“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 (GI) parasites,” says Matt Poore, North Carolina State University (NCSU) animal science professor.

During the past few decades, common anthelmintics — drugs that destroy parasitic intestinal worms — proved so dependable in controlling the parasites attacking the GI tracts of cattle, and the economic return was 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.

According to the 2007-08 Beef report from the National Animal Health Monitoring Service (NAHMS), 82% of producers deworm cows once/year or more; 70% deworm replacement heifers with the same frequency; and 54% deworm stocker calves at least once during the year.

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.

Though anecdotal evidence existed for years, the presence of GI parasites resistant to anthelmintics in American cattle was documented scientifically for the first time last summer by researchers with the Agricultural Research Service (ARS). In a study dating back to 2002, they found parasites resistant to the common anthelmintic classes currently used by cattle producers — avermectins, moxidectin and benzimidazole.

Lou Gasbarre, recently retired ARS parasitologist in Beltsville, MD, and his peers reported in their August paper,¹ “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.”

The ARS research focused upon a Midwestern backgrounding operation that used intensive grazing management and strategically timed deworming for more than 17 years. In the fall of 2002, fecal-sample testing confirmed the producer's suspicions that decreased production and calf diarrhea were due to GI parasites. ARS worked with the producer, designing the multiyear study that documented parasitic resistance in that operation.

Unraveling a mystery

In 2007, NCSU researchers 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 DVM Mark Alley, 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,” Poore says.

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

Poore and Alley explain one of the challenges of Fecal Egg Count Reduction (FECR) tests is the lack of a standard protocol for labs conducting them. Consequently, the same samples sent to different labs can yield dramatically different results.

So, they replicated the study in both herds the following year, utilizing the FECR test protocol used by NAHMS. The overall results were the same: a lack of ivermectin efficacy in their Piedmont herd, while all treatments were effective in cattle at Goldsboro.

To further explain the differences, Poore and Alley submitted samples to 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. A less expensive but less precise method of parasite identification involves hatching eggs and trying to identify the larvae.

In the case of NCSU'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 was the one apparently building resistance to the ivermecti — another species of parasite called Cooperia.

ARS researchers found similar results in their study.

According to Gasbarre and his associates, “Removal of drug-sensitive but highly immunogenic species such as O. ostertagi may favor colonization and retention of less immunogenic but drug-resistant genera such as Cooperia.”

The management keys

Consequently, Alley explains controlling GI parasites is no longer as simple as using a dewormer. Though their research yielded as many questions as answers, Poore and Alley say the findings underscore some clear recommendations.

First, 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/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.”

Second, 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 and effective, but Poore explains it's also an engraved invitation to increased resistance to the anthelmintic by GI 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.”

Alley adds that just as important is getting back to the idea of strategic deworming. 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.

According to the NAHMS 2007-08 Beef report, 85.1% of producers time the deworming of their cattle based on their schedule (Table 1); 83% base efficacy of deworming products on cattle appearance (Table 2). Only 5.7% of operations conducted fecal samples within the previous three years of the study.

“It is likely that the widespread use of the avermectins for more than two decades has altered the face of parasitic gastroenteritis in the U.S., and that in the future the most economically important parasite will not be Ostertagia, but rather less pathogenic forms, which because of their drug resistance patterns thrive in particular production systems,” conclude researchers in the ARS paper. “This will challenge parasitologists to draw less on generalizations about parasite transmission profiles, and instead develop approaches more tailored for a given production location.”