Worming Toward Failure

By Wes Ishmael

Increasing evidence suggests the widespread use
of anthelmintic drugs and their effectiveness in controlling internal cattle parasites are resulting in
site-specific resistance.

Go on, name the technologies that helped revolutionize extraordinary gains in cattle production efficiency during the last century.

You'll be forgiven if the development of reliable anthelmintics — drugs that destroy profit-robbing internal parasites — came way down on your list, if at all. Controlling worms lacks the emotional appeal of genetic technologies like synchronization and artificial insemination. Keeping internal parasites at bay lacks the visibility of growth promotants and long term anti-microbial treatment for bovine respiratory disease.

Yet, increased weight gains and the boost to the immune system fostered by effectively controlling internal parasites continue to help increase efficiency — advantages that also help build an environment for other efficiency-building technologies to succeed.

Amy Radunz, Extension beef cattle specialist at the University of Wisconsin-Madison pointed out in an online educational forum last spring, "Dewormers have provided effective parasite control, which has resulted in returns to farmers between $20 to $200 per head. The cost of these products is reasonable when compared to potential productions gains provided."

This reliable boost makes strategic deworming commonplace. According to the National Animal Health Monitoring System (NAHMS) Beef 2007-08 Report, 81.7 percent of operations deworm cows 1 or more times each year; 53.7 percent deworm unweaned calves at least once during the year; and 54.1 percent of those with stocker cattle deworm at least once during the year.

Widespread and injudicious use spawns resistance

Unfortunately, growing evidence suggests site-specific cattle parasite resistance to anthelmintics is emerging.

Parasite resistance to common anthelmintics in sheep and goats has long been documented in the U.S. and abroad. Anecdotal evidence and suspicion about resistance in cattle has existed for years, too.

But, according to a 2009 study authored by Lou Gasbarre and Agricultural Research Service (ARS) peers, the prevailing notion was that anthelmintic resistance wouldn't emerge in beef cattle because the industry is too diverse and far-flung. Gasbarre is a world renowned parasitologist who retired from ARS.

Turns out, that notion was wrong.

Gasbarre's study offered the first published scientific documentation of this reality. It stemmed from his efforts to help a Midwestern backgrounder figure out why more of his cattle were getting sick and performing worse than normal. This is one of those outfits that does things right and is committed to the details of animal health. As for parasite control, this operation had employed intensive grazing management and strategically timed deworming for almost 2 decades. Yet, fecal samples in 2002 revealed parasitic infection.

Gasbarre and fellow ARS researchers found parasites resistant to the common anthelmintic classes currently used by cattle producers — avermectins, moxidectin and benzimidazole.

"What has been overlooked has been the high efficacy of the macrocyclic lactones (which include avermectins) and the implementation of more aggressive treatment programs aimed at increasing producer profitability by maximizing parasite control," Gasbarre writes.

"The use of highly efficacious, long lasting drugs coupled with very effective strategic dosing regimens has convinced many producers to use the drugs more frequently. Coupled with the demonstrated success and popularity of intensive rotational grazing programs, this has resulted in more producers using more drugs in systems that are designed to better utilize forage resources."

The problem is injudicious use of effective anthelmintics.

Matt Poore, an animal science professor at North Carolina State University (NC State) explains, "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 it's also an engraved invitation to increased resistance to the anthelmintic by gastrointestinal pests.

In 2007, Poore and Mark Alley, DVM, began working with the research herd in the university's Upper Piedmont Research Station at Reidsville, N.C., to understand what was happening. Alley is a clinical assistant professor and Extension cattle veterinarian with NC State.

Although weaned calves in the NC State herd had been dewormed, fecal sampling showed the calves were still shedding the eggs of intestinal parasites. "It used to be that when you treated cattle with an avermectin (a class of dewormer), you'd get 100 percent reduction in the egg count of gastrointestinal parasites," Poore says.

Poore and Alley decided to evaluate the efficacy of various anthelmintics. They 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-percent reduction in fecal egg counts, which is suggestive of resistance. We got 100-percent 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, N.C. All of the treatments reduced fecal egg counts by more than 90 percent.

Poore and Alley explain one of the challenges of Fecal Egg Count Reduction (FECR) tests is that there is no 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 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 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."

Keep in mind, resistance does not suggest some new super-worm. Instead, Dwight Bowman, professor of parasitology at Cornell University, explains, "Resistance is selected for because it is a genetic switch. You are not changing something in the worm. By giving dewormers, we are not inducing resistance, we are simply providing an environment for the resistant [parasites] to flourish and multiply."

Bowman was among the experts speaking at the Anthelmintic Resistance Roundtable in 2005. Intervet sponsored the event to prompt discussion about resistance, its diagnosis, economic impacts on the U.S. cattle business and potential solutions.

Managing against resistance

As evidence of parasitic resistance to anthelmintic resistance grows, so does the need to guard efficacy of the tools that have been so effective for so long. The primary safeguard begins with using current anthelmintics judiciously.

In addition to off-label use, 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."

At the Roundtable mentioned earlier, Gil Myers of Myers Parasitology Services at Magnolia, Ky., pointed out that there are 2 simple practices that producers can do to decrease the risk of anthelmintic resistance. "Know the weights of the animals you are treating, and make sure you are treating them with the proper amount of the drug," he said. "The literature clearly indicates that underdosing can lead to drug resistance."

Even before that, 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 to $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."

According to the NAHMS 2007-2008 Beef Report, during the 3 years prior to the report, only 5.7 percent of operations performed any testing of fecal samples to evaluate the parasite burden in their cattle. That percentage didn't vary statistically between regions or herd size.

"We see a great deal of herd variation, and this stands to reason because no two cattle operations are managed the same, nor is their history of dewormer use the same," Myers explained. "How producers handle the cattle, their stocking rate, age of cattle, pasture contamination level at the start of grazing and weather conditions all help determine parasite challenges more than location. It is generally accepted that the greater the parasite challenge, the harder it will be to have successful treatment. Preventing seasonal buildup of parasite contamination is important to preventing parasite resistance from developing within the animals."

"It is likely that the widespread use of the avermectins for more than 2 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 O. ostertagia, but rather less pathogenic forms, which, because of their drug resistance patterns, thrive in particular production systems," Gasbarre explains in the ARS study. "This will challenge parasitologists to draw less on generalizations about parasite transmission profiles, and instead develop approaches more tailored for a given production location."