Repetitious use of the same deworming drugs has led to a growing number of beef herds showing signs of parasite resistance. A recent set of trials conducted over two years by Mark Alley, DVM, at North Carolina State University’s College of Veterinary Medicine and Matt Poore, PhD, from the Department of Animal Science at North Carolina State University, presented further evidence that resistant worm populations are increasing and the efficacy of popular deworming drugs like avermectin pour-ons is declining. The research findings were presented at the 2009 World Association for the Advancement of Veterinary Parasitology (WAAVP) meeting in Calgary.
When calves placed in a weaning feed trial at North Carolina State University’s Upper Piedmont Research Station (UPRS) in Reidsville, N.C., failed to respond to their genetic potential, random fecal samples taken from the calves showed that they were still shedding a lot of parasite eggs, despite recently having been dewormed.
Alley and Poore set out see if the problem existed in a few calves or if it was a problem in the entire group. In doing so, they decided to evaluate the efficacy of four cattle anthelmintic dewormers, including a generic ivermectin pour-on, Ivomec® pour-on, injectable Ivomec®, and Safe-Guard® (fenbendazole) drench.
Using the Fecal Egg Count Reduction Test (FECRT) protocol used by U.S. Department of Agriculture’s National Animal Health Monitoring System (NAHMS), the researchers pulled fecal samples from 25 calves assigned to receive each product plus an untreated control group. They then applied the assigned dewormers based on exact weight and then pulled fecal samples from each of the calves 14 days later.
“Post treatment, the average egg count from each group varied depending on which product was used,” noted Alley. “In the generic topical, brand-name topical and ivermectin injectable groups, there was less than 80 percent reduction in eggs present after treatment. The fenbendazole treated group had 100 percent reduction in eggs present.”
Parasite Identification
The fecal samples were sent to USDA’s ARS Bovine Functional Genomics Lab in Beltsville Ma., where, in addition to the fecal egg count reduction test, parasite identification tests using the latest polymerase chain reaction (PCR) DNA technology were performed. In the pre-treatment groups, PCR tests confirmed the presence of haemonchus, ostertagia, and cooperia parasites in all groups. Post treatment, PCR tests found cooperia still present in all three avermectin groups and the control, but none in the calves treated with fenbendazole.
Cooperia’s presence in the avermectin treated groups indicates that repeated use of dewormers in the same drug class over time may have resulted in an increasing population of parasites that are resistant to that family of anthelmintic.
Testing is key
Alley and Poore repeated the study at another university farm – the Center for Environmental Farming Systems – in North Carolina. Prior to the test, the mature cows on this farm were only dewormed if testing showed an elevated worm count, resulting in very low product use over time.
In the second study, every product group except for the control showed a greater than 90 percent reduction in average egg counts after treatment.
“That led us to believe that these potential issues are farm specific, and producers need to evaluate what is going on with the parasite populations on their specific farm,” explained Alley. “As a result we will work to identify farms that need to do fecal egg count testing to give them an idea of how their particular deworming program is working, which will allow us to better customize a control program for that farm. Further research is needed to determine the most economic parasite control program for each farm.”
Source: www.cattlenetwork.com