Common occurrence of urethral cysts in older female ranch mink

Cysts were found often in the proximal urethra of female sapphire and pastel mink, Mustela vison, examined at necropsy during studies on slow viral diseases. Their prevalence, the same in both color phases, was age-dependent. They occurred in less than 2% of females under 2 years old but in more than 60% of those over 7 years old. The cysts varied from ovoid vesicles 3 to 4 mm long to multilocular masses 10 to 15 mm across that greatly distended the proximal urethra and sometimes occluded it. Small cysts were not accompanied by clinical signs, but large ones often caused persistent urinary incontinence and occasionally, urine retention. The cysts contained fluid that varied from water-clear to dull yellow. They arose by expansion of small urethral glands normally present in female mink and destroyed much of the urethral wall by pressure atrophy. Although the cysts became larger and more prevalent as the mink aged, the stimulus that caused them to form was not apparent. They appear to have no counterpart in other animals.