Endometriosis in rhesus monkeys

Endometriosis was present in five out of six Rhesus monkeys who had to be sacrificed because of disease in a colony of 90. The monkeys lost appetite and became constipated. The abdomen was distended and often a pelvic mass could be palpated. At autopsy, 300-500 ml of blood-stained fluid was found in the abdomen together with intestinal or pelvic endometriosis. The ethiology of endometriosis in Rhesus monkeys is unknown. The most probable explanation is intraabdominal implantation of endometrial tissue through retrograde menstruation. The lesion is rare in animals in free ranging colonies. It is not known whether this is due to a higher frequency of pregnancies in animals with free access to matings or to its consequences, i.e. more infrequent menstruations.