Drug-Resistant Parasites Are Common In Ranched Bison, Texas A&M Researchers Find

Story by Courtney Price, VMBS Marketing & Communications

July 15, 2025

Parasitic worms are a pervasive problem in United States agriculture, affecting large and small ruminants such as sheep, goats, and cattle. These worms — also called gastrointestinal nematodes or roundworms — feed on their host, weakening the animals’ immune system and, in certain cases, causing death.

Drug resistance in parasites has become increasingly problematic for herd owners seeking to protect their animals, and a new study led by Texas A&M University and the University of Calgary researchers has revealed that the problem also extends to ranched bison. 

“Using new molecular sequencing techniques, we identified several different species of parasites in samples from 16 ranched bison herds in Texas and Oklahoma, including the three most economically important, which are also problematic in other ruminants,” said Kaylee Kipp, a doctoral candidate at the Texas A&M College of Veterinary Medicine & Biomedical Sciences (VMBS). 

Those three parasite species include Haemonchus contortus, commonly known as the barber’s pole worm; Haemonchus placei, a similar nematode species primarily found in cattle; and Ostertagia ostertagi, commonly known as the brown stomach worm. 

“In every herd sampled, we found genetic mutations within the parasites’ DNA that are associated with drug resistance. These mutations had an average relative frequency of 68% resistance in O. ostertagi samples and 70% resistance in H. contortus and H. placei across bison in all 16 ranches,” Kipp said. “In the case of O. ostertagi, the levels we found were significantly higher than in any previous reports, which are typically studies of cattle.” 

While the average frequency of drug resistance mutations was around 70%, the researchers found that parasite populations from some of the herds had much higher levels of resistance.

“In some samples of O. ostertagi, the parasites had 80-100% drug resistance,” Kipp said. “We found similarly high resistance — up to 80-90% — in some samples of H. contortus, a parasite that is common in sheep and goats.”

These findings point to a greater need for research on ranched bison as well as new treatment strategies for parasites.

“Ranched bison are growing in popularity in U.S. agriculture but are understudied compared to other ruminants,” said Dr. Guilherme Verocai, an associate professor in the VMBS’ Department of Veterinary Pathobiology and director of the Parasitology Diagnostics Laboratory. “Our findings are significant not only for bison but also for other ruminant species because of the high possibility of parasite transmission between them.”

Understanding Drug Resistance

Antiparasitic drugs are often the primary way for herd owners to protect their animals — and through them, their own livelihoods — from poor health and herd loss caused by parasites.

Gastrointestinal parasites are common in grazing animals because the parasite larvae are shed in feces and then spread to the grass that the animals eat. The goal of treatment is not to eliminate all parasites but, rather, to keep infection levels low enough that the animals do not experience disease.

In bison — as well as cattle, sheep, and goats — there are three main broad-spectrum antiparasitic drug classes used to treat gastrointestinal nematodes. The recent study — which focused on the most common class, called benzimidazoles — shows that they are losing their effectiveness.

“If bison herd owners migrate to the other existing antiparasitics, then it is likely only a matter of time before the parasites become resistant to those drugs, too. In fact, there is already evidence of resistance to other drug classes,” Kipp said. “Unfortunately, increased or indiscriminate drug use actually gives the parasites more opportunities to develop resistance.”

To avoid a situation in which antiparasitic drugs are no longer effective, researchers recommend several solutions.

“First, we need to develop better management strategies for controlling parasites that don’t involve whole-herd drug treatment or put such selective pressure on the parasite populations, potentially causing them to mutate,” Verocai said. “For example, some bison herd owners also own cattle and they may rotate them through the same pastures, which may allow for increased parasite transmission between livestock species. These are practices we must reconsider, along with overgrazing and overcrowding.

“Additionally, it’s important for owners to avoid trying to get their parasite population down to zero because that leads to overuse of the drugs and can backfire and cause drug resistance,” he said. “In some cases, a veterinarian may decide to give dewormer to only young animals who are more vulnerable to the damage caused by parasites or to those with a high parasite egg count in their feces. This can slow the process of drug resistance and saves the producers money.”   

Continuing The Investigation

In the future, the researchers plan to investigate parasite drug resistance in bison herds across the U.S. to get a better understanding of how geography and climate may affect owners’ management decisions. 

“Here in Texas, we don’t have hard winters, so we have almost year-round parasite problems,” Kipp said. “We suspect that this plays a role in how often herd owners choose to treat their animals for parasites.”

The researchers are also investigating parasite populations in wild bison from six different conservation sites to understand how they differ from ranched bison.Herd owners who would like help tracking parasite levels in their herds can visit the Parasitology Diagnostics Laboratory website for more information about testing services.

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For more information about the Texas A&M College of Veterinary Medicine & Biomedical Sciences, please visit our website at vetmed.tamu.edu or join us on Facebook, Instagram, and Twitter.

Contact Information: Jennifer Gauntt, Director of VMBS Communications, Texas A&M College of Veterinary Medicine & Biomedical Sciences, jgauntt@cvm.tamu.edu, 979-862-4216

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