re dealing with a biological system with many parts.”
McKenzie says the ‘every two months’ routine deworming system has been breaking down. Parasites that have always been considered non-issues are starting to resurface, and some parasites that seemed inconsequential are turning out to be significant.
Where did the practice of deworming every two months come from? McKenzie says in the 1960s, a number of studies focused on large strongyles in horses. “They looked at the effect of deworming on the shedding of large strongyle eggs,” he said, “and they found that with the best drugs that were available at the time, deworming reduced the shedding of large strongyle eggs for about eight weeks. That’s it. That’s where the two month interval came from.”
McKenzie explains the life cycle of the large strongyle and the damage it can cause. “They’re large worms, and easily detected,” he said. “This parasite was a huge problem because it invades the large blood vessels supplying the large intestine. After the parasites are ingested and mature in the horse’s gut, there are larval stages that migrate through the intestinal wall and up into the large blood vessels that supply the intestines with blood. Big, nasty clots would form around the larval stage, and then pieces of the clot would fall off and lodge in the arteries that supply the colon, and the intestine would die.”
Horses infected with large strongyles often underwent surgery for removal and repair of dead intestine, but didn’t usually fare well after the surgery. McKenzie says because the outcome for many horses was death, large strongyles were the focus of parasite control programs at that time.
“Large strongyles are easy to kill and to eliminate from the horse’s environment,” said McKenzie. “The parasite has a simple fecal-oral life cycle. Parasites pass out through manure, are ingested by the horse and go back to the intestine to complete their life cycle. The big problem is still that the larvae migrated from intestine to the bloodstream and damaged blood vessels.”
McKenzie says because it was relatively simple to design a control program for large strongyles, this parasite has become almost a historical footnote. A single deworming treatment would stop the life cycle and subsequent pasture contamination for up to six months, and the larvae could only survive for about a year in the environment. At most farms, routine treatment would eliminate large strongyles from the environment within 18 months.
“Most horses get adequate enough deworming that large strongyles are not an issue,” said McKenzie. “It isn’t that we’ve eliminated them – they aren’t eradicated – but it’s rare that we see a clinical problem from large strongyles.” The new knowledge of how to treat this potentially deadly parasite meant fewer deaths due to complications. However, large strongyles are not the only parasite issue. The anthelmintic (deworming) treatment was effective for that parasite, but not a good paradigm for other parasites.
Although many horse owners don’t recall the days of the veterinarian using a tube to administer a deworming treatment, it was the only means of treating horses for parasites. But a single innovation caused a huge shift in parasite management: the development of easily administered paste dewormers. “This was ground-breaking,” said McKenzie, who recalls the practice of veterinarian-administered dewormers via tubing. “I don’t think anyone has fond memories of tube deworming, but that all stopped when paste dewormers became readily available. The loss is that veterinarians were no longer involved in the horse’s deworming – paste dewormers were available widely and there was no thought to designing a parasite management scheme for an individual farm or horse. Every horse got the same treatment on a schedule.”
Another complicating factor in the deworming issue is ivermectin. “Ivermectin revolutionized parasite management, not just in horses but in all species, including humans,” said McKenzie. “The problem is that when ivermectin came out, they were touting it to veterinarians and horse owners that because of the way it worked, it would be impossible for parasites to become resistant to the drug.” McKenzie explains that basic biology shows that any time strong pressure, such as routine deworming, is applied to a population – it is going to respond. The strongest parasites will survive treatment and potentially become resistant to drug pressure.
Today, 40 years after the development of ivermectin, there’s widespread resistance of certain parasites to it. “The drug companies were wrong,” said McKenzie, “but resistance was slow to develop, and not widespread among all parasites. So it isn’t that ivermectin doesn’t have a place in a deworming regimen, but it isn’t the wonder drug it was touted to be.”
McKenzie says eliminating large strongyles from the assortment of parasites that affect horses has resulted in almost no colic secondary to this parasite. However, after the issue of large strongyles was eliminated, there was more focus on other equine parasites.
Tapeworms are one such parasite. McKenzie says although they were often visible in some post-mortem exams, they didn’t seem to be a problem for the horse. “Small strongyles were around back then, but not in large numbers,” he said. “They didn’t seem to be causing clinical problems, but that has changed. Ascarids, or roundworms, are present in young animals, and by age two to three years, horses no longer have them. Because roundworms were a transient part of every horse’s life, veterinarians thought they weren’t causing too much trouble. But we’ve learned that that isn’t always the case.”
McKenzie says in his experience, the most intensively managed horse farms are the most likely to have resistant organisms. He urges horse owners to understand more about the parasites that infect horses, and develop a program that involves whole farm management combined with strategic deworming – the practice of treating horses for parasites using a variety of tools rather than simply deworming on a schedule.
Parts two and three will include a review of the most common equine parasites and how to develop a deworming strategy that will ensure anthelmintic products that remain effective in the future.