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Benign Prostatic Hyperplasia in Small Animals

By Michelle Kutzler, DVM, PhD, DACT, Banfield Professor of Companion Animal Industries, Oregon State University

Benign prostatic hyperplasia (BPH) is the most common prostatic disorder and is found in most intact male dogs >6 yr old as a result of androgenic stimulation or altered androgen:estrogen ratio. It is not known why some males are affected and others are not. In some dogs, hyperplasia may begin as early as 2.5 yr of age and, after 4 yr of age, cystic hyperplasia tends to develop. Clinical signs may be absent, or tenesmus, persistent or intermittent hematuria, or hemorrhagic preputial discharge may occur. The diagnosis is suggested by physical and historical findings and by a nonpainful, symmetrically enlarged prostate. Radiology can confirm prostatomegaly. Ultrasonography should show diffuse, relatively symmetric involvement with multiple, diffuse, cystic structures. Cytologic examination of massage or ejaculate specimens reveals hemorrhage with mild inflammation without evidence of sepsis or neoplasia. Definitive diagnosis is only possible by biopsy. Castration is the treatment of choice; prostatic involution is usually evident within a few weeks and is often complete in several months.

For males intended for use in breeding, medical therapy with finasteride is effective. Finasteride blocks the action of 5 α-reductase, an enzyme that converts testosterone to dihydrotestosterone. Dihydrotestosterone is the biologically active hormone to promote prostatic hyperplasia in both people and dogs. Giving finasteride at 1 mg/kg/day, PO, for 16–21 wk, to laboratory Beagles resulted in a 50%–70% reduction in prostatic hypertrophy with no negative effect on semen quality. Lower dosages of finasteride (0.1 mg/kg/day, PO, for 16 wk) reduced hypertrophied prostate volume by 43%, resolved clinical signs, reduced dihydrotestosterone concentration by 58%, maintained normal testosterone levels, and had no deleterious effect on semen quality, fertility, or libido in a group of nine dogs with prostatic hypertrophy. However, prostatic hypertrophy returns if finasteride administration is discontinued. The low dosage (0.1–0.5 mg/kg) of finasteride correlates to convenient dosing of one 5-mg capsule/day for dogs weighing 10–50 kg.