Diagnosis and treatment of colic in horses has certainly improved over the last 20 years but recurrent colic is still difficult to diagnose and treat. Dr. Amanda House indicates in the Practitioner magazine that colic is considered recurrent when three or more episodes occur over a period of a few months. The causes of recurrent colic are varied and this makes the problem that much more difficult to diagnose but intestinal parasites, gastric ulcers, sand accumulation, impactions, enteroliths, and inflammatory bowel disease are all possible causes. Certainly a good physical exam is required and your vet must also check all organ systems and not just the GI tract as colic can also be caused by reproductive problems and kidney or liver disease. Because of this, a rectal exam is required as well as blood work to check organ function.
If a cause is not discovered, testing for parasites, collecting an abdominal fluid sample and using a long scope to examine the horse's stomach are good ideas. Also, ultrasound of the abdomen can help identify liver, kidney, and even intestinal problems. A piece of tissue or biopsy can be carefully removed from the rectal mucosa and submitted for testing, which can identify certain intestinal syndromes.
Even after all of this, the reason some horses experience recurrent colic may be unknown. Sometimes preventing recurrent colic can be as simple as a change in diet, even if your horse has been on the current diet for a long time. However, this change must occur slowly over a period of weeks because a rapid change in diet can also lead to colic. If you have a horse that has recurrent colic, it is important to try and find the problem as eventually surgery may be required for one of the episodes.