Common wisdom has long said that by rotating among the various classes of dewormers throughout the year, we're likely to eliminate most, if not all, of a horse's parasite load. However, while a number of information sources still espouse a simple monthly rotation strategy and many horsemen and women still carry out that type of program, parasitologists now have evidence as to why the long-followed rotation tactic just doesn't work, and could be hastening resistance.
 
Rotation was initially recommended in the 1960s as an option for avoiding small strongyle resistance to dewormers, a serious concern since small strongyles (small bloodworms or redworms) are one of the major parasite types affecting horses today.
 
Based on the best scientific knowledge at that time, a rotation strategy was considered to be the 'gold standard' in terms of parasite elimination, since it was believed that by reducing reliance on any particular class of dewormer it would reduce or eliminate the possibility that parasites would build up tolerance, or resistance, to any one dewormer, leaving them all effective in your horse herd.
 
However, there's since been documented evidence of problems with and resistance to dewormer classes such as the benzimadazoles (including oxibendazole, this month's featured dewormer), the pyrantels, and phenothiazine, all dewormers typically included in a rotation plan. A study on oxidendazole resistance in the early 1990s indicated that not only did alternating among several classes of dewormers not affect the development of resistance (the whole reason for rotating in the first place), the efficacy or effectiveness of the drugs used had decreased to less than 70% over the course of the study.
 
It's also now believed that simple rotation deworming, and small exposures to all the various deworming classes, has promoted dewormer tolerance in parasite populations that weren't killed outright by the dewormer, perpetuating the 'resistant' genes in their offspring.

 

The animal health company Merial is developing a deworming education campaign, indicating that 'blind routine and rotational deworming falls short.' The campaign's focus is to urge horse owners to stop arbitrarily using year-round or rotational deworming programs when they don't know what types of worms are present or in what quantities, challenging more than four decades of conventional thinking. To get attention, they're trying a little humor, in a 'Greetings, Human' approach straight from the worm's perspective - we'll include a link in an upcoming issue of Worm Control News after the site goes live.
 
"Merial's educational campaign humorously recognizes that if worms could talk, they'd in fact be telling us to keep on doing exactly what we've been doing for decades," says Frank Hurtig, DVM, MBA, director for Merial's Large Animal Veterinary Services. "In the long run, it's better for the worms if horse owners continue to deworm the way most have been. The last thing a resistant small strongyle would want is for us to become more strategic in deworming practices."
 
Merial now suggests horse owners ask their equine veterinarian about a targeted deworming approach, with fecal testing, selective deworming treatment, and environment management for their operation. That way, you'll know what to treat, and can do so without the excess amounts of dewormer that are part of the developing resistance crisis.