Agility Dogs are athletes. As an athlete they can have subluxations and sprained muscles. Many times an owner brings the dog to the vet and the first response is pain meds, xray and or mri. This approach is like using a shot gun to kill a mosquito.
If the dog is evaluated by a board certified animal chiropractor a common cause for this issue is as mentioned above... subluxation and muscle strain. A course of adjustments and some rehab exercises often gets the dog back.
Your vet will not typically advise you on chiropractic since they have no training on this form of care.
I make house calls for dogs and would be happy to evaluate your dog, call me at 781-337-1180 to discuss your dogs problem.
Many of the agility dogs i see are adjusted on a routine to keep them healthy and better equipped to manage the course and not get injured.
