Parasites are growing resistant to dewormers used by many Missouri farmers to treat their cattle. That’s according to a new study from University of Missouri researchers.
Parasites can cause weight loss and death, making cattle operations less productive. Past studies have shown that these parasites have grown resistant to some types of drugs used to treat them. The yet-unpublished data from MU researchers indicate the same thing is happening on Missouri farms.
The study looked at the number of parasite eggs in cattle feces before and after using different classes of dewormers.
“If we saw a 90% reduction in the fecal egg count at 14 days post-deworming as compared to the pre-deworming test, we would consider that a successful deworming,” said Eric Bailey, an associate professor at the University of Missouri who led the study. He’s also Missouri’s state beef nutrition specialist.
Researchers found that one class of dewormers, made with chemicals called macrocyclic lactones, has decreased in efficacy. After two weeks, they only decreased fecal egg count by 80.1% in cows and 69.4% in calves.
Another kind of treatment, which contains a different class of compounds called benzimidazoles, had a 97.8% reduction when used on cows and a 99.4% reduction in calves.
Bailey said deworming products that use macrocyclic lactones are easier to use and therefore much more common. A 2017 survey from the U.S. Department of Agriculture found that of the operations that dewormed cattle at least occasionally, 95.4% used products with avermectins, a class of macrocyclic lactones.
That’s led to built-up resistance because parasites that naturally resist the products live to reproduce.
“They have the opportunity then to proliferate and to essentially, over time, become a large proportion of the internal parasites in cattle,” he said.
The study was limited in scope. It wasn’t controlled for management practices and was done on real Missouri farms. Samples were collected from 159 operations, but only 51 tests are analyzed in the data.
That’s because some producers had to break from the study’s protocols and only samples with enough worms in them were considered.
“If the samples from that herd didn't really have very many worms in them, it wasn't going to be very easy to determine what the impact of that deworming was,” said Eric Meusch, a livestock specialist with MU extension who worked with farmers to collect data.
That’s part of the reason the results haven’t been published yet. The study is part of a broader effort to study parasite resistance in Missouri, Florida, Kentucky, Oklahoma and Texas.
“We don't have a lot of statistical power at this point, but the thought process is as the other states finalize their observational data collection and we aggregate these data into a larger, more robust data set, then we can really make some statements,” Bailey said.
The effort is being funded by Merck Animal Health. Merck offers farmers free fecal egg count reduction tests and maintains its own database.
“They just wanted to reach out to some universities to get some third-party objective feedback to see if what we saw was the same thing as what their database would show,” Bailey said. He added that his results match Merck’s data.
Merck also sells a benzimidazole-containing dewormer, which the study found is still effective. Bailey said Merck had no control over the study’s set-up or results.
“They told us they wanted us to look at as many farms as we could, regardless of what product was being used by the individual farmers,” he said. “This truly was observational data, and there's no bias essentially in this because Merck was blinded to what products were used.”
Resisting Resistance
Bailey said farmers can combat parasite resistance by cycling through different types of dewormers.
“An important distinction to draw is that just because you buy product A from company A doesn't mean that you should go buy product A from company B instead,” he said. “Paying attention to what the active ingredient is in the deworming product is important in making sure that you're not using the same class – the macrocyclic lactones – over and over and over, year after year.”
Different management practices can help, too. Moving livestock to different areas of pasture, a method called rotational grazing, helps kill parasites.
“Cattle that stay on the same pasture year after year after year are going to graze infected material,” Bailey said.
“But if they drop manure with the eggs in the pasture A and then go on to pastures B, C, D, E and F, and don't rotate back to pasture A for an entire season, then those eggs will mature and hatch, and the larvae will not be consumed by the cattle in the same season, and they'll die,” he added.
Meusch raises cattle himself and employs pasture rotation. He said producers should also make sure they aren’t over-grazing their pastures.
“If we don't graze below four inches, we're not likely to be in contact with very many larvae,” he said. “If we're constantly grazing below that four-inch level, you're getting down in the canopy, where there's the moisture and the coolness that allows those larvae to survive.”
He also currently uses products with macrocyclic lactones. Even though his fecal egg count reduction test showed that his deworming has been successful, he said he might make some adjustments following the results of the study.
“It's probably been since the 1960s or 70s that we've relied so heavily on these pharmaceutical interventions,” Meusch said. “We can manage through a lot of this without some of these pharmaceutical fixes.”
