Dogs and cats can lick under or around their tails for a variety of reasons. Some of the most common are:
Anal gland disease
Anal gland disease is one of the most common reasons a dog will lick under the tail. Dogs and cats have two anal glands (also called anal sacs) on each side of the anal opening. These can become impacted or infected. If they do, the pet will often lick the area to try to relieve the pressure in the glands. Some dogs will also 'scoot' along the floor when they have an anal gland problem.
Reproductive disease or discharges
Female animals will often lick the vulvar area when they are in heat. They will also lick the area if they have a uterine infection (pyometra). Some female animals develop vaginitis which can cause severe itching. A fourth reason a female may lick her vulva is when she is in the process of giving birth.
Fleas often bite or are found at the base of the tail - where the tail attaches to the body. Cats and dogs with fleas may lick or chew at that site.
On occasion, a pet will lick the anal area if it has intestinal worms. However, most animals with worms, do not show this behavior.
Dogs and cats may lick under or around the tail as a normal grooming process. If you see your pet is licking excessively, however, it is probably a sign of a medical problem and you should contact your veterinarian.