“It used to be that when you treated cattle with an avermectin (a class of dewormer) you’d get 100% reduction in the egg count of gastrointestinal parasites,” says Matt Poore, an animal science professor at North Carolina State University (NC State).

During the past few decades, common anthelmintics—drugs that destroy parasitic intestinal worms—proved so dependable in controlling the parasites attacking the gastrointestinal tracts of cattle—the economic return so obvious—that more producers began using them more routinely. Arguably, the efficacy of these deworming products even helped change the way producers manage cattle.

Unfortunately, growing evidence suggests widespread and sometimes injudicious use of anthelmintics may be creating parasitic resistance to them, at least on a site-specific basis.

In 2007, researchers at NC State were trying to uncover the root of poorer-than-expected cattle performance in their cattle herd at the Upper Piedmont Research Station at Reidsville, NC. Though weaned calves had been dewormed, fecal sampling showed the calves were still shedding the eggs of intestinal parasites. In evaluating whether the problem existed across the entire herd, Poore and Mark Alley, DVM, a clinical assistant professor and Extension cattle veterinarian with the College of Veterinary Medicine in the Department of Population Health and Pathobiology, decided to compare the efficacy of various anthelmintics.

In simple terms, Poore and Alley compared an untreated control group to calves receiving a generic label ivermectin pour-on; a brand-name ivermectin pour-on; or a drench of fenbendazole (part of the benzimidazole family).

“We got relatively poor efficacy with the ivermectins, less than a 90% reduction in fecal egg counts, which is suggestive of resistance. We got 100% reduction with the fenbendazole,” says Poore.

Next, Poore and Alley compared the same treatments, plus injectable ivermectin in the NC State research herd at the Center for Environmental Farming Systems at Goldsboro, NC. All of the treatments reduced fecal egg counts by more than 90%.

Ultimately, Poore and Alley replicated the study and also included a treatment group for injectable ivermectin. The results were the same.

To further explain the differences, Poore and Alley submitted samples to the USDA Agricultural Research Service (ARS) to determine which specific parasite eggs were still being shed by treated calves. The polymerase chain reaction (PCR) test for egg identification at ARS identifies parasite species through DNA.

In the case of NC State’s Upper Piedmont herd, ivermectin had eliminated Ostertagia (O. ostertagi)—the brown stomach worm—long believed to be the most pathogenic and economically costly of cattle gastrointestinal parasites. The species left behind, the one apparently building resistance to the ivermectin, is another species of parasite called Cooperia.

ARS researchers found similar results in a study dating back to 2002, which they explained in an August 2009 paper.

Lou Gasbarre, recently retired ARS parasitologist, and his peers reported, “This is an extremely important finding, given the prevailing notion that resistance to ivermectin (part of the avermectin family) and related compounds would be slow to arise if it appeared at all in cattle…

“The argument has been that, because a significant portion of American cattle are raised in areas where environmental conditions preclude heavy parasite transmission, i.e. the arid to semi-arid West, there would not be a blanket of selective pressure placed upon the parasite populations.”
Given this emerging reality, Alley explains deworming programs should be tailored to the unique challenges of individual herds based on fecal testing and identifying specific parasite species that pose the primary challenge. He understands such testing costs $10-$15 per head, but points out the entire herd needn’t be tested. “Test a subset of the herd, at least the most susceptible animals,” Alley says. “The recommendation is to test a minimum of 20 head.”

Secondly, evidence of anthelmintic resistance suggests producers safeguard the products that are working for them through judicious use.

As an example, Poore says, “One of the biggest culprits to increasing resistance we see in this part of the world is producers using low doses of ivermectin to control horn flies in the summer.” It’s cheap. It’s effective. But Poore explains its also an engraved invitation to increased resistance to the anthelmintic by gastrointestinal pests.

Other risky practices include dosing based on weights taken with the eyeball rather than a scale and dosing based on average weights rather than individual cattle weights.

“Use moderation. Don’t deworm cows every month because you see an effect,” Poore emphasizes. “Focus most of your efforts on the youngest stock: they’re the most susceptible, shed the most eggs and require the least amount of product to deworm.”

It’s important to get back to the idea of strategic deworming, Alley says. Rather than rely solely or almost exclusively on anthelmintics, he recommends producers work harder at such things as rotational grazing and timing the use of dewormers when they’re most effective rather than when they’re most convenient to administer.

In a recent article on the topic
(fyi.uwex.edu/repetitive-cattle-deworming), Amy Radunz, a beef cattle Extension specialist at the University of Wisconsin-Madison, says internal parasite management should include:

  1. Fecal Egg Count Reduction Test (FECRT) protocol, used by USDA National Animal Health Monitoring System (NAHMS), a valuable tool to determine when animals need to be dewormed. In addition, this can be used to detect worm resistance in a group of cattle. The test can cost between $10-15 per head, but the entire herd does not need to be tested. A farmer should test at least 20 of the most susceptible animals in the herd.
  2. Calves are more susceptible, shed the most eggs, and require less product to deworm than cows in the herd. Farmers could reduce cost and overuse of product by only deworming calves in the herd at the appropriate times.
  3. Farmers should avoid overusing dewormers and use these products in moderation. This can occur by deworming when the drugs are most effective instead of when convenient to administer. Furthermore, the dewormers should be given using the correct dose. This requires knowing the actual individual weight or average body weight of the animal when administering the drug.
  4. If resistance is detected through testing, changing drug class is recommended. Another alternative could be rotating chemicals in the deworming program.
  5. Good pasture management is also critical to reduce resistance. Rotate cattle through pastures to reduce re-infections; it is recommended to let the pastures rest for 3-4 weeks before re-introducing cattle.
For more details on the findings of Poore and Alley, see Worm Wars in the April issue of BEEF Magazine.