The anal sacs are a pair of scent glands located along each side of the anus. The secretion from the glands is foul-smelling and excreted in small amounts on the feces during bowel movements to mark a pet’s territory. Dogs and cats can function normally without these scent glands.
Cancer can develop in the anal sac glands in dogs, but rarely in cats. In most cases, this tumor affects only one anal sac, but occasionally both left and right sacs are affected. Anal sac cancer is usually malignant and has a high tendency to spread to the other body parts. If it spreads, regional lymph nodes and the liver are commonly affected; but, the cancerous tumor can also spread to the lungs.
You may not see any signs of a problem at home, yet your pet’s veterinarian may find a mass in the anal sac during a routine physical examination. Other signs may include a large visible swelling of the anal region, straining during bowel movements, thin ribbon-shaped stools, difficulty passing urine and arching of the back. Anal sac tumors can cause elevation of the calcium level in the blood, which results in increased thirst, increased urination, decreased activity, decreased appetite, weight loss, vomiting, muscle weakness and low heart rate.
The diagnosis of an anal sac mass is made upon digital rectal palpation by your veterinarian. Confirmation of the type of tumor necessitates a biopsy, which is frequently done after the tumor has been removed. Tests done prior to surgery may include a complete blood count, blood chemistry profile and urinalysis to check internal organ health. Chest radiographs (X-rays) and abdominal ultrasound are used to identify visible spread of the cancer. (Please note that microscopic spread of a tumor to other organs cannot be detected with radiographs and ultrasound.)
If the cancer has spread to the lungs, liver or other organs, surgery may only be a palliative measure (to relieve signs of straining to have bowel movements), but the survival time may not be improved.
If lymph nodes inside the abdomen are enlarged, a sign that the cancer may have spread, they should also be removed by making an incision on the lower part of the abdomen. Very large, invasive lymph nodes may not be safely removed; therefore, chemotherapy may be recommended prior to surgery to shrink the nodes. The lymph nodes will be evaluated using ultrasound every two to three weeks, and surgery will commence once the nodes have become small enough to be safely removed.
Surgical removal of the anal sac and its associated tumor has been shown to improve survival times and to help minimize clinical signs associated with the tumor.
Chemotherapy is usually recommended if the tumor is malignant. Consultation with a veterinary oncologist is recommended to determine optimal therapeutic options. Unlike humans, most dogs do not lose their hair and usually have only mild side effects from chemotherapy, which may include transient loss of appetite and vomiting.
Radiation therapy may be used to shrink an inoperable tumor to kill residual cells that may be present. Skin burns and irritation of the colon (side effects of radiation treatment) may be temporary or permanent complications of this treatment.
For more information on this subject, speak to the veterinarian that is treating your pet.