Resistance is not a new topic when it comes to drugs such as antibiotics, but one class of drugs—anthelmintics, also known as dewormers—is often overlooked. And that’s not good.
An anthelmintic is the term for drugs used to treat and prevent infections of animals with parasitic worms. Worms are not only a welfare issue in the livestock industry, but also result in an economic burden.
Parasitic worms of relevance include flatworms, tapeworms, and roundworms. Ostertagia ostertagi, commonly known as the medium or brown stomach worm, is the most important cattle parasite in North America, according to Dr. Thomas Craig of Texas A&M University.
Resistance to deworming products is caused by the ability of parasites to survive doses of drugs that would typically kill similar parasites. These drugs can be administered in the form of a pour-on, paste, drench, or injectable.
“Anthelmintic resistance is an inevitable consequence of the use of anthelmintics over time,” said Louisiana State University Extension Veterinarian Christine Navarre in an Extension publication.
The short answer to “why does resistance happen?” is an over-reliance on anthelmintics.
Resistant parasites have genes that protect them from being affected by an anthelmintic. Drug-resistant worms exist in a small proportion of all populations. After deworming an infected animal, the only worms that survive in a population are resistant to the drug. It’s a simple case of only the fittest survive. The resistant worms will then pass their genes onto the next generation, creating an increasingly drug-resistant population.
Wormy details
Adult parasitic worms live in the gastrointestinal tract of cattle and lay eggs that are shed in manure, according to Navarre. The eggs hatch into larvae that crawl onto grass and are ingested by grazing cattle. The larvae then develop into adults in the gastrointestinal tract and continue the cycle.
There is vast genetic diversity in parasite populations. A specie can lay up to 5,000 eggs a day, and with 300 females infecting each host in a herd of 100, this can result in upwards of 1 billion eggs total per day on an individual operation.
Cooperia sp. and Haemonchus placei are common worms found in cattle but tend to affect only calves. They are warm-season worms, so large numbers build up in summer months. Cattle tend to build immunity to these parasites as they mature, but adult cattle will continue to carry low numbers that can contaminate the pastures for calves.
Brown stomach worms are significant because they impact both young and adult cattle. The parasite thrives in cool weather from October to May.
Cattle are less commonly identified with anthelmintic resistance, as treatments aren’t given frequently enough to stoke resistance. Small ruminants are the most prone to resistance worldwide and horses also have a higher incidence of resistance as they are more regularly dewormed.
In addition to anthelmintic resistance, rainfall, environment, grazing management, animal maturity, and individual immunity determine the severity of parasitic outbreaks on operations.
Delaying resistance
Resistance is a natural biological consequence of drug treatment. Overusing a product and ignoring resistance issues could result in a serious case of drug-resistant parasites. Frequently treating animals with a dewormer more than a few times a year, underdosing, and other management practices can increase the onset of resistance.
By treating all animals at the same time—a common management practice to save labor and time—resistance is more prone to happen. In addition, treating animals when there is a small incidence of larvae in the pasture, or treating and turning out onto a clean field, can encourage resistance even further.
However, attempts can be made to prevent the inevitable. For example, by only treating animals that display a need for treatment (especially before turning out into clean pastures), avoiding deworming older cows going into summer, not using the same pastures for young stockers year after year and selectively breeding and culling, resistance can be delayed.
Increasing overall herd immunity will decrease the potential for parasites, which can include implementing a proper nutrition program. Using cows as “vacuum cleaners” for calf parasites by grazing cows on pasture after calves can reduce the amount of Cooperia and Haemonchus parasites.
Implementing a proper parasite control program is also important to delaying resistance. Ensuring proper product storage, administration, dose, and route can help achieve the expected production responses. Alternating product use will keep parasites from building immunity against a certain product.
Maintaining and increasing “refugia” in the worm population is one of the most critical control measures to delaying resistance. This is effectively selecting for weaker worms.
“Parasites in refugia do not have genes for anthelmintic resistance—they are still susceptible to anthelmintics,” Navarre said. “The more refugia in a population, the more the resistance genes in a population are diluted and the more effective anthelmintics will be.”
By deworming an entire group of cattle, refugia is eliminated in the animals and the only parasites that survive are drug resistant. After mating and reproducing, the parasites take over and eventually dewormers fail to work. By selectively treating animals, refugia can be maintained and worms that are not drug resistant will be capable of reproducing other non-drug-resistant offspring.
The same idea goes for pastures. If an entire group of dewormed animals are put onto clean pasture, the only parasitic inhabitants of that pasture will now be drug resistant and contaminate the entire pasture.
“Cattlemen should work with their veterinarian to find a balance between keeping overall parasite levels low enough to prevent economic losses while at the same time retaining some refugia to slow the progression of anthelmintic resistance,” Navarre said. — Anna Miller, WLJ editor
