Research shows risks with bute


Research shows that prolonged use of phenylbutazone (bute) can cause some adverse effects in horses as soon as three days after initial treatment.

The effects include protein loss, lowered white blood cell counts, blood-flow changes in the right dorsal colon and changes in volatile fatty acid activity.

“This research will hopefully help people to stop and look at what’s going on in a particular horse,” says lead researcher Rebecca McConnico, Associate Professor at Louisiana State University’s School of Veterinary Medicine.

“The research will serve its purpose if we take a moment to look at individual horses and assess the possible toxic effects of this drug.”

During the study, eight horses were either administered bute at 8.8 mg/kg for 21 days, or were part of the control group given corn syrup. The horses were closely monitored using physical examinations, blood samples, arterial blood flow analysis examinations and samples collected from the right dorsal colon as well as other analyses.

Horses receiving bute experienced:

  • Abnormally low protein concentrations in the blood stream beginning as early as three days after the initial administration of bute.
  • A low number of white blood cells starting three to six days after initial treatment.
  • Concentrations of one type of volatile fatty acid lowered significantly in just two weeks.

Dr McConnico says volatile fatty acids are largely thought to be responsible for water absorption in the distal part of the colon in horses. In addition to these results, two horses developed colitis while receiving bute and were removed from the study and hospitalised.Without the detailed measurement undertaken in this research, the outward signs of these effects would be subtle, Dr McConnico says.

“It’s not necessarily what the average horse owner would see; it’s almost what you wouldn’t see,” Dr McConnico says. “You wouldn’t see the horse at its best physically. These fluctuations could really affect a horse’s ability to perform.”

With the support of the Grayson Jockey Club Research Foundation, the study’s five authors addressed these questions and have made strides to determine the commonplace medication’s potential effects.

“This research brings to light individual animal variability,” Dr McConnico says. “There are a whole lot of medications where we don’t quite know what they’re doing to the animal’s entire system. Hopefully, we’ll continue down this path of research and sort that out for the horses’ benefit.”

The labour-intensive study encountered a few surprises, including an interruption by Hurricane Katrina in 2005. Despite setbacks, the findings begin to confirm anecdotal evidence gathered over the course of Dr McConnico’s career.

“During my graduate programme and working in referral practices, we would tend to see cases that were a little more unusual,” Dr McConnico says. “In many of those cases, it seemed to many of us that bute has not been tolerated well in a large number of horses.

“However, some horses could tolerate it. We felt we needed to look at bute use in the long term, which is what this research accomplished.”


McConnico RS, Morgan TW, Williams CC, Hubert JD, Moore RM. Pathophysiologic effects of phenylbutazone on the right dorsal colon in horses. AJVR 2008;69(11):1496-1505.

First published May, 2009

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