Deworming Q & A
I’m looking for the 3-way rotation chart. Can you repost this on your website?
We actually removed the rotation chart from our website and catalog because experts now agree that switching between classes of dewormers based on the calendar is neither the best way to protect our horses from parasites nor prevent resistance from developing in the worms themselves. While rotating between the three chemical classes of dewormers made sense when it was introduced years ago, it’s no longer the best defense against parasites and, in fact, may be doing more harm than good.
First, each of the three chemical classes of dewormers (benzimidazole/pyrantel/ivermectin) has a different egg suppression period. That is, each class prevents eggs from being laid by adult female worms for a different amount of time. While some dewormers “last” for just four weeks, others are effective for six weeks, eight weeks or even more. So a parasite control program that rotates between these classes every eight weeks isn’t taking into account the differences in how long each dewormer “lasts.”
Second, experts have detected resistance to almost all of the chemical classes of dewormers. Throughout the country, there are pockets of parasite resistance where the benzimidazoles or the pyrantels are no longer effective against small strongyles in adult horses. Scientists have also found that, on some farms, ivermectin is ineffective in controlling ascarid (roundworm) infections in young horses. Therefore, it’s important to use a chemical class on your farm that you know works by conducting fecal egg counts.
Third, parasites have life cycles that depend on external environmental cues like temperature and moisture. When it’s below about 40°F, eggs continue to be laid in manure, but they are no longer able to hatch into infective larvae. So there’s no need to deworm after the first hard frost in the fall in northern climates because horses aren’t picking up any new parasites. However, when temperatures begin to warm up in the spring, eggs that were lying dormant on your pasture all winter now hatch, which is why you should begin deworming again when temperatures remain consistently above freezing (it’s the opposite for southern climates). This is also why it’s a good idea to deworm your herd in the spring before they go out on pasture and deposit even more eggs that can then hatch and infect the horses.
I encourage you to sit down with your veterinarian and develop a parasite control program that is appropriate for your horse and geographic area. Take into account whether your horse is a low, medium or high egg shedder and also incorporate other parasite control measures like manure management.