Prevent Parasite Resistance

March 25, 2010 07:22 AM


Bonus Content:

Strategic Deworming Resources

Video: Learn more about hte cost/benefits of strategic deworming from Harold Newcomb, technical service veterinarian for Intervet/Schering-Plough Animal Health.

Link to USDA National Animal Health Monitoring System's Parasites on U.S. Beef Cow-calf Operations, 2007–08.

Link to USDA National Animal Health Monitoring System's Parasite Control Practices on U.S. Cow-calf Operations, 2007–08.

Consider your deworming strategy an investment. In order to get the most benefit, you have to carefully consider treatment timing and frequency for that investment to pay dividends. There are indications, however, that producers may not be getting the most from their investment and, over time, this could lead to the development of parasites resistant to current deworming products.

In December, USDA's National Animal Health Monitoring System released the results of its survey of cow–calf operations across the U.S. in regard to parasite control practices. The survey found that among operations with unweaned calves or weaned stocker calves, more than half usually deworm these animals one or more times per year (53.7% and 54.1% of respondents, respectively). About 7 out of 10 operations (69.5%) deworm replacement heifers one or more times per year, and 8 out of 10 operations (81.7%) deworm cows one or more times per year.

The survey also revealed that only 5.7% of the operations perform any testing on fecal samples to evaluate parasite burdens in cattle.

These results confirm that animals are most often dewormed at the start of the grazing season and at the end, but this may not be sufficient for parasite control, according to W. D. Whittier and John F. Currin, Extension specialists at Virginia-Maryland Regional College of Veterinary Medicine.

"One deworming in the spring is not cost-effective because it does not prevent a buildup of the worm burden later in the grazing season. Deworming in the fall may prevent the ‘sleeping' larvae from doing damage the following spring. However, this is only the case if the right drug is used and cattle are kept off contaminated pastures following deworming,” Whittier says.

Studies show that strategic deworming programs can provide an extra 30 lb. to 100 lb. pounds of gain per grazing season, Whittier says. But that's dependent on using the right dewormer at the right time for the type of cattle you are raising.

Implement an effective strategy. While deworming treatments focus on the animal, an effective strategy also looks at the pasture and timing of the parasite's life cycle. Strategic deworming is one way to accomplish this. It involves developing a program that maximizes the economic benefit of deworming cattle while also removing the larvae from infected pastures.

"The main problem is out there in the pastures,” says Harold Newcomb, technical service veterinarian for Intervet/Schering-Plough Animal Health. "If you target the animal only, without recognizing what's out in the pasture, you're not going to have successful deworming.”

Understanding the life cycle of the parasite can help you break the cycle of infestation and get the most effective treatment out your products. It also helps prevent the development of parasite resistance through ineffective use of products.

Younger animals tend to be more susceptible to parasites, so you may need a different treatment protocol for younger animals such as heifers and stockers, versus older animals in the herd.

Cows, for exam-ple, may be fine with a treatment at spring turnout. But younger animals may require a treatment (possibly with more than one product) at spring turnout, again in the summer to break the parasite life cycle and then a follow-up treatment with another class of product in the fall. Your veterinarian or state parasitologist can help determine specific strategies.

Stocker facilities. Tom Craig, Texas A&M University parasitologist, says stocker operators may also want to check their facilities for parasite control.


Younger animals are more susceptible to parasite infestations and may require a different treatment program than adult cattle in the herd.

"When stockers are commingled on arrival, treated with an anthelmintic and kept in a small receiving area, there are indications this could lead to developing of resistant parasites through the years of using those same pens,” Craig says.

Cattle will avoid grazing in areas where they defecate, he says, unless they have no choice. That's where overgrazing or grazing in areas following a drought could lead to higher populations of parasites. Also, location will determine parasite infestations—temperature, humidity and types of parasite must all be evaluated to determine the most effective deworming strategy.

Test fecal samples. Testing fecal samples accomplishes two things. First, it allows you to determine the number of parasite eggs present in the herd. Second, testing before and after treatment can help determine the efficacy of a treatment as well as indicate if there might be a resistance issue.

Craig recommends testing 20% of the herd, with fecal samples about the size of a fist. Samples from each animal should be kept separate, in individual bags. Craig says he has seen producers turn in "composite” samples from a number of animals, but that won't help to determine product efficacy.

After you take samples, treat the animals and send the samples to a lab as soon as possible. Wait 14 days and gather a second round of fecal samples for analysis.

The fecal sample comparisons will look at whether or not parasite egg counts were reduced. A 90% or greater reduction indicates the product is working. If not, follow up with a coproculture, which grows the worms to the infective stage to identify which parasites are in the herd and pastures. A parasitologist can then evaluate whether resistance is an issue.

Use effective products. Just as with other animal health products, basic storage instructions must be followed to ensure the efficacy of the product. Mac Devin, professional services veterinarian with Boehringer Ingelheim, offers these tips:

  • Watch expiration dates.
  • Store products according to label directions. Avoid extreme heat or cold conditions.
  • Use products obtained from reputable sources.
  • Follow label indications for dosages. Know animal weights to determine the proper dose. However, if you can't weigh individual animals, treat based on the heaviest weight, not the average. Treating to the average weight means some of the animals will be underdosed, resulting in lower performance due to remaining parasites.

Consult with your veterinarian. "We recommend producers work with their veterinarian to determine the most effective products and the times of year to treat,” Newcomb says. "Their veterinarian probably knows their situation best and can help tailor a parasite program that works best for them.” BT

To contact Kim Watson-Potts, e-mail


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