ORTHOPEDICS The causes of stringhalt
Q: This year I bought a horse to prevent him from going to slaughter. I immediately recognized a stifle issue, perhaps a locking patella, but because I have the means and the space for a pasture pet, I purchased him from the auction. I will never know his history, but there is evidence that he has had an accident. He looks like he got into sheep wire and has a healed injury to one hoof that gives him no problems. His issue is that occasionally his hind leg locks in the up position. He appears to be in no pain. I am hoping you will outline treatment options for me. He is a very kind, respectful 8-year-old horse who deserves a chance to live his life to the fullest. I don’t need him to be a riding horse, but if he could be sound enough to ride it would be good. Lynn Howland Maple Valley, Washington
A: A locking patella (upward fixation of the patella) will cause the rear leg to lock in extension. The toe will often drag on the ground because the horse is unable to bend the stifle or the hock. What you have described sounds like the opposite condition: “The leg locking in the up position” is most compatible with a diagnosis of stringhalt, which is characterized by excessive contraction of the digital extensor muscles---sometimes to the point of the horse kicking himself in the belly with each step. It is not thought of as an overtly painful condition, but it is an extreme muscle contraction or spasm.
There are two major categories of stringhalt:
• Australian stringhalt (which also occurs in the United States) results from ingestion of a toxic plant. Flatweed, common dandelion and little mallow have all been implicated in this form of the disease, although these weeds are not consistently toxic. It is possible certain environmental conditions may increase toxicity because this form of the disease seems to occur more often in late summer or fall. Horses with Australian stringhalt are often affected in both rear legs. Spasms can also sometimes occur in the front legs and the muscles of the larynx. These horses usually recover when they stop ingesting the toxic plant, but it may take months.
• Classic stringhalt is not related to plant toxicity. It may result from a neck or back injury, but most commonly the cause remains a mystery. Horses affected with classic stringhalt rarely recover and often become progressively worse with time. The disease usually affects one rear leg but some cases progress to involve both. Many treatments, including muscle relaxants, anticonvulsant drugs, Botox (botulinum toxin) and surgery, have been tried with variable success. These horses are not appropriate for riding and are often very limited in their ability to perform groundwork maneuvers.
Your horse is very lucky to have found a home that does not require him to be an athlete. If you do decide to ride this horse I would strongly suggest a thorough neuromuscular exam first. Due to safety concerns, I would never advise my clients to ride a horse affected with stringhalt.
BIG STEP: In stringhalt, the digital extensor muscles contract excessively, causing overflexion of the hind limb.