Dr. Cox fielded an early morning phone call from a distressed owner in late November, 2015. The client went to feed her horses at 6:30 AM and found her Quarterhorse gelding to be lethargic, sweating profusely, trembing all over his body, and his head hanging very low with no interest in eating hay or grain. The horse was fine yesterday, and was cross country hacked and jumped by her daughters. He did seem quieter than normal under saddle, and refused 2 jumps, which was abnormal for this forward and willing athlete. 

 

After a short conversation between Dr. Cox and the owner, it was evident that the gelding had a no fever, was mildly interested in hay (would lip up hay half-heartedly), had passed normal manure overnight, drank approximately 1" of water from his bucket, and had not urinated overnight. No overt signs of colic were present and the gelding was not willing or seemingly able to move around in his stall. He just stood in the corner with his head down. If forced to move, his respiratory rate would rise to 40 to 50 breaths per minute and his muscle trembling would worsen. The horses have enjoyed 24/7 turnout in large pastures with some wooded areas for the past few weeks. The 2 other pasture mates were normal and seemingly unaffected. 

 

During the drive the farm, the owner called in distress. The gelding now had green hay material coming from his nostrils. It seemed possible a simple hay choke had ensued in the early AM and was causing his inappetance. Upon arrival Dr. Cox examined, sedated, and resolved the gelding's choke. A rectal exam revealed a moderately large urinary bladder and a large volume of feces that had been present in the rectum for a long period of time (many hours due to mucosal tags adhered to the fecal balls). 

 

After the sedation wore off, the gelding was mildly interested in handgrazing. He remained quiet but responsive to his surroundings, and his muscle fasiculations (trembling) waxed and waned, always worsened by moving him around the stall or paddock. He proceeded to choke for a 2nd time on green grass while handgrazing. 

 

The decision to refer the gelding to Cornell University for futher diagnostic evaluation was made due to multiple worsening muscle deficits (esophageal, triceps/quadriceps, and rectal muscle), difficulty swallowing, mild facial swelling, and an increasingly quiet/lethargic patient. 

 

During the next 8 hours at Cornell, his condition deteriorated despite aggressive medical treatment. His muscle fasiculations worsened and failed to respond to medications. He became recumbant and his heart rate exceeded 90 beats per minute. His muscle enzymes were profountly elevated and his urine was coffee colored (myoglobinuria) due to a myoglobin - a sign of rhabdomyolisis or muscle degeneration. 

 

The current working differential diganosis for the failing gelding was Red Maple toxiticy, and he was not responding to the appropiate medical therapy. Due to his increasing pain level and worsening condition he was humanely euthanized. 

 

His owner was devastated. Desperate to find a source of Red Maple trees on her property she surveyed her pastures in the daylight the next morning. Instead of Red Maple -- she found something worse. 

 

Box Elder trees. 

 

The seed pods of these trees contain an amino acid called hypoglycin A, which is exceedingly toxic to horses. Affected horses are reluctant to move, have muscle weakness and stiffness that progresses into muscle tremors, have increased heart rates, choke frequently, and exhibit myoglobinuria. Most affected horses die within 72 hours. Other than supportive care, there is no 'treatment' or specific antidote. 

 

Most cases of seasonal pasture myopathy occur in the fall when pastures are grazed down, horses are not yet being fed hay while pastured, horses are pastured for over 12 hours daily, and rainy or windly conditions occur just prior to clinical presentation. All of these conditions applied to this case. 

 

The only possible good that can come from this family's heartache is that another horse may be spared from hypoglicin A toxicity. Box Elder trees, Red Maple trees, and European Sycamore trees should be removed from horse pastures. See pictures of each of these trees in the images to the right. 

 

Top picture - Box Elder tree seed pods. From www.foragingtexas.com 

 

Middle picture - Maple tree leaves. From http://gk12glacier.bu.edu/wordpress/sandersdemott2012/2012/11/02/leaf-idenification-lesson-part-2/

 

Bottom picture - European Sycamore tree with leaf and seed pod. From www.equineworld.co.uk

 

Thanks for taking the time to read about seasonal pasture associated myopathy. Sometimes, knowledge can prevent tragedy. Many thanks to our clients for allowing us to share this story.