By T. J. Dunn, Jr., DVM
Few topics raise dog owners’ eyebrows (and lower dogs’ tails) faster than the subject of anal sacs! Even though anal sacs are often also called anal glands, they technically are not truly glandular in structure. These two small repositories of foul smelling material technically are small pouches lined with cells whose job it is to continually produce an oily semi-liquid substance that is stored within the sac. But why?
For a credible response I draw on my extensive knowledge of comparative anatomy, pathology, microbiology and biochemistry and respond, "I have no idea."
"You think it’s his what?" asked the incredulous owner of the uncomfortable Basset Hound on the exam table. I restated my diagnosis, knowing the owner probably never heard the term anal sacs before.
"Barkley has an anal sac infection. See how swollen and inflamed it is next to his anus. In fact the abscess is nearly ready to break through to the surface of the skin. We’ll need to open this abscess and allow the infection to drain, and then we’ll flush it, check the other anal sac, and send him home on antibiotics. Warm compresses for a few days will speed up the healing, too."
Anal sacs are small paired pockets located between the internal and external anal sphincter muscles, one on each side of the anus at the 4 and 8 o’clock position. The sac empties through a short and narrow duct to the surface near the inside edge of the anus. Each sac is lined with abundant sebaceous (oil) glands and numerous apocrine (sweat) glands. The secreted substance is a semi-oily, brownish fluid that packs an odor strong enough to gag a maggot!
For unknown reasons some dogs' anal sacs produce a rather thick, semi-solid material which is much more prone to impacting the sac due to the sacs' inability to pass this semi-solid material through the narrow duct to the outside. In many of these cases the sac will become infected, cause pain and inflammation and even break through to the skin surface.
The abscessed anal sac may need surgery to provide drainage and curettage of the damaged and infected sac and surrounding tissue. Antibiotics are indicated in abscessed anal sac disease.
One study indicated that anal sac disorders affects about 12 percent of dogs. There are a number of theories why dogs, cats, skunks, and other mammals have anal sacs and what possible use they may have. One theory states that anal sac contents, when excreted with the passing stool or by anal sphincter muscle contraction, act as a powerful territorial scent marker somewhat akin to humans posting a No Trespassing sign. Another theory states that the anal sac material lubricates hard stool, which makes passage easier. Humans do not have anal sacs. It’s bad enough that some humans get hemorrhoids, which are dilated, irritated blood vessels at the anus; luckily, dogs do not get hemorrhoids.
There are no predictable antecedents to painful anal sac disorders. Infections, obstruction and subsequent overfilling (called impaction), and rupture with drainage through the perianal skin are the most common clinical presentations. Anal sphincter muscle dysfunction, enlarged anal sacs that are not compressed properly by the anal sphincter muscle, hypersecretion from the anal sac lining, obstructed or constricted anal sac ducts all may be predisposing circumstances for anal sac problems.