Anthelmintics (dewormers) are among history's miracle drugs. Since first introduced in the 1960s, these products have literally helped feed a growing planet by increasing the efficiency and sheer volume of cattle, sheep and goat production.

Much like with antibiotics or Roundup-Ready technology, however, maximum effectiveness is considered to be a fleeting thing. The process of natural selection in the target population eventually leads to lessened effectiveness as populations resistant to the formulations develop over time.

Much documentation exists of resistance among target populations of antibiotics used in human medicine, as well as the target organisms of Roundup-Ready technology in crop production. There's no debate that such development of resistance will come with anthelmintics, but the question today is whether that “when” is “now.”

Where small ruminants are concerned, the consensus is that it's here. Large penetrations of resistance have been documented in sheep and goat herds worldwide, as well as in the U.S. Where cattle are concerned, the situation isn't as numerically alarming — there is only one well-documented cattle case in the U.S.

Which leads some folks to contend that any crisis in U.S. cattle is 15-20 years away and producers are being panicked unnecessarily. Louis Gasbarre, research leader at the U.S. Agricultural Research Service's Bovine Functional Genomics Laboratory in Beltsville, MD, isn't among them. What's at stake, Gasbarre says, is the future access to these important livestock-production tools.

“If we don't address the issue that there is a potential problem today when we have our first examples of it, hiding our heads in the sand will just perpetuate it and make the problem worse, to the point where we won't be able to manage it.

“The prudent thing to say is we know it exists, it's been demonstrated; the extent is unknown, but being looked at as we speak. This information will tell us how to better manage these products so that we can keep the drugs we have, because there isn't much else out there in the pipeline, particularly in the pour-on formulations.”

The practice of deworming livestock received a big boost in the early 1980s with the commercial arrival of ivermectin, the leading product in a class of anthelmintics known as macrocyclic lactones. This broad-spectrum anthelmintic was effective against both internal and external parasites across many species, even in humans, where it was heralded for its effectiveness in eliminating River Blindness in the Third World.

In animals, its topical application made it convenient to use, and it soon dominated the market worldwide. When ivermectin went off patent in 1996, it spawned a whole slew of generic formulations with a resulting dramatic drop in dosage price.

Ivermectin was tailor made for production systems where livestock are intensively stocked and grazed. Administration was cheap, easy and convenient, particularly in year-round grazing, multiple-treatment situations for sheep and goats, where the first resistance problems began to occur.

A Wisconsin wakeup?

While anthelmintic resistance is well documented throughout the world in small ruminants, widespread resistance in cattle nematodes has been confirmed only in New Zealand, with cases also reported in South Africa, England and South America. The wakeup call concerning U.S. cattle occurred three years ago when a stocker operation in Wisconsin intensively grazing Southeast calves found significant numbers of parasites resistant to a battery of macrocyclic lactones.

A resulting two-year test period utilizing fecal egg counts to measure efficacy and resistance found that:

  • The major species present in both years were Haemonchus contortus and Cooperia punctata.

  • H. contortus was resistant to all the macrocyclic lactones and the bendzimizoles tested.

  • Cooperia punctata and H. placei were resistant to all the commonly used macrocyclic lactones.

More recently, preliminary data from a university bull test in West Virginia has shown similar results, with efficacy of ivermectin falling anywhere from 20-70%, depending on the source of the cattle.

“People expected this to happen at some point,” Gasbarre says, “but it's scary in that it came about so quickly.”

High stakes

James Hawkins, DVM, PhD and cattle parasitology consultant for Merial, originator of ivermectin, suggests that the Wisconsin situation is not representative of the U.S. cattle industry. For one thing, he says, U.S. producers deworm their cattle “on average” slightly more than once annually, not multiple times during a grazing season, as in the Wisconsin case.

He says the southern-origin calves were raised on pastures intensively grazed for a number of years and placed with contract graziers on a gain basis. Pushed for maximum gain, the cattle were wormed as many as five times during the grazing period prior to shipment to Wisconsin. Once in Wisconsin, the cattle were grazed under intensive stocking in a rotational-grazing system, and dewormed several more times.

“If you're a producer with fairly standard management and treat 2-3 times annually, there should be no concern,” Hawkins says. He adds that Merial offers a satisfaction guarantee on all its Ivomec and Eprinex products, and will work with producers' veterinarians and the nearest veterinary school to help solve any problem that may develop.

The resistance stakes are high. Bert Stromberg, a University of Minnesota professor of veterinary pathobiology, says the effects of clinical parasitism are easy to see — diarrhea, bottle jaw, anemia and rough hair coat. But subclinical parasitism is more insidious, chipping away with reductions in weaning weight, milk production, reproductive performance and growth, not to mention increased susceptibility to disease. All told, industry estimates of losses due to subclinical parasitism in cattle are $190/head, Stromberg says.

In today's climate of high gain costs, Stromberg says, it's imperative that cattle be able to perform to their genetic ability. An effective parasite-control program is part of that.

Gasbarre is quick to reassure producers that resistance is not epidemic, though there are more reports from the countryside of what he terms “treatment failures.” It's dependent on a number of factors, he says, including frequency of use of the product, the stocking density and worm load of the pastures. More southern latitudes, which tend to be warmer, wetter, greener and have longer grazing seasons, are typically at more risk than northern latitudes.

“Areas where you essentially have year-round grazing, which means there is no rest period so the drugs have to be used more times per year, are more at risk. In addition, these areas use pastures more intensively,” he says.

Fecal egg count

“From a producer's standpoint, the chance of seeing resistance is going to be directly related to how often and on what proportion of the animals a drug is used. So if you use it more often and on a higher percentage of the animals ? in other words, if you're deworming everything ? your chances of seeing resistance are going to be higher,” Gasbarre says.

Still, Gasbarre advises producers to not panic. “If your program is performing the way you expect it to perform, then I would not do too much extra, except think about ways that you can perhaps cut down the selective pressure.

“But if you're using a dewormer and not getting the control that you're used to seeing — or expect to see — then you need to evaluate whether that's because of the way you're deworming, or because the dewormer isn't working as it should.”

One diagnostic is a fecal egg count reduction test (FECRT), Stromberg says. Offered as a free service by most commercial anthelmintic companies, an FECRT utilizes fecal samples taken prior to, and 14 days following, anthelmintic treatment. It's a standard protocol to quantify parasite burden and possible anthelmintic resistance in the cattle industry. Gasbarre calls it “the gold standard and the only non-lethal method” for determing anthelmintic efficacy.

FECRT is also a component in the 2007 National Animal Health Monitoring System (NAHMS) Beef 2007-08 Study currently underway. Established in 1990 by USDA's Animal and Plant Health Inspection Service, the charge of NAHMS (http://nahms.aphis.usda.gov/) is to collect and report accurate information on animal health and management in the U.S.

Producer options

As part of its latest survey focused on the cow-calf segment of the beef industry, the main results of which are due in June, NAHMS hopes to quantify parasite burden and evaluate response to deworming in the cattle herds of its producer-volunteers. An expected 200 volunteer participants will submit samples on 6- to 18-month-old calves for FECRT analysis to one of three laboratories — the University of Minnesota, Colorado State University and Beltsville, MD.

Dave Dargatz, a DVM and NAHMS beef cattle specialist, says the results will provide input in formulating future research priorities in a number of areas including internal parasites on cow-calf operations. He says samples will be submitted through December 2008, with a full analysis of the results expected in early 2009.

Gasbarre says he's excited by the added knowledge the project will bring to the issue. “It's the first time we will have a large enough, and representative enough, sample to address the U.S. industry as a whole,” he says.

So what should producers do?

“If a producer doesn't think he's getting the production bounce on cattle that previous deworming has had, he needs to contact one of the commercial firms that offer free FECRT evaluation, his state or local parasitologist or myself about evaluating the product,” says Gasbarre, who can be reached at 301-504-8509 or Louis.Gasbarre@ars.usda.gov.

If results indicate their worming product isn't performing to its expected level of activity, it will “take a little bit of finesse and skill in determining what works for a given locale,” Gasbarre says. “Among the mistakes we've made in the past is telling producers there's one program that will work for everyone.”

The objective is to use the drugs judiciously, and only where needed.

“Whenever possible, producers need to balance the sustainability of their program with immediate, short-term production goals. There are times in the production scheme when the system should be left to rest,” he says. That might include moving cattle to residue areas or pastures that haven't seen much grazing pressure, or that haven't hosted highly susceptible animals.

“You can change the drugs you're using at times to again cut down on the pressure with one drug. You can treat parts of the herd at times. For instance, in the case of a cow nursing a calf,” he says.

He adds that producers in general need to perhaps “rethink some of the assumptions of modern beef production.

“As an industry, we've gotten kind of lazy, opting to let the drugs do everything rather than paying attention to some management factors,” he says.

“Instead of saying that we're going to do it all with the bottle, there are management practices we can utilize to keep parasites down on pastures, things we rarely see done any more since the advent of inexpensive, broad-spectrum drugs.

“You don't see people dragging pasture to break up fecal pats when it's dry, for instance. And you don't see people rotating different groups of animals onto pastures to try to cut down transmission.

“I think there are a lot of things we can do, but we definitely want to be careful about oversimplifying the situation by saying there is one answer for every locale.”

In some cases, switching drugs once in a while is good; in other cases, it won't have any effect at all, Gasbarre adds. In some cases, research shows there are situations where combinations of products have been very effective (see “Research Roundup,” page 87).

“We also need to differentiate in terms of practices that might help spread the resistance that's already out there. For instance, the chance of something spreading from a feedlot is extraordinarily low, because the parasites won't be carried to someone else's pastures. On the other hand, if you're selling calves or a cow, remember that what you're doing could have a tremendous effect on the next guy in the chain.”

So, as Ebenezer Scrooge asked: “Are these the shadows of the things that will be, or are they shadows of things that may be, only?”

Gasbarre says: “Most people don't remember what they did before they had these important management tools. If we act judiciously, we can maintain them.”