Question

I understand how using fecal egg counts to guide deworming schedules helps with parasite resistance. But why doesn’t rotating deworming products do the same thing?

Answer

It is true that rotational deworming was recommended and used for decades. The goal was to avoid exposing the parasites to the same medication with the same mode of action over and over. This wasn’t an invalid approach because repeated treatment with the same drug will inevitably lead to resistance. And treating with different medications with different mechanisms of action would, in theory, prevent drug resistance, because those parasites that survived one treatment would get killed by the next. However, that theory wasn’t the reality, and the idea of rotational deworming has now been completely abandoned. Here’s why.

Drug resistance is complicated

Decades of rotational deworming did not prevent dewormer resistance for multiple reasons. First of all, parasites are complex organisms with large genomes. They have strategies to evade a relatively crude and simple approach consisting of throwing three or four different medications at them repeatedly. Secondly, drug resistance is complicated and multifactorial. It is not a matter of just one or even a few genes. The genetic mechanisms are many, and we don’t even know them all yet. And it is not just a matter of which genes a given parasite may have. It is also a question of gene expression. Which genes are upregulated or downregulated? And are some of these mechanisms operative between drug classes?

Horses can carry many parasites

Further, horses can easily harbor 15 to 20 different parasite species at the same time. And these are biologically different in many aspects, including susceptibility to anthelmintics and possibly even mechanisms for resistance. All in all, we are dealing with complex and sophisticated organisms here—and rotational deworming is anything but sophisticated.

Multi-drug resistance makes rotation impossible

In fact, despite decades of rotational deworming, multi-drug resistance is becoming more common among equine parasites. As a result, we often do not have effective dewormers to rotate between. Three drug classes are available for treatment of nematodes (roundworms) and only two for tapeworm treatment. That is a limited number of options. And all of these classes have drug resistance issues in one or more equine parasites, reducing the number of options even further.

People often rotationally deworm without any sort of testing. Without the testing, they will not know which parasites they are actually treating and whether the dewormers even work against them. In other words, people live on a false sense of security, thinking that they have good parasite control while they may not.

No horse anywhere in the world needs to be dewormed every eight weeks. In fact, most mature horses don’t need more than two annual treatments. Treating every eight weeks is putting a high selection pressure for drug resistant worms. If you are rotationally deworming, you are selecting for multidrug resistance.

In summary, rotational deworming is not an alternative to fecal egg count testing. In fact, it is not possible to design a proper parasite control program without routine fecal egg count testing. 

Martin Nielsen, DVM, PhD
Aarhus University
Viborg, Denmark