New Research Demonstrates Phenylbutazone Risks
RELEASE: March 18, 2009
AUTHOR/ADMINISTRATOR: By Natasha Joseph
New research demonstrates that prolonged administration 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. [1]
“Every veterinarian should be aware of this important research,” said Frank Hurtig, DVM, MBA, Director, Merial Veterinary Services. “We applaud Dr. McConnico and her team for their hard work and contribution to our knowledge of this subject.”
The research team’s goal was to focus on the effects in individual horses, noted lead researcher Rebecca S. McConnico, DVM, PhD, Dipl. ACVIM, associate professor, Louisiana State University 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,” said Dr. McConnico.
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. [1]
Horses receiving bute experienced: [1]
· 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 noted that 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 hospitalized. [1]
Without the detailed measurement undertaken in this research, the outward signs of these effects would be subtle, Dr. McConnico noted.
“It’s not necessarily what the average horse owner would see; it’s almost what you wouldn’t see,” Dr. McConnico said. “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 said. “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 labor-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 program and working in referral practices, we would tend to see cases that were a little more unusual,” Dr. McConnico said. “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.”
Merial is a world-leading, innovation-driven animal health company, providing a comprehensive range of products to enhance the health, well-being and performance of a wide range of animals. Merial employs approximately 5,400 people and operates in more than 150 countries worldwide. Its 2008 sales were over $2.6 billion. Merial Limited is a joint venture between Merck & Co., Inc., and sanofi-aventis. For more information, please see