"If parasites are such a health threat to my cattle, why not deworm all of them?" Here’s why you shouldn’t.
By Tom Van Dyke, Merial Veterinary Services
If parasites are such a health threat to my cattle, then why wouldn’t I want to deworm all of them? This is a logical and typical follow-up question by producers learning about the concept of refugia. Now, instead of eliminating all internal parasites, maintaining refugia means intentionally allowing some worms to survive.1,2,3
Internal parasites have long been recognized as one of the most costly health challenges in the livestock industries.3,4,5 Dairy calves and replacement heifers on grass are at greatest risk from the costly negative effects of internal parasites.6,7 Parasitism reduces weight gains in dairy calves and yearlings, affects body condition scores, delays time to first breeding and impairs milk production during first lactation.6,7
Dewormers, or anthelmintics, have been recognized as one of the most cost-effective pharmaceutical technologies.5 Anthelmintics have improved in spectrum and efficacy over time.3 There is a growing concern that repeated use of products from the available dewormer classes may lead to widespread parasite resistance.1,2,4,5,8
Resistance means that an increasing percentage of parasites in a population survive treatment with a dewormer that has been effective in the past. With each successive treatment, a few resistant parasites may survive to pass on their genetics to the next generation. After repeated treatments, the percentage of resistant parasites on a farm may outnumber the more susceptible ones, and then the dewormer loses effectiveness.1,3,4,8
Refugia is being advocated as an important tool to slow the progress of resistance and maintain effectiveness of the dewormers currently available.1,2,3,4 Refugia is the part of the total parasite population which is not exposed to the antiparasitic drug being used. The worms, and their genes, in "refuge" could hide out in non-treated animals, eggs and larvae on the pasture or parasite stages not reached by a specific dewormer.1,2,4 If there is sufficient refugia and if a very effective dewormer is used, then the proportion of resistant genes in the population can be kept low.1,9 The resistant parasites are diluted by susceptible parasites. Animals will more likely be re-infected with susceptible parasites and continue to shed more eggs from susceptible worms back to the pasture. The dewormer remains effective.1,9
Studies and computer models where 10% of the sheep in a herd were left untreated indicated that a significant delay in resistance development can be accomplished.9 Refugia in herds of sheep can also be maintained by using a method to detect and treat only the severely infected animals.1 But cattle are not sheep, and the primary parasites of concern are different between the species.
Management practices used to minimize the effect of parasites and resistance may be similar, but don’t translate exactly from sheep to cattle.10
Unfortunately, there is not a total consensus among the parasitology experts regarding the best practices to forestall anthelmintic resistance.1,3,4,8,9,10,11
So what does all this mean to the producer who wants optimal health and production for all cattle while seeking to maintain efficacy of the dewormers? "How many untreated animals are needed for refugia? Which class of animal needs treatment the least? On which class of animal does treatment have the biggest impact? Which class is the cheapest to treat? Which is the easiest to treat?"
These are all questions South Dakota State University’s Dr. Mike Hildreth suggests will need to be answered at the individual farm level.11
Merial is a world-leading, innovation-driven animal health company, providing a comprehensive range of products to enhance the health, well-being and performance of a wide range of animals. Merial employs approximately 6,200 people and operates in more than 150 countries worldwide with close to €2 billion of sales in 2013. Merial is a Sanofi company. For more information, please see www.merial.com.
1 FDA.gov. Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It. Helpful Information for Veterinarians. Available at http://www.fda.gov/downloads/AnimalVeterinary/ResourcesforYou/UCM347442.pdf. Accessed January 39, 2014.
2 Beef + Lamb New Zealand. Worms in refugia as a tool to delay drench resistance. Beef + Lamb New Zealand R & D Brief; Number 130: August 2007. Available at http://www.beeflambnz.com/Documents/Farm/Managing%20drench%20resistance.pdf . Accessed June 11, 2014.
3 Wolfgang D. Can Parasite Control be More Effective and Reduce Resistance at the Same
Time? Penn State Extension. April 24, 2013. Available at
http://extension.psu.edu/animals/dairy/news/2013/can-parasite-control-be-more-effective-and-reduce-resistance-at-the-same-time. Accessed June 11, 2014.
4 Navarre CB. Best Management Practices: Internal Parasite Control in Louisiana Beef Cattle. June 2013. Available at http://www.lsuagcenter.com/NR/rdonlyres/6BC02F27-60B6-43BA-B2FF-DD5F58FD1F9C/91919/BestManagementPracticesParasitesControlinBeefCattl.pdf. Accessed June 11, 2014.
5 Lawrence JD, Ibarburu MA. Economic analysis of pharmaceutical technologies in modern beef production in a bioeconomy era. 2007. Iowa State University. Available at http://ageconsearch.umn.edu/bitstream/37560/2/confp05-07.pdf. Accessed June 11, 2014.
6 Elsener J, Villeneuve A, DesCoteaux L. Evaluation of a strategic deworming program
in dairy heifers in Quebec based on the use of moxidectin, an endectocide with
a long persistency. Can Vet J 2001;42;38-44.
7 Boyles S, Johnson LJ, Slanger WD, Dreft BJ, Kirsch JD. Effect of Deworming Heifers on Gain and Reproduction. NDSU Institutional Repository. Farm Research: Vol. 49, No.6(Winter 1992-1993)
8 Cima G. Worms’ adaptation a critical problem. JAVMA news, May 01, 2013. Available at c. Accessed June 11, 2014.
9 Leathwick DM, Waghorn TS, Miller CM, Candy PM, Oliver MB. Managing anthelmintic resistance - Use of a combination anthelmintic and leaving some lambs untreated to slow the development of resistance to ivermectin. Vet Parasitol. 2012;187:285-294.
10 Maday J. Parasitologist Gasbarre response to FDA report. Drovers CattleNetwork. April 29, 2013. Available at http://www.bovinevetonline.com/news/Parasitologist-Gasbarre-responds-to-FDA-report-205302581.html. Accessed January 30, 2014.
11 Hildreth M. Trichostrongyle Parasite Management in Cows, Calves and Stockers from the Northern Great Plains. Presented Merial Veterinary Symposium; Sioux Falls, South Dakota. January 24, 2014.