Question

I own a 10-year-old Quarter Horse mare diagnosed with recurrent exertional rhabdomyolysis (RER) via bloodwork with two episodes of tying-up in the past eight months. Both episodes were associated with a short period of no work and a weather change. She had dark-colored urine after both incidents. Her diet consists of brome/orchardgrass hay, available at all times, and a handful of sweet feed with salt mixed in. Supplements include dried raspberry leaves (to even out estrus-related behavior), prebiotics, and probiotics. She’s an easy keeper and generally calm except just prior to running barrels and poles. How should I manage her going forward?

Answer

Your mare doesn’t fit the usual profile for RER, as the condition tends to be observed most often in young, excitable Thoroughbred fillies. She may be predisposed to sporadic exertional rhabdomyolysis rather than recurrent rhabdomyolysis, but I would still recommend a diet appropriate for horses diagnosed with RER to help reduce the risk of further episodes. Be on the lookout for situations that may precipitate another episode, such as a change in exercise, a drop in water intake, or specific weather patterns.

General dietary recommendations for RER include:

• Concentrate: no greater than 20% daily digestible energy (DE) from nonstructural carbohydrates (NSC), 20-25% from fat.  Concentrates should contain 12-18% NSC, 10-13% fat, 3.2-3.4 Mcal/kg DE. Sources of highly digestible fiber such as beet pulp or soy hulls.
• Forage: variable, though frequent provision of hay with moderate NSC and mixed alfalfa content. Many horses with RER are predisposed to gastric ulcers, so the alfalfa may be beneficial.

In this specific instance, feeding suggestions for your mare include:

• Hay: continue free-choice access to the brome/orchardgrass hay.
• Concentrate: add a balancer pellet to the mare’s diet to make up for nutritional deficiencies in hay; either continue to feed the small amount of high-fat concentrate or discontinue. I understand it helps with mixing in the salt.
• Electrolytes: continue adding salt to meals. Supplement with research-proven electrolytes, such as Restore SR, for competitions or heavy workouts when you expect sweat loss to be significant. A supplement containing just electrolytes (primarily sodium, chloride, potassium) and no unnecessary ingredients such as sugar (e.g., dextrose) would be most effective.
• Supplements: continue the dried raspberry leaves, prebiotics, and probiotics if you think they work for your mare. Add Nano-E as a source of natural vitamin E (potent antioxidant for horses with myopathies); the recommended amount for myopathies is 2,000-3,000 IU/day, so 10 ml/day would be a logical starting point.

Recommended exercise for horses diagnosed with RER involves daily exercise. Beginning approximately24 hours after an RER episode, hand walk or turnout daily. Prolonged stall rest is counterproductive. However, if your mare’s tying-up episodes are not due to RER, follow your veterinarian’s recommendations for rest when appropriate. With the dark urine indicating myoglobinuria during previous tying-up episodes, your vet is the best reference for advice.

Dietary recommendations for myopathies are not set in stone and are usually quite individual depending on the horse’s nutritional needs and the particular diagnosed disease. If your mare suffers another episode after dietary and management changes, a muscle biopsy might offer more information on her condition but that, of course, would be a discussion between you and your veterinarian.