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Parasite resistance to anthelmintics grows across North America

by Wyoming Livestock Roundup

In the last 10 years, the efficacy of macrocyclic lactones, a class of drug used to treat parasites in cattle, has decreased, and retired research leader of the Bovine Functional Genomics Laboratory of USDA’s Agricultural Research Service Louis Gasbarre says, “The first real signs of anthelmintic resistance were occurring at the end of the last century.”
    “I think we’ve become complacent because we had an almost unprecedented run of about 30 years where we had a variety of very, very effective anthelmintics with very low toxicity and high efficacy,” explains Gasbarre. “This subject is very appropriate for what is going on in the cattle industry today.”
Recognizing the problem
    Gasbarre notes that when one producer who utilized a strategic and previously effective parasite control program started seeing his cattle decrease in productivity and others die in the pasture, he contacted the USDA.
    “The animals were necropsied and came back diagnosed with parasitic gastroenteritis,” he says. “At that point, parasites were being controlled almost exclusive with macrocyclic lactones.”
    Though the incident seemed isolated, the intensive management of the operation led Gasbarre’s team to begin a research study on the property, treating five groups of cattle with different treatments and leaving one untreated.
    “From the macrocylic lactones, we had about 100 eggs per gram,” explains Gasbarre of the results of the first fecal egg counts of the study. “This is a classic negative binomial distribution, and it is very characteristic of parasites.”
    The following year, the team conducted the study again, increasing the number of animals necropsied to look at worm recovery.
    “We were convinced that there were significant worm populations surviving treatment,” he notes. “With the exception of one group receiving a combination therapy, we had lots of worms left after treatment.”
    “We are leaving large populations of worms behind, and it appears the bulk of the worms are small intestinal worms – not the classic parasite we have dealt with for years and years,” Gasbarre adds.
Increasing prevalence
    Another case popped up at West Virginia University’s Extension research facility that also showed fecal egg reduction of less than 50 percent, which is significantly less than expected.
    “The important thing about this study was that it wasn’t in one particular operation, it was across the state of West Virginia,” he explains.
    By the end of 2009, Gasbarre’s lab had identified parasites with a true resistance to macrocylic lactones in Kansas, North Carolina, South Carolina, Wisconsin, West Virginia and Wyoming, and suspected resistance in Alabama, California, Nebraska, North Dakota, Iowa, Tennessee, Texas and Virginia.
    “We aren’t just looking at one part of the country,” adds Gasbarre. “We have it in the Southeast, where everyone thought we would find it, but also in the Northern plains and West coast.”
Parasitic culprits
    “If we look to see what was surviving, we were seeing Cooperia punctata, which was somewhat surprising,” explains Gasbarre. “Until this point, C. punctata was a minor parasite pretty much restricted to warmer climates.”
    The researchers expected to see Ostertagia, Cooperia and Haemonchus species, but C. punctata was the only species surviving treatment with an anthelmintic.
    “Cooperia has now become the dominant parasite in North America, largely because it is showing very strong levels of resistance to treatment with macrocyclic lactones,” Gasbarre says.
    He also adds that Nematodirus populations in young animals are becoming increasingly prevalent because there is little competition in the gut.
    “We are removing other nematode species, so we are getting an overgrowth of this parasite in the clean gut,” comments Gasbarre.
Treatment paradigms
    Because parasite treatment strategies have focused on the control of Ostertagia species, strategic deworming protocols dictated treatment early in the growing season, during the optimal transmission phase for the species.
    “If you look at C. punctata, it gets going most heavily in the summertime. If animals have been treated with a drug in April or May, that drug is long gone by the time those worms show up,” Gasbarre says. “All of our treatment programs have missed Nematodirus, as well.”
    With deworming treatment that remove other gut flora, Cooperia or Nematodirus species, depending on the area, are left without competition to grow uninhibited.
Shifting the strategy
    “We can’t build on a repeated administration of a single drug,” he mentions. “We need to be doing a lot of things, including integrating management of the host immunity and smart use of drugs.”
    To address increasing immunity of parasite to anthelmintic drugs, Gasbarre says that producers should work with their veterinarians to look at the timing of treatments, the parasite of focus and monitoring.
    “Most importantly, we cannot assume treatments have solved a problem. We’ve got to look behind and check the efficacy of treatments,” he comments. “We can’t build on a single class of anthelmintics. We’ve got to start looking at a combination. A strategic worming is a good idea, but programs have to be sustainable over time.”
    “The parasite populations are continuing to change,” Gasbarre emphasizes. “We need to monitor that change, and we need to change to meet new challenges.”
    Gasbarre presented this information in a webinar sponsored by Merck Animal Health and Bovine Veterinarian at Saige Albert is editor of the Wyoming Livestock Roundup and can be reached at

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