02/21/2024
Interesting case to share with you!
Valli is a 3 yr QH mare who presented at MPEH with a several week history of unusual behaviors (irritable, decreased appetite, cinchy) and most recently was noted to have an abnormal gait characterized as sluggish, short behind, and looked like she was shuffling. On initial exam she was bright with normal vital parameters but confirmed to be walking very stiffly with pain on palpation of big muscle groups. Also of note in her history is recent vaccination 4 weeks prior and muscle atrophy. We quickly ran blood work on Valli in house which showed very elevated muscle enzymes (CK, AST, LDH). This confirmed that Valli was "tying up", which is called rhabdomyolysis.
Valli was hospitalized at MPEH for treatment. When horses "tie up", no matter the reason, the muscles start breaking down and the products of this get processed through the kidneys. The kidneys often struggle to deal with this load of toxins so a major part of treatment is high rate IV fluids to diurese or flush the kidneys. We also started Valli on muscle relaxants and additional kidney and muscle support and she was on stall rest. Valli also had concurrent mild gastric ulcers and hepatitis which she was treated for.
When we diagnose a horse with rhabdomyolysis, we need to know why this is happening. Rhabdomyolysis can be exertional or non-exertional. An example of exertional rhabdomyolysis is PSSM1/PSSM2. Valli's episode was not related to exercise at all, which left us with two main differentials: Selenium/Vitamin E deficiency or Immune Mediated Myositis. Valli was tested and found to have normal selenium blood values but was vitamin E deficient. However, this alone was not enough to explain her rhabdomyolysis. While undergoing initial treatment, she continued to have muscle damage - the fluids and supportive care was not enough to stop it.
Valli was presumed to have Immune Mediated Myositis and later confirmed via genetic testing to be a carrier of the MHYM gene (from her dam's side). Immune Mediated Myositis is a genetic form of non-exertional rhabdomyolysis that is recognized in Quarter Horses and related breeds. Horses can be carriers (heterozygous with one affected gene copy) or homozygous (two affected gene copies), or negative (two normal gene copies). Horses affected by IMM will have episodes of rhabdomyolysis in response to a triggered immune system, such as by significant systemic illness or vaccination. They will have severe muscle atrophy which can result in loss of 40% of muscle mass within 48 hours. Treatment involves supportive care, treatment of any underlying illness, and corticosteroids to stop the IMM rhabdomyolysis.
Valli improved drastically once started on a course of steroids. She quickly become much more comfortable and was able to go home on an extended course of medications as well as a long a slow rehab program to help her recover her muscle.
IMM genetic testing for the MHYM gene is now available in AQHA panels and is encouraged by anyone breeding Quarter Horses, mares and stallions. It is not recommended to breed horses with two copies or carriers with only one MHYM gene as even these horses, like Valli, can become symptomatic.
Vitamin E deficiencies are also a large problem in the Spokane/North Idaho region. Horses must ingest vitamin E, and the only natural source is fresh pasture grass. All hays as well as most grains and even vitamin/mineral supplements do not contain much or any vitamin E. When supplementing, horses only absorb the natural form (d-alpha-tocopheryl acetate) and should receive about 3-4IU/kg of bodyweight. For an average sized horse this is about 1500-2000IU per day. Horses in heavy work, broodmares, and foals will require more. Vitamin E is a powerful antioxidant is involved in critical pathways of the reproductive, muscular, nervous, circulatory, and immune systems.
* Photo of Valli taken in summer 2020 before her IMM episode.
* Video of Valli shortly before being presented at MPEH can be seen in separate post.
* All those with questions about Vitamin E, Dr Craven will be speaking about VitE and Selenium at our upcoming 20th Annual Equine Education Day, March 9th! See prior post for details!
