Anne C. Kelleher, Barbara Good, Theo de Waal, Orla M. Keane
5 min
As Irish beef production relies heavily on pasture-based systems, the control of gastrointestinal nematodes (GIN) is critical for calf health and economic viability. While anthelmintic resistance (AR) is well-documented in small ruminants, data on cattle in Ireland has been sparse. This study aimed to quantify the prevalence of resistance to the three main classes of broad-spectrum anthelmintics—benzimidazoles (BZ), imidazothiazoles (levamisole), and macrocyclic lactones (ivermectin and moxidectin)—on commercial dairy calf-to-beef farms.
Researchers monitored 36 farms over two years, performing herd-level faecal egg counts (FEC) to identify when parasite burdens reached a threshold suitable for testing. Once a farm met the criteria, a Faecal Egg Count Reduction Test (FECRT) was conducted. This involved treating groups of calves with specific anthelmintics and comparing pre- and post-treatment egg counts. Larval cultures were also performed to identify which nematode genera (specifically Cooperia and Ostertagia) were present before and after treatment, allowing for genus-specific efficacy calculations.
The results indicate a high prevalence of resistance across all tested drug classes. Ivermectin resistance was identified on every farm tested (100%), while moxidectin resistance was confirmed on 73% of farms. Benzimidazole resistance was found on 60% of farms. Levamisole was the most effective treatment, with resistance detected on only 18% of farms. Post-treatment analysis confirmed that both Cooperia and Ostertagia species were surviving treatment, indicating that resistance is not limited to the commonly cited dose-limiting genus, Cooperia.
These findings demonstrate that the current reliance on prophylactic anthelmintic treatment is unsustainable in the Irish cattle industry. The widespread presence of multi-drug resistant nematode populations threatens the health of first-season grazing calves and necessitates a shift toward more integrated parasite management strategies that do not rely solely on chemical intervention.
Alex: So even if a farmer is doing everything they think is right—treating on schedule, using the recommended drugs—the study suggests the worms may have already evolved past those treatments.
Sam: That's the core concern. And there's a difficult cycle at work here. Farmers are understandably worried about their calves getting sick, so they treat early and often. But that constant pressure is precisely what drives the parasites to evolve resistance faster. The fear of the disease is accelerating the very problem that makes the disease harder to treat.
Alex: That does sound like a hard cycle to break. Are there any limitations to the study we should keep in mind?
Sam: One worth noting is that the study didn't include an untreated control group—a set of animals left without medicine to compare against the treated ones. That's standard in drug trials, but here the farmers were understandably unwilling to leave calves unprotected. Without that comparison, it's harder to pin down the exact degree of resistance with complete precision. The study also relied on farms that volunteered to participate, so the results may reflect the practices of those specific operations rather than every farm in Ireland.
Alex: So it's a trade-off—animal welfare over perfect experimental conditions. What do the researchers suggest should change?
Sam: They point toward what they call precision parasite management. Rather than treating animals on a fixed schedule regardless of whether they actually need it, the idea is to use diagnostics—like egg counts—to decide when treatment is genuinely necessary. Only treat the animals that actually have a significant worm burden.
Alex: Like only taking medicine when you're actually sick, rather than taking it every day just in case.
Sam: Exactly. They also discuss a concept called refugia. The idea is to deliberately leave a small portion of the worm population unexposed to drugs—worms that are still susceptible to treatment. By keeping those susceptible worms in the mix, you dilute the resistant ones over time, which slows the spread of resistance and keeps the drugs effective for longer.
Alex: That's a counterintuitive idea—intentionally not killing some of the parasites in order to preserve your ability to kill them later.
Sam: It is. And alongside that, the paper points to grassland management as another lever. Something as practical as not moving calves onto pastures that are already heavily contaminated with worm larvae can meaningfully reduce how much the animals are exposed in the first place—reducing the need for treatment at all.
Alex: So the message isn't just "the drugs are failing"—it's that the whole approach needs to shift. Less reliance on chemicals, more targeted decision-making.
Sam: That's the conclusion. The study is a clear warning that if farming practices don't adapt, the industry risks losing effective control over these parasites altogether. The tools are still there, but they need to be used more carefully, and supported by other strategies, if they're going to last.
Alex: It's a useful reminder that biology doesn't stand still—and that the way we respond to it has to keep pace. Thanks for walking us through it.
Sam: Thanks for listening to ResearchPod.