Louis Gasbarre, a retired research leader in USDA’s Bovine Functional Genomics Laboratory who is now a consultant, says drug resistance in some worm populations has shown that we can’t completely control parasites.

“Prior to the 1980s, producers controlled parasites with good management – rotating classes of livestock on pastures, monitoring animals, and not overstocking pastures. When the new wonder drugs became available, we thought we didn’t need to do anything except deworm the animals,” Gasbarre says. “But, if we keep using dewormers indiscriminately, we may lose the efficacy of these tools.”

Yazwinski adds that to avoid the development of drug resistance by parasites, the treatments must be targeted for when they are most effective, and done as infrequently as possible.

“This utilizes the refugia (population of pathogens on your farm that hasn’t been exposed to drugs); they are the most vulnerable and easiest to control,” he says. “Refugia parasites cross with resistant parasites and help keep the total population vulnerable to chemical control.”

The larger this refugia population, the more they will intermingle with any worms that have developed drug resistance, thus diluting them.

“The refugia helps make sure current and future worms can be removed. If you just treat the cow one time, and the rest of the year let her get some parasite exposure (to worms that aren’t resistant), and shed some eggs, she contributes to the population that hasn’t been selected for drug resistance,” Yazwinski says. This also allows calves to be exposed and start to develop immunity to worms.

In stocker cattle, which are prone to several types of worms, it’s more complicated. In a stocker operation, it’s important to know which worms you’re dealing with and what they are doing, when choosing drugs. “You must think about drug resistance, which you generally don’t need to worry about on a cow-calf operation,” Yaswinski says.

“Resistance in some worms is enough of a concern at the stocker and feeder levels that some people are combining drugs to maintain efficacy,” Yazwinski says. An example is using a “white” dewormer (oxfendazole, albendazole or fenbendazole) in conjunction with a macrocyclic lactone (ivermectin, doramectin, eprinomectin or moxidectin).

“Since you’re combining drugs that have been available awhile, you’re selecting for dual resistance. But, to get the most economic effect, many people are going this route today.” When combining drugs of different classes, be sure to use full doses of each, he cautions.

Sidebar: Drug classes

We have three families of dewormers,” says Thomas Craig, Texas A&M University veterinary pathobiologist. “The benzimidozoles (white paste oral dewormers) have a broad spectrum of activity, but no residual effect.” They are very effective, however, against worms in the digestive tract.

Levamisole also is only effective against adult worms, has no residual effect, and can’t reach arrested larval stages. “It’s given as an injectable or oral. It’s a good drug, but not as good as newer products. There are some situations where it’s useful in stocker calves – where there’s drug resistance to other dewormers,” Craig explains.

“Another class is the macrolides (ivermectin, moxydectin) that retain high blood level for a period of time, so any incoming worms will be killed,” he says.

The best drug depends on your region/climate, and class of cattle, but be careful. “Products like Ivomec are now off patent, so there are many generics available. They all have ivermectin in them, but some don’t do the job,” Craig says.

Heather Smith Thomas is a rancher and freelance writer based in Salmon, ID.