The anal sacs sit on either side of the anus and are normally emptied when the dog passes stool. They can become overfull, inflamed or in some cases develop an abscess. The dog may lick its rear, scoot on its bottom, smell strongly or react when it sits down.
A dog that drags its bottom does not necessarily have anal sac problems. Itching, skin irritation, parasites and other problems around the anus can cause similar signs. The vet examines the area and decides whether the sacs are full, inflamed or infected and whether there is an underlying cause for recurring problems.
Contact a vet if your dog is in pain, develops swelling or fluid in the area, or if the problem keeps coming back. A tender lump next to the anus can be a sign of an abscess and should be assessed quickly.