Worm Control Strategy
Arkansas animal scientist recommends using refugia principle
to minimize drug resistance to anthelmintics.
You don't want to overdo deworming, warns a University of Arkansas professor of animal science, pointing to the drug resistance developing in some worm populations.
"Target treatments for when they are most effective and you can get the most benefit, and as infrequently as possible," recommends Tom Yazwinski. "This takes into consideration the refugia principle. This term refers to the population of pathogens on your farm that has not been exposed to any chemical products. They are therefore the most vulnerable (and 'unselected' for resistance) and easiest to control. Refugia parasites cross with resistant parasites and help keep the total parasite population vulnerable to chemical control. They are the gene pool for drug susceptibility."
The larger the refugia population, the more it will intermingle with the worms that have developed resistance and thus dilute the population of resistant worms, he explains. Every time you treat a cow, you are killing all the easy worms but leaving the resistant ones, and after awhile the resistant worms are the only ones left because they are the only ones reproducing. Then you have a big problem.
"The refugia population mixes with the resistant population and can help make sure that current and future worms can be removed effectively each time you use a dewormer,"says Yazwinski. "If you just treat the mama cow one time, the rest of the year you let her get some parasite exposure (to worms that are not resistant), shedding some eggs, contributing to the population of worms on the pasture that has not been selected for drug resistance."
This allows calves to get some exposure and start to develop some immunity to the worms.
"We are long past the time when we tried to get rid of all the worms (an impossible goal). Let them have their quarter, but try to keep it to a quarter," he explains. Dealing with worms is always a numbers game.
"You can't get rid of all of them, but you can keep their numbers low enough that they have very little economic impact on your operation," says Yazwinski. You can live with them in a balance that you can handle.
"We can still do this at the cow-calf level, but it often becomes impossible at the stocker level, because we have such a concentration of susceptible animals on restricted pastures. You have to periodically retreat the same animals to try to keep economics on your side. Young cattle are usually on pastures that have been heavily contaminated with worms for years, and you are usually bringing animals in from other places, including animals from other areas with resistant worms," explains Yazwinski. This tends to concentrate the resistance.
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