Anal sacs are paired small pouches located on either side of the anal opening in dogs, cats, ferrets and other animals. The lining of the sacs produces a smelly brown liquid that is usually eliminated in small quantities during defecation. The purpose of the anal sac is unknown, but it is thought that they function in communication about the animal or its territory.

Anal sacs can become inflamed, infected, or impacted (blocked). With these conditions, it is common for your pet to scoot their bottom on the ground and lick or chew at the area.

Tumours of the anal sacs (apocrine gland adenocarcinoma is the most common tumour type) are a serious but uncommon problem because they tend to invade surrounding tissues and metastasize (spread to distant tissues) even when the primary tumour is very small. The tumours are almost always only on one side. In approximately 25% of cases, the tumour can cause hypercalcemia (elevated blood calcium) which can cause kidney failure.

Anal sac tumours occur in male and female dogs with roughly an equal incidence. They occur in any breed but are more common in Spaniel breeds. Ten years is the average age of affected dogs. Anal sac tumours are very rare in cats.

Signs and Symptoms:

Figure 1

The signs of anal sac tumours can be variable:

  • An external swelling in the perianal region (Figure 1).
  • A mass may be felt during a rectal examination.
  • Constipation.
  • Pain or straining to defecate.
  • Blood in the stool.
  • Excessive thirst and urination may occur if kidney failure is present from elevated calcium levels.
  • Without enlarged lymph nodes (in the abdomen), even a large tumour often produces limited symptoms associated with defecation.

Animal owners may observe symptoms of kidney failure from the elevated calcium (referred to as hypercalcemia of malignancy).

These symptoms generally include:

  • Increased thirst.
  • Increase in urination.
  • Vomiting.
  • Loss of appetite.
  • Lethargy (weakness/tiredness).


In general, the following tests are recommended to diagnose the tumour, provide a clear clinical picture of overall health and evaluate for metastasis:

  • Aspiration: A small needle is inserted into the tumour to obtain a few cells that can differentiate cancer from infection or inflammation.
  • Blood tests: Assess overall health. Evaluating for hypercalcemia and kidney failure
  • Chest x-rays: Evaluate for metastatic nodules and other heart and lung problems
  • Abdominal ultrasound: Examination to evaluate for enlarged lymph nodes or tumour spread into other organs such as liver, kidneys, etc. These enlarged lymph nodes are often what produce symptoms associated with defecation.


Consultation with your primary care veterinarian may result in a referral to a veterinary surgeon to fully explore your options.

  • Surgery is the mainstay of treatmentIt is the only proven method to influence the survival of dogs with these tumours. The tumour is removed through an incision adjacent to the anal opening directly over the tumour. Wide and aggressive removal is not feasible due to the adjoining rectum and anus. With large tumours, additional tissue attached to the tumour may need to be removed. This may result in some of the complications noted in the section “Aftercare and Outcome.”
  • If there are enlarged lymph nodes in the abdomen, they are removed through an abdominal surgical approach on the underside of the dog. These nodes are enlarged in about 50% of cases. This can be done at the time of the primary tumour removal, shortly after that, or later if these nodes enlarge. This procedure is done to alleviate constipation and difficulty defecating.
  • If kidney failure or hypercalcemia is present, therapy with intravenous fluids and medications may be needed before surgery to make your dog a more suitable candidate for anaesthesia. In some cases, kidney failure can be permanent.

After surgery, chemotherapy and/or radiation treatment may improve the life expectancy of your pet.

Aftercare and Outcome:

Most animals are discharged one-two days after surgery. There is usually a follow-up appointment to see how your dog is doing and to remove skin stitches or staples (if present). Pain can be well-controlled with owner-administered medications.

Restrictions following surgery usually are:

  • Use of a restrictive collar for 10-14 days after surgery to prevent the natural tendency of dogs to lick and chew at a wound. This behaviour can cause a breakdown of the wound and infection.
  • Stool softening medications may be needed until swelling resolves.
  • Limited and restricted activity is indicated for about two weeks to allow recovery and incision healing.

Postoperative complications can include:

  • Incision infection.
  • Wound breakdown (dehiscence).
  • Fecal incontinence can occur in up to 33% of dogs, especially with the removal of larger masses. This is usually temporary, but owners need to be aware of this problem. If the tumour is only on one side, the incontinence is typically partial in that the dog has difficulty controlling bowel movements but not continuous dropping of stool.
  • Continued kidney problems.

The prognosis with apocrine gland adenocarcinoma depends on the type of treatment, size of the mass, presence of hypercalcemia and presence of lymph node involvement. Surgical removal of these nodes can produce long-term relief of constipation. Some animals have had multiple surgeries to remove recurrent lymph nodes to alleviate obstructions successfully.

It is important that your veterinarian examine the anal sacs as part of your dog’s routine examination. Early detection can significantly improve survival.

Anal sac adenocarcinoma tumour removal: