Back pain in horses has always been and still is a difficult diagnosis. Many horses are diagnosed incorrectly with it and treated by chiropractors and acupuncturists and even veterinarians even though the horse probably doesn't even have back pain. Lots of horses get injections in their backs because back pain is difficult to prove. Certainly horses can have back pain but determining for sure which ones do and which ones don't is difficult. And the very low back, or sacroiliac joint, is also difficult to diagnose as a cause of pain.
To correctly diagnose the condition, your vet will have to numb or block the joints and then see if the horse is clinically improved. However Dr. Rob Nan Wessum indicated at a recent AAEP meeting that there are several gaits that suggest sacroiliac joint disease. Many horses with sacroiliac disease will track narrow when at a walk or a trot: To avoid rotating the sacrum, the horse places the hind feet closer together than normal when walking, and it appears the horse is walking on a line with each hind foot landing in front of the other. Also, some horses with sacral pain will walk with both limbs advancing at the same time while normally the walk is a four-beat gait and all limbs move at different times. This is especially evident when you walk the horse in a serpentine or zig zag pattern. Another key for sacral pain is when the horse is trotting in a circle, the hind limbs do not follow the front limb and are either in a wider or narrower path. Many of these horses will also hold their tail to the same side when walking and will also bunny hop or move both back legs together when cantering. If your horse has any of these characteristics, ask your vet about sacroiliac pain.