Anal glands

The anal glands or anal sacs are small, paired sacs located on either side of the anus between the external and internal sphincter muscles. Sebaceous glands within the lining secrete a foul smelling liquid that is used for identification of members within a species. These sacs are found in all carnivora except bears and sea otters.

Dogs
In dogs, these glands are occasionally referred to as "scent glands", because they enable the animals to mark their territory and identify other dogs. The glands can spontaneously empty, especially under times of stress, and create a very sudden unpleasant change in the odor of the dog. The anal glands normally empty when the dog defecates. Failure to empty results in discomfort from the full anal gland pushing on the anus. The glands can be emptied by the dog's keeper, or more typically by a groomer or veterinarian, by squeezing the gland so the contents are released through the small openings on either side of the anus. Discomfort is evidenced by the dog scooting its posterior on the ground, licking or biting at the anus, sitting uncomfortably, difficulty sitting or standing, or chasing its tail.

Discomfort may also be evident with impaction or infection of the anal glands. Anal gland impaction results from blockage of the duct leading from the gland to the opening. The gland is usually nonpainful and swollen. Anal gland infection results in pain, swelling, and sometimes abscessation and fever. Treatment is by expression of the gland, lancing of an abscess, and oral antibiotics and antibiotic infusion into the gland in the case of infection. The most common bacterial isolates from anal gland infection are E. coli, Enterococcus faecalis, Clostridium perfringens, and Proteus species.

Anal glands may be removed surgically in a procedure known as anal sacculectomy. This is usually done in the case of recurrent infection or because of the presence of an anal sac adenocarcinoma, a malignant tumor. Potential complications include fecal incontinence (especially when both glands are removed), tenesmus from stricture or scar formation, and persistent draining fistulae.

Anal gland fluid is normally yellow to tan in color and watery in consistency. Impacted anal gland material is usually brown or gray and thick. The presence of blood or pus indicates infection.

Opossums
Opossums use their anal glands when they "play possum". As the opossum mimics death, the glands secrete a foul-smelling liquid, suggesting the opossum is rotting. Note that opossums are not members of the carnivora, and that their anal sacs differ from those of dogs and their relatives.

Skunks
Skunks use their anal glands to spray a foul-smelling and sticky fluid as a defense against predators.

Weasels
Research by the Institute for Pheromone Research and the Department of Chemistry, Indiana University suggests that weasels may well use their anal glands in gender recognition.