Excerpt from Ask the Vet About Dogs: Easy Answers to Commonly Asked Questions

What are anal sacs, and why does my dog need to have hers emptied?
On either side of your dog’s anus, just under the skin at the two o’clock and ten o’clock positions, are two small sacs, each with a duct that opens into the rectum just as it exits the body. The walls of these two sacs are lined with cells that secrete a smelly, oily substance. These make up the anal glands. As a dog defecates, the stool that passes through the anus places pressure on the two sacs, and their smelly contents are smeared onto the feces. This is thought to be a method by which dogs communicate with each other: A dog who encounters another dog’s feces can tell something about that dog by the unique smell of the material secreted by his or her anal glands.

Some dogs seem to be losing their ability to empty their anal sacs. Small breeds of dogs, whose physical characteristics have been artificially altered by human selection, often have sacs whose ducts are too small for proper emptying. Individual dogs of all types may experience similar problems. When the sacs become full, or impacted, they are quite painful, and most dogs try to relieve their discomfort by scooting on the floor or licking and chewing at their anal area. Your veterinarian can periodically manually empty the anal sacs, and determine whether there is any sign of infection or other disease.

Two things may help: First, be sure your dog has firm stools. You may wish to add some high-fiber food to her diet. Ask your veterinarian about foods made for overweight or diabetic dogs. The fiber in these diets can firm up your dog’s stools, ensuring that the stools place pressure on the anal sacs as they pass. Second, be sure your dog gets plenty of exercise. Strong muscles help dogs have normal bowel movements and empty the sacs normally.

Surgical removal of the sacs is an option, but it requires an incision into the tissues around the anus, and there may be permanent nerve damage after the surgery that prevents your dog from controlling her bowel movements. I recommend surgery only as a last resort for a dog who suffers from chronic infections of the anal sacs.