Stringhalt can be caused by a vertebrae in the spine and or the pelvis being subluxated or a poison from the sweet pea plant. If the sweet pea plant is ruled out then an evaluation by an Animal Chiropractor will help. Stringhalt is when your horse flexes the hind limb like a spasm. If it is a subluxation complex then this is similiar to sciatica in people and I could adjust your horse and give you some exercises which will help. Giving your horse Omega 3 will also keep the inflammation down.
Test your horse by backing him or her up. This is when the stringhalt is more evident, since your horse has to use these muscles and nerves to activate the back up process and thus any nerve irritation will show itself.
Contact me at 617-472-0661 or IndekChiropractic.com {horse and rider chiropractic}