Texas A&M Study Highlights Need For More Targeted Parasite Control In Bison

Researchers found that common deworming practices could contribute to drug resistance, highlighting the need for bison-specific parasite management strategies.

Gastrointestinal parasites can cause serious health problems in North American bison, but the drugs producers rely on to control them can become less effective when used too frequently or without knowing which animals actually need treatment.

A new study led by researchers at the Texas A&M College of Veterinary Medicine & Biomedical Sciences (VMBS) examined how bison producers manage parasites, providing new information that could help researchers develop more targeted approaches to deworming and slow the development of drug resistance.

The findings provide researchers with a clearer picture of how ranched bison are managed across the southern and central United States and highlight opportunities to improve parasite control while protecting the limited drugs available to producers.

“Gastrointestinal nematodes are one of the most economically important health concerns for bison,” said Dr. Kaylee Kipp, a recent VMBS doctoral graduate and associate professor of professional practice in ranch management at Texas Christian University. “We have both anecdotal and diagnostic evidence showing that bison seem to have more parasitic issues compared to beef cattle.”

These parasites, commonly called gastrointestinal worms, can cause diarrhea, poor weight gain, and reduced production. In severe cases, Kipp said, parasite infections can even contribute to the deaths of adult bison.

That makes controlling parasites important — but researchers say simply deworming more often isn’t necessarily the answer.

Learning How Producers Manage Bison

Despite the importance of parasites to bison health, relatively little is known about how producers manage parasites in ranched bison, particularly in the southern U.S.

Most previous bison research has focused on larger herds in northern states and Canada, leaving researchers with less information about operations in Texas and neighboring states.

“We needed to understand the basics of these operations,” said Dr. Guilherme Verocai, an associate professor in the VMBS’ Department of Veterinary Pathobiology and director of the Parasitology Diagnostic Laboratory. “Are we talking about a herd of 500 animals or 30? Those can be very different operations.”

To better understand those differences, the research team surveyed bison producers about herd demographics, pasture management, nutrition, animal sourcing, and parasite control practices.

Nearly 90% reported deworming their bison at least once a year, and almost one-third dewormed more than twice annually. At the same time, diagnostic tools that can help determine whether treatment is necessary were rarely used.

The survey also showed that producers largely manage bison using practices adapted from the cattle industry.

Although many practices are similar, bison present unique challenges.

Unlike cattle, bison can be more difficult and dangerous to handle, making frequent trips through handling facilities challenging. As a result, some producers turn to alternatives such as medicated feed to administer dewormers.

Another challenge is that researchers still have limited information about how deworming drugs should be used specifically in bison.

“There is only one product that is FDA approved to treat gastrointestinal nematodes in bison,” Verocai said. “For most treatments, producers have to adapt products and practices developed for cattle or other ruminants.”

That means producers may start with recommendations developed for cattle even though researchers don’t yet know the ideal doses or treatment strategies for bison — a knowledge gap that can complicate efforts to use dewormers effectively.

When Parasites Fight Back

The researchers are particularly concerned about anthelmintic resistance — when worms develop resistance to the drugs used to kill them.

The process is similar to antibiotic resistance: when a herd is treated, susceptible parasites are killed while parasites carrying traits that allow them to survive can remain and reproduce. Repeated treatment can gradually increase the proportion of resistant parasites.

“The concern is that over time, we could reach a point where the majority of those parasites are resistant,” Verocai said. “That’s why treatment needs to be one of several tools we use to manage and control parasites.”

That approach is especially important because producers have relatively few classes of dewormers available, and few options are expected in the near future.

One tool that can help producers use those drugs more strategically is diagnostic testing. Fecal tests can show which parasites are present and help determine whether treatment is necessary, allowing producers and veterinarians to make more targeted decisions.

“What might look like extra labor — collecting the fecal sample, sending it to the lab, and waiting on those results — most likely means you would treat fewer animals and save money on drug use,” Verocai said.

Using diagnostics to guide treatment can also reduce pressure on parasite populations to develop resistance, helping preserve the effectiveness of available dewormers for longer.  

Managing Bison And Their Parasites

Another strategy researchers hope to introduce more widely to bison producers is refugia, which involves maintaining a population of parasites that has not been exposed to a dewormer. One way to do this is by intentionally leaving some animals untreated, allowing drug-susceptible parasites to reproduce with resistant parasites and helping slow the spread of resistance.   

Fewer than 10% of surveyed producers reported intentionally leaving animals untreated for this purpose.

“Refugia is a huge topic and needs to be talked about more,” Kipp said. “We can’t only be thinking about the next growing season, the next breeding season, or the next calving season. We have to be thinking about five, 10, or 20 years down the road. Is the practice you’re implementing on your property sustainable?”

The goal, researchers emphasize, isn’t to stop treating parasites but to make treatment decisions based on the needs of each herd.

“Every time you buy an animal, you are buying parasites,” Kipp said. “You are essentially not only a bison manager, but you’re also a parasite manager in a way, because they’re not going to go away.”

Because every herd differs in its size, environment, management, and parasite populations, there isn’t one approach that will work for every operation. By establishing a baseline of current practices, the researchers hope their work will contribute to bison-specific recommendations that protect animal health while preserving the effectiveness of available dewormers.

“It’s a double-edged sword,” Kipp said. “Parasites can cause serious problems in bison, so the natural response is to deworm them, but more treatment isn’t always better. We need to use these drugs strategically and be good stewards of them because we only have so many options.”