Infection and Tumors of the Breasts in Cats

The cat normally has four pairs of mammary glands. The upper two pairs have a common lymphatic channel and drain into the axillary (armpit) lymph nodes. The lower pairs also have a common channel and drain into the inguinal (groin) lymph nodes. Infections and tumors of the breasts may cause enlargement of the corresponding lymph nodes.

Mammary Hyperplasia

This condition is most commonly seen in young cats. Unspayed female cats can experience an enlargement of one or more breast glands that begins one to two weeks after their first heat cycle. This condition, called mammary hyperplasia or mammary hypertrophy, is caused by high levels of progesterone. Pregnant queens also may experience mammary hypertrophy that begins during the first two weeks of pregnancy. In a pregnant cat, this needs to be distinguished from mastitis and mammary cancer. The condition has also been reported in neutered females and males receiving progesterone therapy for another condition, and will appear two to six weeks after the therapy.

The breast enlargement may or may not be painful. In severe cases, characterized by rapid increase in breast size, there is reddish-blue discoloration, warmth, pain, and ulceration of the skin overlying the swollen breast. The hind legs may become swollen.

One very severe complication of mammary hyperplasia is clotting of the veins in the breast. This occurs in a small number of cases. The clotting process extends centrally. If the clots break free and are carried to the lungs, these cats may die suddenly from pulmonary thromboembolism. Secondary infections can lead to septic shock.

Treatment: The best treatment for mammary hyperplasia is spaying the cat. When the condition is related to pregnancy or progesterone therapy, consider either allowing the queen to deliver or discontinuing the progesterone. The risks must be balanced against the benefits. Breast biopsy is indicated only when breast swelling or enlargement occurs in a cat who is neither pregnant nor experiencing her first estrous, or when the swelling does not disappear after the source of the progesterone has been removed.

A new medication, aglepristone, is a progesterone blocker and removes the hormonal stimulus for the problem. It has been used experimentally with good success, but at this time it is not approved for use in the United States; it may become available in the future.