This benign condition is characterized by rapid abnormal growth of one or more mammary glands. There are two basic types of hyperplasia of the feline mammary gland—lobular hyperplasia and fibroepithelial hyperplasia. Lobular hyperplasia is seen as palpable masses in one or more mammary glands in intact cats 1–14 yr old. Fibroepithelial hyperplasia occurs in young, cycling, or pregnant cats; in old, intact females and males; and in neutered males after treatment with progestins.
Feline mammary hypertrophy is considered to be a hormone-dependent dysplastic change in the mammary gland. Hyperplasia occurs within 1–2 wk after estrus or 2–6 wk after progestin treatment. The tremendously enlarged glands may appear erythematous, and some of the skin may be necrotic. Edema of the skin and both hindlegs is common, and the condition can easily be confused with acute mastitis.
Ovariohysterectomy or mastectomy is curative, although spontaneous remissions occur. Ovariohysterectomy is followed by regression of the glands and prevents recurrence.