Intensive use of anthelmintic drugs is reshaping parasite populations in cattle, and a new study from Argentina has captured that evolutionary process in unprecedented genetic detail. Researchers have combined deep amplicon sequencing with the Faecal Egg Count Reduction Test (FECRT) to investigate gastrointestinal nematode communities and benzimidazole resistance across six commercial beef farms. Their results reveal not only the spread of resistance-associated mutations, but also a troubling mismatch between the genetic profile of some parasites and their ability to survive treatment.
Gastrointestinal nematodes are among the most important health and productivity challenges facing cattle producers. These parasitic worms inhabit the digestive tract, where they can impair nutrient absorption, reduce weight gain and, in severe infections, cause anemia or death. Benzimidazoles, including commonly used cattle dewormers, have been central to controlling these infections for decades. However, repeated exposure creates strong selective pressure: susceptible worms are eliminated, while individuals carrying protective genetic variants survive and reproduce.
The Argentine study examined parasite populations from six commercial beef farms and identified seven nematode species through sequencing of the internal transcribed spacer 2, or ITS2, region of ribosomal DNA. This genetic marker acts as a molecular barcode, allowing closely related worm species to be distinguished even when their eggs appear almost identical under a microscope. The communities were dominated by Haemonchus placei and Cooperia punctata, two parasites with major implications for cattle health and drug-control programs in tropical and subtropical production systems.
To investigate resistance, the researchers focused on the isotype-1 β-tubulin gene, known as tbb-isotype-1. This gene encodes β-tubulin, a structural protein involved in the formation of microtubules. Benzimidazole drugs bind to β-tubulin in nematode cells, disrupting essential processes such as cell division and intracellular transport. Changes in the protein’s structure can reduce drug binding, allowing the parasite to survive. Because of this mechanism, specific alterations in the β-tubulin gene are widely used as molecular indicators of benzimidazole resistance.
Deep amplicon sequencing detected four single-nucleotide polymorphisms, or SNPs, associated with resistance. These included F167Y, caused by a TTC-to-TAC substitution; E198A, caused by GAA-to-GCA; E198L, caused by GAA-to-TTG; and F200Y, caused by TTC-to-TAC. The mutations were found in several species, including C. punctata, Ostertagia ostertagi, Haemonchus contortus and Cooperia oncophora. Rather than simply recording whether a mutation was present or absent, the sequencing approach measured the relative abundance of different genetic variants within each parasite community.
The genetic data were compared with treatment performance using the FECRT, a field method that estimates how effectively a dewormer reduces the number of parasite eggs shed in cattle feces. A substantial decline in egg counts after treatment generally indicates that the drug remains effective, whereas a poor reduction suggests resistance or treatment failure. In this study, failures following benzimidazole administration were associated mainly with C. punctata and, to a lesser extent, O. ostertagi. The latter species carried approximately 71.4 percent of the resistance-associated β-tubulin amplicon sequence variants detected in the study, indicating strong directional selection at this locus.
Yet the most unexpected finding emerged from the surviving C. punctata. After some benzimidazole treatments, the worms that remained frequently carried predominantly susceptible β-tubulin alleles. In other words, their genetic profiles did not match the observed treatment outcome. This discordance challenges the assumption that resistance-associated mutations in tbb-isotype-1 can serve as a complete explanation for benzimidazole failure. A parasite may survive despite lacking the canonical mutations, while another carrying a resistance-associated allele may still be removed by treatment.
The researchers suggest several possible explanations. Resistance may involve additional genes or biological pathways that are not captured by β-tubulin screening. Changes in drug metabolism, cellular transport or stress responses could help nematodes tolerate exposure. Ecological factors may also influence the result. Parasites occupying different regions of the gastrointestinal tract could experience distinct drug concentrations, while variation in feeding behavior, host physiology and parasite developmental stage might affect susceptibility. Pharmacokinetic differences between cattle could further alter how long and at what concentration a drug remains active.
The pattern changed when cattle received a combination of benzimidazole and macrocyclic lactone treatments. In these animals, surviving C. punctata predominantly carried β-tubulin variants associated with benzimidazole resistance, suggesting that multidrug exposure intensified selection for resistant genotypes. Combined treatments may remove a wider range of susceptible parasites and leave a smaller group in which resistance-associated alleles become more concentrated. Although combination therapy can improve control in some circumstances, the findings show that it may also accelerate evolutionary change when used without careful resistance monitoring.
The study provides the first broad genetic characterization of gastrointestinal nematode communities and benzimidazole resistance in large-scale cattle systems in Argentina. Its central message is that molecular surveillance and field-based treatment tests are most powerful when used together. FECRT reveals what happens at the population level after treatment, while sequencing helps identify which species and genetic variants are involved. Neither method alone can fully explain the complex biology of resistance.
For cattle producers, the findings underline the risks of relying repeatedly on the same drug class or interpreting treatment failure through a single genetic marker. Sustainable parasite control will require species-level surveillance, regular assessments of drug efficacy and a better understanding of how resistance develops across mixed nematode communities. As anthelmintic use continues to shape parasite evolution, studies such as this one offer an early warning that resistance is not a single mutation or a simple yes-or-no trait, but a dynamic interaction between genetics, ecology, treatment practices and host biology.
Subject of Research: Gastrointestinal nematode communities and benzimidazole resistance in commercial beef cattle from Argentina
Article Title: First genetic characterization of gastrointestinal nematode communities and benzimidazole resistance in cattle from Argentina
Article References: Maté, M.L., Canton, C., Redman, E. et al. “First genetic characterization of gastrointestinal nematode communities and benzimidazole resistance in cattle from Argentina.” Heredity (2026). https://doi.org/10.1038/s41437-026-00871-4
Image Credits: AI Generated
DOI: 10.1038/s41437-026-00871-4
Keywords: gastrointestinal nematodes, cattle, Argentina, benzimidazole resistance, anthelmintic resistance, Cooperia punctata, Haemonchus placei, β-tubulin, deep amplicon sequencing, Faecal Egg Count Reduction Test
Tags: anthelmintic resistancebeef cattle health managementbenzimidazole resistancecattle parasite controldeep amplicon sequencing in veterinary parasitologyevolution of anthelmintic resistancefaecal egg count reduction testgastrointestinal nematode communitiesgenetic diversity in parasitic wormsimpact of drug use on parasite populationsmolecular identification of gastrointestinal nematodesmolecular markers in parasite resistance
