Spooked by the increasing anecdotal reports of resistance to dewormers, which was relatively unheard of just a decade ago, veterinary researchers are urging the cattle industry to take a hard look at the habit of rote, unmonitored use of dewormers, hoping to help protect their effectiveness for the future. So-called “targeted selective treatment” is one umbrella tool they’ve proposed for slowing the development of resistance. It aims to cut the number of deworming treatments by selectively treating only those cattle that will benefit most from treatment without at the same time causing either clinical disease or excessive productivity losses.
How? Some recently published and soon-to-be-published studies detail exactly what targeted selective treatment might look like—some effectively; some, not so much. For example:
EBVs for deworming? British researchers in an upcoming article scheduled for publication in the journal Veterinary Parasitology mathematically modeled the possibility of developing and using Estimated Breeding Values to select for genetic resistance to common cattle worms. Although they demonstrate EBVs, particularly those developed from genetic markers vs. those based on phenotypic traits, could work, their practical application still lies a long way off. Obviously, using genetic markers to predict a calf’s resistance to worms requires a number of technological advances still to come. Several research studies have identified numerous genetic markers (in sheep) that can be linked to resistance; however, they likely only apply to averaged predictions for the population or, at best, family lines as a whole. At this point they can tell us little about individual animals. And as far as that average goes, our ability to predict host resistance under current technology is only moderate. Meanwhile, funding to improve and then apply that technology, as with any EBV, has to fall on somebody. In the case of parasite resistance, EBVs may ultimately work, but whether they would be an economical tool that warrants seedstock producers or associations investing in them remains to be proven.
Targeted deworming by daily gain? Swedish University of Agricultural Sciences researchers report on a three-year trial in that country testing whether holding off deworming on stocker calves until they showed signs of depressed gain would work in selective treatment. They subjected groups of pastured calves to either no deworming, heavy deworming with an injectable at turnout and once every month after, or deworming only calves in a group that had gained less than the average for the bottom half of the dewormed group after eight weeks on grass. Although the study’s results posed on interesting thought experiment--in that they did succeed in cutting the use of dewormer by 92 percent without suffering unacceptable losses in gain (although they were statistically significantly lower), the study’s authors recognize a couple of complicating factors that make the application on typical U.S. operations problematic:
- All calves were weighed on two consecutive days before turnout, and once every second week throughout the grazing period. That handling intensity makes the method unworkable for the average operation, at least until more reliable tools for automatic weight monitoring come down the road.
- Despite a deworming schedule that would be considered excessive by most standards, the study caused no signs of short-term selection for dewormer resistance in the animals that were treated. That is to say, the problem the study was in search of solving never materialized.
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