Worms that can fight off the effects of the chemicals you rely on to protect your horse? Superworms? It sounds like a nightmare. But it's becoming a reality: Already some dangerous equine parasites have found ways to resist widely used deworming medications, and the problem is spreading.
Luckily, people are smarter than worms. Here's how one farm beat resistant worms and how you can apply the same strategy to protect your horse from this threat.
Several years ago, Dr. Dianne Little and other researchers from the North Carolina State University College of Veterinary Medicine began following horses on a breeding farm in central North Carolina. For years the farm had dewormed horses every eight weeks, switching drugs each time from fenbendazole to pyrantel to ivermectin. Yet high levels of worm eggs—specifically, small strongyle (cyathostome) eggs—were turning up in manure just a few weeks after treatment. In addition, some weanlings were showing signs of a disease condition called larval cyathostomiasis, which develops when small strongyle larvae become encysted in the gut wall.
Further checks showed that the worms were resistant to two of the three broad classes of deworming medications: benzimidazoles (such as fenbendazole) and pyrantel salts (pyrantel pamoate or pyrantel tartrate). Resistance develops when a few worms survive treatment and pass the traits that helped them survive to their offspring. On this farm, only the third class of dewormers—macrocyclic lactones (ivermectin and moxidectin)—worked against small strongyles, currently the most widespread and dangerous equine internal parasite.
The NCSU veterinarians wondered if they were watching that last bastion fall. Ivermectin was still killing worms on this farm, but the interval it took for eggs to return to high levels after treatment was shrinking—from eight weeks to three or four weeks and even two weeks in some horses. Shorter egg reappearance time is thought to be an early sign that resistance is developing. Were the worms on this farm turning into superworms, unscathed by all available medications?
To get a handle on the problem, veterinarians mapped out a two-part plan. First, the farm switched from rotational deworming to a strategic program, treating each horse only when its fecal count rose above target levels (200 eggs per gram for mares and 100 epg for foals). Horses aren't equally susceptible to parasites and produce variable amounts of eggs in their manure. On a typical farm, 20 percent of the horses shed 80 percent of the worm eggs. The idea was to identify the high shedders, treat them effectively and manage the others with fewer treatments. Reducing treatments would save money and might help reduce the chance that worms would develop resistance.
Second, the veterinarians recommended changes in pasture management. Strongyle eggs are passed in manure. Larvae hatch in the field and are picked up by grazing horses. Reducing the number of horses grazing each pasture, rotating and resting pastures to interrupt worm life cycles and stopping the practice of spreading uncomposted manure on fields could reduce reinfection.
These steps weren't taken right away, and problems continued. But about a year after new managers took over and made the changes, egg reappearance times began to increase. Ivermectin, it turned out, still worked against the worms. The problem had been rapid reinfection from pasture that was heavily populated with worms due to poor management.