Mammary Hyperplasia

Disorders of the mammary glands can be worrisome in cats.  Mammary gland cancer is the third most common cancer in the cat.  Unlike dogs, in which only half of all mammary tumors are malignant, mammary cancer in cats has a much worse prognosis, with 80 to 90% of mammary tumors being malignant. While most cases of mammary cancer occur in older cats (most commonly 10 to 12 years of age), mammary cancer has been reported in cats as young as 9 months of age.

There is another condition of the mammary glands called mammary hyperplasia, which is characterized by rapid growth of mammary tissues. Although the condition is considered to be benign, the condition can mimic mammary cancer, which has a much worse prognosis. Mammary hyperplasia, however, tends to occur in young, unspayed female cats instead of older cats. It is important to differentiate mammary hyperplasia from mammary cancer, as the ages of affected cats with these conditions can overlap. Mammary hyperplasia occurs as a result of the influence of progresterone on the mammary glands.  When a young cat becomes pregnant (ovulation followed by fertilization) or pseudo-pregnant (ovulation, but no fertilization), progesterone levels begin to rise.  Mammary tissues in cats have many progesterone receptors, and the binding of progesterone to these receptors can stimulate mammary gland development.   Although the condition is generally regarded as benign, in some cats, this stimulation can lead to severe swelling, pain, ulceration, and infection of the glands.  Treatment involves removing the source of the progesterone, although frequently, removal of the progesterone doesn’t result in regression of the swelling.  In the case of unspayed females, removal of the progesterone source is best accomplished by spaying.  The extent of the mammary gland swelling, however, can make a relatively simple surgery like spaying much more difficult.  The swollen mammary glands tend to be very painful, and in my experience, the incision site often becomes bruised and very tender afterward, causing a lot of discomfort for the cat.  My last case of mammary hyperplasia was a disaster. The client had unfortunately waited several days before bringing in the cat for examination, and by the time I had a chance to examine the cat, several of the glands had ulcerated and had become seriously infected.  When this happens, it is sometimes necessary to surgically remove the swollen and infected gland(s).  Unfortunately, this cat, with a high fever and severe dehydration, was in no condition to be anesthetized for any type of surgery. Emergency care with intravenous fluids and antibiotics was unsuccessful, and the cat developed septic shock followed by cardiac arrest.  She could not be resuscitated.  She was only 7 months old.

Questions as to why only a minority of cats develop mammary hyperplasia after exposure to high levels of progesterone, and why only young cats seem to develop the condition have yet to be answered, and the pathogenesis of this condition requires further study.

Although mammary hyperplasia is an uncommon and relatively benign condition, many cats develop significant clinical signs of illness, including lethargy, loss of appetite, and painful, uncomfortable mammary glands. Despite the benign description, most of the cases that I’ve encountered have been fraught with complications. The presence of progesterone receptors on feline mammary tissue offers the potential for targeted endocrine therapy with drugs that block progesterone receptors.  In fact, aglepristone, a progesterone blocker, has been successfully administered in cats with this condition, with excellent results. Unfortunately, the drug is not licensed for use in the United States.

Updated 4/1/2016