The task of battling parasitism in beef cattle has been a widespread success. Animal scientists generally agree the fight got easier with the introduction of effective and convenient commercial products like fenbendazole and ivermectin in the 1980s.

The major internal parasitic threat to cattle health and performance comes from nematodes found in the stomach and intestines. Numerous university trials have shown paybacks from effective internal parasite control of $25 to $200/head, says James Strickland, DVM, retired University of Georgia Extension veterinarian.

He says the Ostertagia species (primarily the brown stomach worm) is the most common and most damaging among the bovine gastrointestinal (GI) parasites causing severe symptoms and the highest economic loss.

Beef producers need to be aware though, that not all is copasetic in the deworming world.

Resistance to the modern dewormers (anthelmintics — compounds designed to destroy or cause the expulsion of parasitic intestinal worms) is an increasing problem, primarily in sheep and goats, throughout the world. Until recently, there has been no data indicating drug-resistant ruminant nematodes in North American cattle herds.

That picture seems to be changing and questions are being raised about the efficacy of the more common anthelmintics.

“We're definitely beginning to see some resistance to anthelmintics showing up in cattle in this country,” says Louis Gasbarre, Beltsville, MD. He's a research leader with USDA's Agricultural Research Service. “And, we do know that resistance is widespread among the anthelmintics. But, we're not sure, geographically, how widespread resistance has become in the U.S.”

Indications are that resistance is showing up in multiple areas of the U.S.

Real-world resistance

Gasbarre says a Midwest back-grounder who acquired calves from the Southeast in 2002 experienced lower than expected weight gain during the fall, as well as what appeared to be parasitic gastroenteritis in his cattle.

“The operation used intensive grazing management and had practiced strategically timed deworming for several years,” Gasbarre explains. “Fecal sample results supported the idea that decreased productivity was the result of GI nematode parasitism.”

Subsequent field testing by Larry Smith, DVM, Smith Research and Development, Inc., Lodi, WI, found the cattle harbored populations of parasites resistant to two different classes of anthelmintics. Gasbarre and Smith reported their data and observations to the American Association of Veterinary Parasitologists' (AAVP) annual meeting last summer.

They found the producers' pastures contained substantial numbers of barber pole worms (Haemonchus contortus) with resistance to both avermectins and benzimadoles. The large stomach worm (Haemonchus placei) and small intestinal worms of the Cooperia species were resistant in varying levels to all the commonly used avermectins.

The “good” news is that resistance has not shown up in Ostertagia, Gasbarre says. He notes that while no significant Ostertagia resistance has shown up yet, there are some “hints” of resistance.

While little is known about the efficacy of particular branded products, Gasbarre and Smith have met with the various drug companies to express their concerns.

A vocal critic

One person who's been forward about anthelmintic efficacy is parasitologist-industry consultant, Don Bliss, president of MidAmerica Agricultural Research, Verona, WI.

Bliss' criticism is mainly focused on the endectocide pour-ons popular among cattle producers because of ease of application and reduced cattle stress. He believes that, in some cases, the pour-ons lack sufficient efficacy to be considered efficient dewormers.

In a strategic deworming “white paper” published under the Intervet, Inc. banner, Bliss says the reduced efficacy of pour-ons is due to reduced absorption of the active ingredients into the bloodstream compared to injectable formulations.

“This reduced blood level indicates many animals may not be receiving a therapeutic dose of the active ingredient following treatment,” he states. “This renders these pour-on products unsuitable for use in a strategic deworming program.”

It's important to note Bliss' work generally condemns all classes of avermectin endecticides (including Ivomec® Pour-on, Ivomec® Injection, Ivomec® Plus Injection, Dectomax® Injection and Ivomec® Eprinex® Pour-on) and promotes fenbendazole (particularly Safe-Guard®, an Intervet product).

Bliss says if dewormers lack efficacy and leave even a few parasites to reproduce, the parasites can cause significant production losses and possibly lead to resistant strains.

He bases some of his findings on work with a “sizeable cow-calf operation” in central Georgia he says experienced a severe case of parasitic gastroenteritis in 2003. He reported his observations to the 2003 AAVP gathering.

“Over a period of 60 days beginning in early March 2003, 45 cows died,” Bliss says. “Post-mortem examination revealed severe abomasal damage characteristic of ostertagiasis.”

Bliss' field data indicate heavily infected cattle from the herd continued to shed worm eggs following treatment with an avermectin pour-on formulation. He says it appears from eggs shed that the remaining worms may have become resistant to avermectin treatment.

“Fecal samples taken two weeks after the last treatment demonstrated the continued presence of parasitic nematodes with several samples showing high levels,” Bliss says.

Countering the criticism

In the Georgia case, Bliss concludes his data “demonstrates avermectin resistance on the ranch was present in four parasite species and not just Ostertagia.”

James Hawkins, DVM, Merial Veterinary Professional Services, says Bliss' data and conclusions may be flawed. Merial is the maker of the Ivomec® (avermectin) family of dewormers.

Hawkins visited the Georgia herd twice and had numerous follow-up conversations with the herd manager. “There is no data to justify his conclusions and there is clearly no reason to make the assumptions he made,” he says.

Hawkins says he's never seen data showing that, with proper product administration, there's less than an adequate amount of active ingredient getting into the animal's bloodstream.

“Whether you look at our products or the other manufacturers' products, there's little difference in blood levels between the pour-ons and the injectables,” Hawkins says. “The difference is certainly not enough to the point of reducing the efficacy of the product.”

Hawkins says it's important to remember these are Food and Drug Administration-approved products with proven efficacy and blood-level profiles.

“Egg count data doesn't tell you the whole story,” Hawkins explains, and he calls Bliss' work “suggestive,” at best.

“Finding worm eggs passing from the animal post-treatment does not constitute proof of resistance,” he says. “It's important to remember that none of the anthelmintics are 100% effective — and none provide zero post-treatment egg counts in every animal.”

Gary Zimmerman, a Livingston, MT, DVM, has done extensive independent field research into ruminant parasitism. President of Zimmerman Research, he's a former Oregon State University veterinary parasitologist.

Zimmerman has conducted numerous controlled studies, including field efficacy trials, with basically all the anthelmintic compounds used today.

He wouldn't comment directly on Bliss' conclusions, except to say that abstracts or papers presented at AAVP don't undergo peer review.

“I have not seen any difference — under the highest levels of testing you can do — in the efficacies between the pour-ons and the injectables,” Zimmerman says. “If there was a lack of active ingredient in blood plasma samples, the FDA wouldn't allow the product to be licensed.”

Zimmerman says he's never seen a situation in groups of cattle with parasites — everything else being equal — where it hasn't been an economic advantage to treat them. “But, you have to do some delving into the individual situation,” he adds.

If resistance in cattle happens, it's most likely to happen in situations where ranchers have young weaned animals treated multiple times.

“And, this is not likely to be a cow-calf problem,” Hawkins adds.

Resistance explained

Bill Kvasnicka, retired University of Nevada-Reno Extension veterinarian, has worked for some time with Bliss and supports Bliss' conclusions.

“I think you're going to see situations increasingly arise where efficacy in terms of parasite resistance is a problem,” Kvasnicka says. “It may be a worldwide problem that's only now beginning to surface in this country. It's certainly something producers and veterinarians should be aware of.”

Gasbarre says, if a parasite resistance problem indeed exists, it could be due ironically to the success of the limited number of anthelmintic products on the market.

“Most producers see the advantages of de-worming because drugs currently on the market have worked so well and have shown a payback,” he says. “Many animals today see the same products 3-4 times a year.”

This intensive use of a narrow drug spectrum, in turn, feeds a basic biological cycle of selection for resistant mutants of the target parasites.

“We believe the resistance beginning to surface is directly related to the intensity of drug usage. We know some stocker systems can select for resistance fairly quickly,” Gasbarre says. “Also, the pipeline for new drugs has slowed, which could present short-term problems for the cattle industry.”

Gasbarre has some advice for producers and their vets:

  • Carefully assess parasite control strategies and make every attempt not to overuse a particular compound.

  • Pay close attention to how the anthelmintic drugs are working — and look for efficacy problems.

  • If resistance is suspected, first get in touch with the drug company and obtain some guidance from its representatives.

  • Producers might have to revert to using some of the “old” drugs, but be aware of their limitations and treatment windows.

“Local vets also need to know that resistance is coming,” Gasbarre concludes. “And they should be on the front edge of any suspected resistance problems.”