Overview 

The anal sacs (also called anal glands) are two small structures that are positioned under the skin, one on each side of the anus. The inner lining of each sac produces a substance (or discharge) that is released through a small opening near the anal opening. This substance has a strong, unpleasant odor, and it can vary in color and consistency.  

Under normal circumstances, it is released as the dog defecates — when there is pressure against the anal sacs. There can also be a spontaneous release of this discharge when a dog is nervous or excited. As such, it's likely that this substance serves as territorial scent marker.  

In addition to benign disease conditions of the anal sacs including impaction, sacculitis and abscessation, the anal sacs can also be affected by cancer. The most common type is known as anal sac adenocarcinoma or apocrine gland anal sac adenocarcinoma (AGASACA). It makes up 2% of skin tumors in dogs.  

Which dogs are most commonly affected? 

These tumors are found mostly in older dogs with an average of about 10 years of age. There is no difference in occurrence between males and females.

Breeds most commonly affected include Alaskan Malamutes, Cavalier King Charles Spaniels, Dachshunds, English Cocker Spaniels, English Springer Spaniels, German Shepherds and Golden Retrievers.  

What are the signs of a problem? 

Some dogs may not show any clinical signs, but a veterinarian may detect a mass during a routine rectal examination. Sometimes a swelling near the anus may be noted. 

Other signs include lethargy, straining to defecate, constipation, decreased appetite, vomiting, and increased thirst and urination. Weakness or swelling in the hind limbs may occur.   

How is it diagnosed? 

The diagnosis generally begins when your veterinarian finds a mass in the area of one or both anal sacs while performing a rectal examination. At the same time, your veterinarian will feel for enlarged lymph nodes (which won’t be present in all dogs or able to be reached in larger dogs). Blood tests and urine tests are often normal, but abnormal findings can include elevated calcium levels in the blood (hypercalcemia) or evidence of kidney disease.  

A sample of the mass can be collected with a needle and examined microscopically for cells that are consistent with this type of tumor. However, a biopsy is needed for a definitive diagnosis, which can be obtained by taking a piece of the mass or by removing the entire mass.  

Other tests include radiographs of the chest and an ultrasonography of the abdomen to look for signs that the tumor may have spread (metastases). Tumors can be found in the lungs in about 2-13% of dogs. In the abdomen, metastases will be identified as enlarged sublumbar lymph nodes in about 50% of dogs. A smaller percentage of dogs will have evidence of metastases in other abdominal organs. A CT scan or MRI are alternate forms of imaging that may be available in some facilities.  

How is it treated? 

Dogs with hypercalcemia may need to be stabilized with intravenous fluids and medications, such as corticosteroids or diuretics.  

Surgery to remove the mass and the entire anal sac (anal sacculectomy) is the treatment of choice. Complete removal can be challenging for larger masses due to their proximity to the rectum. One of the major complications of surgery is fecal incontinence, which may be temporary or permanent. Other complications include infection, poor wound healing and anal stricture.  

If enlarged lymph nodes are present, they can be removed via a separate approach through the abdominal wall. This can be a complicated procedure, and it carries risks of severe hemorrhage and nerve damage that can result in temporary or permanent urinary incontinence. The overall complication rate is approximately 10%. 

Radiation therapy may be used preoperatively, intraoperatively or postoperatively on the anal sac mass and the lymph nodes — reducing the risk of recurrence in cases where complete removal is not possible. It can also be used palliatively in cases where surgery is not an option.  Complications, including life-threatening ones, can occur.  

Chemotherapy has not been proven to improve survival, but may provide some benefits, especially when combined with radiation therapy.   

Palliative care will include pain medications, such as nonsteroidal anti-inflammatories (NSAIDS), corticosteroids (not if NSAIDS are being used), fluid therapy and diuretics. 

Outcome 

Survival times are affected by a number of variables, including the size of the tumor, lymph node metastases, distant (e.g. lung) metastases and the treatment protocol. 

Some generalizations can be made for dogs who are treated with surgery: 

• Some dogs may experience an average survival time of more than 3 years. 

• Recurrence of tumor at the surgical site occurs in up to 45% of cases.  

• Tumors less than 2.5 centimeters in size have a better prognosis. 

• Dogs without enlarged lymph nodes or other metastases have a better prognosis. 

• Dogs with enlarged lymph nodes have improved survival times if the lymph nodes are also removed. 

• Dogs with hypercalcemia have a shorter average survival time. 

• Radiation therapy can improve clinical signs and survival time. 

Dogs not treated with surgery may benefit from chemotherapy and/or radiation therapy.  

Prevention 

Because of possible complications, routine anal sac removal is not recommended as a preventive measure. No other preventive measures are available.  

Routine check-ups including rectal examination are recommended at least once per year and twice annually in older dogs.