Anal sac disease occurs in progressive stages: impaction, inflammation, and abscess formation. Impaction of the anal sacs occurs when the anal gland secretions contained in the anal sac thicken so that the sac is unable to empty during a bowel movement. Inflammation of the anal sac, or anal sacculitis, is an infection usually resulting from impaction; it may also be caused by bacterial growth within the anal sac.
During the sacculitis stage, the impacted fluid may become thinner and fill with pus. Abscess occurs when the inflammation of the anal sac has reached an extreme stage; at this point, a red-brown substance will be seen coming from the sac, which will be enlarged, hot, red, and very painful. Usually, the abscessed sac will rupture, leaving a hole near the side of the rectum that oozes a foul-smelling liquid. The tissues surrounding the abscess will swell up, and this will worsen the dog's inflammation and pain.
