These effects can ultimately impair physiological parameters of the animal and negatively affect daily weight gain, weaning weights, milk production, feed efficiency and reproductive performance.

To date, we’ve largely been able to avoid losses in productivity through the routine application of dewormers and implementation of other control measures. In fact, the belief that anthelmintic treatment improves numerous aspects of cattle production is widely accepted.

However, due to modern management practices (e.g., higher stocking density, intensive grassland management, etc.), gastrointestinal (GI) parasites potentially represent a greater threat than ever before.

Following the advent of modern anthelmintics, there was a notable shift toward the use of chemical compounds as a predominant measure for GI nematode control. Early on, products were used in salvage attempts to save the life of the parasitized animal; today, however, the routine use of dewormers has become commonplace, ingrained in many operations.

The development of generic formulations of certain anthelmintics has further contributed to their appeal and frequency of use, and has made them not only cheap to produce but also easy to obtain. As time goes by, what effects will continued use of dewormers have on GI nematode populations? And what risks will we encounter if we continue using dewormers in the way we are currently?


Anthelmintic resistance in the U.S. cattle industry

One risk of the indiscriminate use of anthelmintics is the potential for development of anthelmintic resistance. Anthelmintic resistance has been described as a measureable decrease in the efficacy of a compound against the targeted parasites.

Giving dewormers does not induce resistance in GI nematodes, but it does provide a selective environment in which nematodes with the already-present genetic capability to withstand the compound will survive.

The susceptible parasites are, consequently, wiped out of the population. Thus, eggs passed onto pasture will be from the resistant population. With time, the resistant population on pasture will outnumber the susceptible population and anthelmintic efficacy will suffer.

Anthelmintic resistance may not be initially obvious; it takes time to develop and to become evident. Producers may slowly begin to notice that, after a period of quality grazing, calves are not gaining as well as they have in the past or are not responding to dewormers in the way they once did.

These kinds of scenarios will hit the individual producer very hard. Producers may think using increasing amounts of anthelmintics will fix the problem, convinced of the profitability of treating, but this approach will only compound the issues as increased frequency of use and increased dosing have been shown to select for resistance.

There continues to be more and more anecdotal evidence that treatments are failing or at least not living up to producers’ expectations. Ultimately, individual producers will need to evaluate whether their own program is performing up to their standards and expectations.

If it is working, will it be sustainable over the next five, 10 or 20 years? And, if it’s not working – why not? Is the failure related to the deworming protocol, or is there an underlying issue with the effectiveness of the dewormer itself?

For example, a failed response to treatment could be due to improper dosing practices rather than an ineffective product. In the end, long-term control will need to be carefully balanced with the immediate and underlying goals of the operation.

Which worms will be most problematic?

In the past, the prevailing opinion was that anthelmintic resistance would be slow to arise in cattle if it appeared at all. However, reports of anthelmintic resistance in the U.S. began surfacing as early as 2003.

In 2009, researchers confirmed the presence of resistance in GI nematodes in cattle originating from the southeastern U.S. and shipped to a stocker operation in Wisconsin. In 2010, resistance in cattle that were shipped to an Idaho feedlot from California was documented.

In 2012, researchers showed resistance in nematodes of cattle obtained from Arkansas and Oklahoma and shipped to a Texas feedlot.

Resistance to one or more compounds has been reported for most of the major genera of GI nematodes in the U.S. cattle industry. The five major genera of bovine strongyles are Ostertagia, Haemonchus, Trichostrongylus, Cooperia and Oesophgostomum. The development of resistance may be more problematic in certain of these genera than in others because of their pathogenicity.

For example, morbidity and mortality could particularly affect the cattle industry if resistant populations of Ostertagia ostertagi become predominant.

Many may wonder why it is important to monitor resistance at all when it doesn’t seem to be directly affecting them. The effects of resistant populations will become evident in time, and producers will be directly affected. It is important to remember that resistance is not restricted by geographic location or type of operation.

Individual farms must critically evaluate their control practices and take necessary steps to prevent resistance from developing in their herd. While it is impractical to think that GI nematodes can be eradicated in cattle, determining which ones are present in the herd, as well as which ones remain after treatment, will allow for more optimally designed control programs to achieve both biological and economic balances over time.

How do I determine presence in my herd?

Previously, to determine which GI nematodes were present in a herd, eggs were recovered from the feces of cattle and differentiated through larval culture and identification, a process that took two to three weeks. The use of molecular tools has revolutionized how the identification of GI nematodes is now approached.

At the Colorado State University Veterinary Diagnostic Laboratory, we provide a 3+2 PCR test capable of differentiating the five major genera of ruminant strongyles: Ostertagia, Cooperia, Haemonchus, Trichostrongylus and Oesophagostomum. Results are available in as few as two to three days.

Samples can be submitted prior to treatment to determine which GI nematodes are present in the herd, or after treatment, to find out which ones are left behind.

Our test allows for the pooling of up to five individual samples, combined for a composite PCR. The three-portion of the test detects the genera considered to be the most problematic: Ostertagia, Cooperia and Haemonchus. The two-portion of the test can be added on request, for the detection of Trichostrongylus and Oesophagostomum.  end mark

Ashley. K. McGrew is a post-doctoral fellow at Colorado State University’s College of Veterinary Medicine and Biomedical Sciences, Colorado State University. Kraig Peel is associate professor of ag sciences at CSU. L.R. Ballweber is professor of clinical parasitology at the CSU College of Veterinary Medicine and Biomedical Sciences.

References omitted due to space but are available upon request. Click here to email an editor.

Photo courtesy of Paul Marchant.

How to submit samples
To submit samples to CSU’s Veterinary Diagnostic Laboratory, 3 to 10 grams of feces need to be collected per rectum or immediately after being passed. In order to adequately represent the herd, collect samples from 17 to 20 individual animals. Include egg counts, if known.

Place each sample in a clean enclosed container. A plastic, sealable bag serves as a good container. Be sure to squeeze out as much air as possible before sealing. If samples cannot be shipped immediately, storage overnight in the refrigerator is acceptable until packaged for shipping.

Ship samples cold but do not allow them to come into direct contact with ice. Arrange for the samples to be received within two days. Please feel free to contact the CSU Veterinary Diagnostic Laboratory if you have any questions.

Send samples to:
Fort Collins Veterinary Diagnostic Laboratory
300 West Drake Road
Fort Collins, CO 80526
Phone: (970) 297-1281
Fax: (970) 297-0320

Colorado State University Veterinary Diagnostic Laboratories