The anal sacs are located on each side of the anus at roughly the 4 and 8 o'clock positions, just under the skin. They open to the outside by tiny passageways or ducts. Glands within the anal sacs produce a dark, foul-smelling substance. The sacs normally empty as the animal has a bowel movement or when the muscles around the anus are flexed (sometimes happens with nerves or stress). As presently understood, they have no known necessary function, although presumably they may have served a territorial marking and/or defensive function in ancient times. (A common example of useful anal glands seen today is in skunks.) As such, your pet can do well without them.

Disease of the anal sacs fall into 3 categories:

Impaction: The anal sac fluid is abnormally thick and cannot escape.

Infection: Bacteria produce a yellow or bloody pus (or sometimes can be more subtle changes), and infection may or may not exist in other areas.

Abscessation: As a result of infection, a hot (and usually very painful) swelling may occur near the anus in the area of the anal gland.  This may rupture and discharge pus and blood.

Signs of anal sac disease may include "scooting" (dragging the anus on the floor), excessive licking under the tail, tenderness near the tail or anus, and/or bloody or sticky drainage from the anal area.

Treatment depends upon the reason for the scooting. Inflammation of the rectum, vulva/scrotum or skin might require veterinary examination and tests. 

For anal sac disease treatment might include

• Manual expression (squeezing) of the sac contents.
  
  
• Flushing the sacs and instilling antibiotics into them.
  
  
• Surgical drainage or removal of the sacs.

The latter two should be done by a veterinarian.  Any animal with real discomfort or persistent scooting should be examined by your veterinarian.