Parasite Resistance Confirmed in Texas Bison Herds

A study by Texas A&M University researchers confirmed parasite resistance to a common treatment in 16 ranched bison herds in Texas and Oklahoma. The results place new emphasis on management practices for bison, cattle and small ruminant producers — all of whom use the same core treatments for parasites.

“The research idea actually started because of the bison producers themselves,” said Kaylee Kipp, MS, Ph.D., a recent doctoral graduate at the Verocai Parasitology Lab at Texas A&M University, who conducted the research. “Texas A&M University Parasitology Diagnostic Laboratory had an influx of samples from producers dealing with really high levels of parasitism in their herds, saying they felt the dewormers they were using were not working as well. We wanted to get a baseline.”

Three parasite species with resistant mutations

A 2025 study conducted by Kipp and other researchers showed that wild conservation bison herds showed no resistance to extremely low levels of benzimidazole resistance. On the other hand, ranched bison are dewormed more frequently. Exposure to treatments actually selects for the resistance parasites.

Using advanced molecular sequencing techniques, the research team tested for resistance to benzimidazole. They identified several different species of parasites in samples, including the three most economically important, which are also problematic in other ruminants. The three parasite species found to carry drug resistance mutations were:

• Haemonchus contortus, commonly known as the barber’s pole worm and a parasite shared with small ruminants and cattle;
• Haemonchus placei, a similar nematode species primarily found in cattle; and
• Ostertagia ostertagi, commonly known as the brown stomach worm, which is also a cattle parasite

Resistance levels were moderate to high

“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,” Dr. 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% to 100% drug resistance, and many samples had similarly high resistance — up to 80% to 90% — of H. contortus.

“Part of the scary news is the resistant parasites we found are not unique to bison but are shared with cattle or small ruminants, said Gui Verocai, DVM, MSc, Ph.D., DACVM (Parasitology), associate professor and director of the Verocai Parasitology Lab at Texas A&M University. “It’s not a bison problem; it’s a livestock problem.”

On-farm resistance effects

Dr. Kipp emphasized the research was a screening and did not include the gold standard test for parasiticide efficacy: fecal egg count reduction tests (FECRTs). In addition, the research did not test for resistance to macrocyclic lactones, which is a class that includes drugs like ivermectin, doramectin, eprinomectin and moxidectin.

“We can’t connect this research to on-farm drug efficacy, but it’s reasonable to suspect that benzimidazole products may have reduced efficacy based on the molecular markers in such high abundance,” Dr. Kipp noted. “We based on this research on markers that are confirmed for resistance to benzimidazole.”

Management practices are key

Battling resistance requires emphasizing best practices for bison, cattle and small ruminant producers, including:

• Ensuring correct dosage for the animal’s weight is used
• Maintaining a refugia population of parasites not exposed to drugs

In bison herds, these management practices have additional challenges, Dr. Kipp noted.

“In bison, every drug is extra-label, so we don’t truly know the right dosage to use. There has been speculation that the recommended dose for cattle is not enough for bison,” she said. “In addition, ranchers may not have a chute with the capability to run bison through.”

As an alternative, many bison producers administer parasiticides via feed additives, medicated blocks or putting treatments in water sources. This left the research team questioning if high parasite levels were a result of ineffective or misapplied treatments, Dr. Kipp said.

“Bison are the last species to receive innovation across the livestock species,” Dr. Verocai noted. “Most treatments are off-label, and we’re adapting as we go. It would be great to be able to recommend a more appropriate dosing scheme or protocols.”

More research on the pharmacokinetics of parasite treatments specifically for bison would be useful in helping stave off resistance and keep treatments available into the future.

A wake-up call to all livestock producers

Next, Dr. Kipp is working with bison ranchers across eight southern states to evaluate management practices for parasite control and evaluating fecal samples for more markers of parasite resistance. Findings thus far indicate widespread resistance, especially across the states where warmer temperatures make parasite threats consistent year-round.

“This is the highest O. ostertagi resistance level detected to date, regardless of hosts — even in cattle,” Dr. Kipp said. “This is a wake-up call and warning to reaffirm that resistance is here and is not going away on its own.”

Photo Credits:

istockphoto.com/Jaden Hilgers

Photo courtesy of Texas A&M