Managing resistance to internal parasites in cattle

Producers need to tailor parasite control to align with their herds and management systems

Managing internal parasite resistance starts with asking the right questions and understanding the principles of antiparasitic resistance and the range of control products. From there, a producer needs to develop a deworming strategy and pin it to the spring and summer grazing calendar. A veterinarian can help.

Antiparasitic resistance is typically defined as the genetic ability of parasites to survive the effects of an antiparasitic drug to which they were previously susceptible. Antiparasitic resistance becomes a problem when an increasing percentage of the parasite load carries resistance genes, decreasing the efficacy of control products and increasing the production-limiting influence of internal parasites.

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The performance of cattle on pasture often depends on the availability of effective broad-spectrum anthelmintics to remove, or prevent infection with gastrointestinal nematodes. The degree to which internal parasites affect performance depends on the age of cattle entering pastures, the number of parasite eggs that overwintered on pasture, the parasite load and degree of egg shedding from animals moving onto pastures, time of year, weather, overall effectiveness of the deworming product being used and the application timing.

Controlling nematodes resistant or partially resistant to commonly used anthelmintics adds another dimension to deworming practices, that being the preservation of effective drugs. Preservation means being astute about the timing of administration of deworming products and using appropriate combinations of products each season. Again, veterinary input is important.

Resistance to antiparasitic drugs has developed more slowly in cattle than in small ruminants (sheep and goats). However, the number of reports in scientific literature over the past five years suggests a rapidly escalating problem in cattle, especially with some species of economically important parasites. Australia, New Zealand, South Africa and South America have struggled with antiparasitic resistance in livestock species for several decades. Recent scientific data indicate significant antiparasitic resistance is emerging in livestock species, including cattle, in North America.

Genetic mutation, environmental influences, egg production and the parasite’s ability to survive and reproduce contribute to resistance. Management practices can also increase or decrease selection pressure for resistance. The following practices can be factors in increasing resistance:

  • Treating too often.
  • Treating the entire herd at the same time.
  • Treating animals when there are few eggs on pasture such as after a harsh winter or hot, dry summer boosts the proportion of resistant eggs.
  • Failure to isolate and deworm new additions and cull heavy shedders. Eighty per cent of eggs are shed by 20 per cent of the animals in a herd.
  • Failure to monitor egg counts.

Drug factors — under-dosing, using the wrong drug or failing to use combinations of drugs when indicated — all contribute to the development of antiparasitic resistance.

One important aspect of the drug’s behaviour is whether it’s short- or long-acting. Long-acting drugs that persist in the animal’s body a long time after initial treatment tend to select for resistance more quickly than short-acting drugs. Long-acting drugs may “fuel the fire” of antiparasitic resistance by not only putting selection pressure on the parasites present in the animal at the time of treatment, but also on parasite larvae ingested after treatment. Also, as the level of a long-acting drug declines in an animal’s body over time, parasites are exposed to gradually decreasing drug levels, which may accelerate the development of resistance.

Remember: “The solution to pollution is dilution” and “dilute the undesirables” by using antiparasitic drugs along with non-chemical control methods such as good pasture management practices, adequate isolation and treatment of new arrivals, and routine culling of heavy egg shedders. These principles may not mesh with traditional management practices on some ranches.

One can also consider preserving refugia. Refugia represent the proportion of the total parasite population that hasn’t been selected for antiparasitic drug resistance — essentially, parasites in “refuge” from the anthelmintics in use. In effect, there’s no selection pressure on these parasites to develop resistance. Refugia occur both inside the animal and in the environment. They include:

  • Parasites in untreated animals, called host-based refugia.
  • Eggs and larvae already on the pasture when animals are treated, called environmental refugia.
  • Life stages of the parasite that are unaffected by drug treatment, such as some larval stages.

The purpose of preserving refugia is to maintain a greater proportion of drug-sensitive parasites. Two strategies to preserve refugia include:

  • Leaving some animals untreated by targeting only animals that need treatment, based on fecal egg counts, or miss several animals that are shedding eggs. Use highly effective anthelmintics on the rest.
  • Delay deworming until there are more eggs and larvae on the pasture, when environmental refugia is higher. This may not coincide with other management practices on some ranches, such as branding, unless the timing changes.

It’s important to keep in mind that animals don’t have to be completely parasite-free to be healthy and productive.  To reduce the risk of resistance, it’s critical to use the most effective product at the right time. Talk to your veterinarian.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).



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