Q. A mare I am planning to breed had shoulder surgery almost two years ago (part of her scapula was removed due to a fracture). Although she healed well, she recently slipped and reinjured the shoulder. She's on Bute twice a day. I have read that Banamine is safer for pregnant mares than Bute. Should I consult with my veterinarian about switching her to Banamine instead of Bute?

Brenda, via e-mail

A. This is a very good question, and amazingly, we don't have a really good answer. Despite being around for years, phenylbutazone (Bute) has never been fully investigated in pregnant or lactating mares. There is a lot of research regarding its toxicity in the gastrointestinal tract and the kidneys. Because of these toxicities, the only reasons for using Bute are because it's cheap, and it's effective for musculoskeletal pain. There are a couple of reasons that Bute is the most toxic of the non-steroidal anti-inflammatory drugs (NSAIDs).

All of the NSAIDs are metabolized by the liver before being eliminated. But with Bute, giving high or repeated doses saturates the enzyme system that the liver uses to get rid of the drug. Once the system is saturated, the Bute starts accumulating in the horse and the blood concentrations of the drug become very high and unpredictable. So, it takes longer than 24 hours to get rid of a whole dose.

If Bute is dosed twice a day for multiple days, as it often is administered, there is a large accumulation of drug in the horse's blood. Continuous high blood concentrations of Bute can quite easily create gastrointestinal and kidney damage.

All the newer NSAIDs, like flunixin meglumine (Banamine), ketoprofen (Ketofen, or Anafen in Canada), or vedaprofen (Quadrisol, available currently in the United Kingdom and Canada) don't have the same accumulating toxic effects.

These drugs are very effective because most of them move quickly out into the inflamed tissues, which gives very effective pain relief for 24 hours. However, blood levels of the drugs are low after a few hours, so the gastrointestinal tract and kidneys are far less likely to incur damage with daily dosing than with Bute.

The other key factor with NSAID toxicity has to do with the horse's hydration status. Since so much of a dose of Bute stays in the blood, if the horse becomes dehydrated for any reason, the drug concentration is higher and more likely to be toxic. In fact, the model for studying Bute toxicity is to give horses "normal" doses of phenylbutazone and restrict their water intake.

Because the newer NSAIDs are so much more expensive, they are almost never used for chronic pain control in horses with ongoing pain such as yours, or horses with laminitis.

The only recent information that I can find on Bute and pregnancy and lactation is in Holstein cows. When cows were given 4 grams of Bute/day, their calves were born with blood levels of Bute approximately one-half of the cows' blood concentration of the drug. I would expect Bute to behave in the same manner in mares and foals. So given this significant risk of exposure of the foal to Bute, I think it's prudent to use Bute only if absolutely necessary for pain control in mares while they are pregnant or lactating.

To minimize the risk of toxicity to both the mare and the foal, make sure that water intake is maximized and the lowest possible dose of Bute should be administered with the longest possible dosing interval. Certainly, twice-daily dosing should be avoided--it's actually safer to give the total daily dose once per day. And it is even safer if the mare can be dosed on an every-other-day or every-third-day basis.

About the Author

Trisha Dowling, DVM, MS, Dipl. ACVIM, ACVCP

Trisha Dowling, DVM, MS, Dipl. ACVIM, ACVCP, is a professor at the University of Saskatchewan, Western College of Veterinary Medicine.

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