Anal Sac Tumors in Dogs

Elizabeth Riley, Veterinary Student Class of 2023
Date Published: 07/22/2021

Photo courtesy of BigStock

If your dog has ever scooted their rear end across your carpet, they were probably trying to empty their anal sacs. Humans don’t have anal sacs, but dogs and cats, along with many other animals, do. So, what are they?

Anal sacs, also called anal glands, are basically small balloons filled with a brown liquid notorious for its repulsively metallic, fishy smell. Dogs have two anal glands that sit just inside their anus at the 4 and 8 o’clock positions. When the dog defecates, some of the stinky fluid in these sacs is squeezed out with the feces. This liquid may play a role in territorial marking and communication amongst dogs.

There are many problems that can affect a dog’s anal sacs, including tumors. Anal sac tumors are abnormal, usually cancerous growths in the anal sac. They are uncommon but serious and left untreated are generally fatal. They can affect any dog but older dogs and certain breeds such as spaniels, German Shepherds, and dachshunds are at higher risk. These tumors cause problems as they spread locally and to distant sites of the body in a process called metastasis. They can bring illness, discomfort and a decreased quality of life to your dog. Thus, it is essential to catch these tumors early and to treat them as soon as possible.

What causes anal sac tumors?

As with most cancers, a cause has not been identified. However, genetics and environmental factors probably play important roles.

Although we can’t pinpoint a cause, we do have some understanding of what’s happening at a microscopic level. For whatever reason, cells in the anal sac start dividing and multiplying in an uncontrolled manner. This cell division creates a mass that is usually cancerous. The mass makes the anal sac larger, but this alone usually doesn’t cause issues, at least until the mass gets quite large. The big problem is when cancer cells break away and travel to other parts of the body to make new tumors. The cells often spread to lymph nodes, which are small lumps of immune tissue scattered throughout the body. They can also spread to essential organs, such as the lungs, liver and kidneys.

The other big problem with anal sac tumors is that they often increase the amount of calcium in the blood. High calcium levels can damage the kidneys, which are the organs that filter blood and make urine. High calcium levels can also disrupt the function of the nervous system, intestines, and heart. As you might imagine, this disruption can cause a range of symptoms and issues for your dog.

What are the signs of anal sac tumors?

It depends on the behavior of the tumor and where it has spread. Many dogs won’t show any signs. However, when the tumor spreads to nearby lymph nodes or the mass gets large, your dog may strain and struggle to defecate, or not even defecate at all. They can also have ribbon-like stool and swollen back legs. When the tumor increases blood calcium levels, your dog may drink and pee more, eat less, vomit and seem weak and tired.

Other possible signs include:

  •        Swelling at the rear end
  •        Licking the rear end more than usual
  •        Bleeding around the anus
  •        Bloody stool

Dogs with anal sac tumors may also scoot, meaning they sit and drag their rear end against the ground. However, scooting much more commonly indicates full or inflamed anal glands that need to be expressed (squeezed) by your veterinarian, rather than cancer.

How are anal sac tumors diagnosed?

Your veterinarian will talk to you about the history of your dog’s problem as well as perform a physical exam. As part of this, they may conduct a rectal exam to feel for the tumor. During this procedure, they will stick a gloved finger through your dog’s anus into the rectum, which is the tube carrying feces out of the body. If they feel or see a growth, they might do a fine needle aspiration of the mass. This means they will poke the mass with a needle, suck out some of the cells, put them on a glass slide and send it to a lab for evaluation. This information will help narrow down what the mass is. If these results are suspicious of cancer, your veterinarian will likely surgically remove the mass and send it to a lab to confirm it is cancer.

Your veterinarian will also do additional tests to figure out if and where the tumor has spread and if it has affected the body in other ways. For example, your veterinarian will do blood and urine tests to check for high blood calcium and kidney damage. They will also do chest x-rays and abdominal ultrasound to see if the tumor has spread to the lymph nodes and organs, such as the lungs, liver and kidneys.

How are anal sac tumors treated?

Until more serious treatment can be pursued, your veterinarian may recommend stool softeners to make it easier for your dog to defecate and give fluids and medications to lower blood calcium. They may also refer you to a veterinary surgeon more well-versed in treating these serious tumors. If it is possible, treatment generally involves surgical removal of the mass and any enlarged lymph nodes in the area. Surgery has been shown to lengthen the survival time of dogs with anal sac tumors. However, tumors may eventually reappear if cancer cells are left behind and the cancer cells may have already spread to other sites at the time of removal. After surgery, some dogs won’t be able to control their bowel movements for a while and there is a risk the surgical site, especially it is near the route feces take out of the body.

Because anal sac tumors have such a high risk of spread, chemotherapy is often recommended after surgery. It involves giving medications, often via the mouth or blood, to kill or inhibit cancer cells throughout the body.

Radiation therapy may also be recommended after or instead of surgery if surgery isn’t possible. It involves using radiation to destroy cancer cells. It is only offered at certain facilities, such as veterinary schools and some specialty practices, due to the expense of the equipment. Many people travel out of state to get radiation services.

Both chemotherapy and radiation therapy can have serious side effects that your veterinarian will discuss with you.

What is the prognosis?

Survival times for dogs with anal sac tumors are variable but on average range from about 1-2 years. However, every dog’s situation is different because prognosis depends on many factors such as:

  • The size of the tumor
  • Spread throughout the body
  • Blood calcium levels
  • Type of treatment

Dogs who receive surgery or any type of treatment generally have a better prognosis whereas the outlook is often less promising for dogs with larger tumors, high blood calcium levels, and extensive spread of the tumor.

How can I prevent anal sac tumors?

There’s no proven way to prevent anal sac tumors from developing. Early detection is important and may be achieved through routine rectal exams performed by your veterinarian. In fact, sometimes anal sac tumors are an incidental finding during a seemingly healthy pet’s physical exam.