Survey finds routine bison deworming often outpaces testing

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A new producer survey is adding detail to a growing concern in bison medicine: many ranched herds are dewormed routinely, but relatively few operations appear to be testing first. In the Veterinary Parasitology study, researchers surveyed 79 bison producers across 15 states and found that parasite control was largely drug-centric, with most respondents treating annually and many doing so more often. The authors warn that this pattern could accelerate anthelmintic resistance in gastrointestinal nematodes affecting ranched bison. (pubmed.ncbi.nlm.nih.gov)

That concern didn’t emerge in a vacuum. Anthelmintic resistance is already a well-established issue in other grazing livestock, and bison have long had less species-specific evidence to guide parasite control decisions. USDA’s Bison 2022 study showed similar management patterns at the industry level, including limited fecal testing and continued use of dewormer rotation, a practice the report notes is no longer recommended by livestock parasitologists as a way to reduce resistance. Separately, a Texas A&M news release this year highlighted molecular evidence of drug-resistance-associated mutations in parasites collected from ranched bison herds in Texas and Oklahoma. (aphis.usda.gov)

In the new survey, 87.3% of producers reported deworming at least once annually, 31.6% treated in both warm and cool seasons, and nearly one-third dewormed more than twice per year. Commonly used products included ivermectin, moxidectin, fenbendazole, and albendazole, and 28.1% of surveyed producers who answered that question reported administering multiple dewormers simultaneously. Most respondents managed herds on fewer than 300 acres, a detail that matters because stocking density, pasture use, and co-grazing can all shape parasite exposure and reinfection pressure. (pubmed.ncbi.nlm.nih.gov)

The testing gap is one of the clearest signals for clinicians. USDA data found that only 23.2% of bison operations performed laboratory fecal testing for intestinal parasites from July 2021 through June 2022, with testing more common on larger operations than smaller ones. Among operations that did test, private veterinarians performed the majority of fecal testing most often. Taken together, that suggests veterinarians already play a central role in the minority of herds using diagnostics, and may have room to influence a broader shift toward evidence-based parasite control. (aphis.usda.gov)

The industry backdrop makes the survey harder to dismiss as just a management snapshot. In a 2025 paper, Kaylee Kipp, Guilherme Verocai, and colleagues reported moderate to high frequencies of benzimidazole resistance polymorphisms in three economically important gastrointestinal nematodes of ranched bison in Texas and Oklahoma, including Haemonchus contortus, Haemonchus placei, and Ostertagia ostertagi. In Texas A&M’s summary of that work, Kipp said every herd sampled had resistance-associated mutations, with especially high levels in some O. ostertagi and Haemonchus samples. Those findings don’t prove that every calendar-based deworming program is failing, but they do strengthen the inference that repeated, non-targeted treatment could be compounding a resistance problem already underway. (pubmed.ncbi.nlm.nih.gov)

Why it matters: For veterinary professionals, especially those advising food animal, mixed animal, or exotic livestock clients, this is a reminder that bison parasite control can’t simply borrow cattle habits without scrutiny. The evidence base is still developing, but the direction is clear: routine whole-herd deworming without fecal surveillance risks selecting for resistant parasite populations while obscuring whether treatment is needed or effective. Refugia-based strategies, selective treatment, and follow-up fecal monitoring are already part of the broader resistance-mitigation conversation in ruminant parasitology, and the bison data suggest those principles are becoming more relevant in this species as well. (pubmed.ncbi.nlm.nih.gov)

There’s also a systems issue here. Bison are a growing production sector in U.S. agriculture, but they remain comparatively understudied, and parasite transmission between bison and other grazing livestock is a concern raised by researchers. That means resistance management in bison isn’t just a niche herd-health issue; it may have implications for co-grazed or neighboring cattle, sheep, and goats, depending on local management. (stories.tamu.edu)

What to watch: The next step is likely more than awareness. Watch for additional resistance surveillance, more discussion of bison-specific treatment guidelines, and stronger veterinary emphasis on fecal diagnostics before and after treatment, especially as producers and clinicians look for workable alternatives to calendar-based deworming. (pubmed.ncbi.nlm.nih.gov)