Genital tract smooth muscle tumours are common in zoo felids but are not associated with melengestrol acetate contraceptive treatment

In a survey of gynaecological lesions in female zoo felids conducted to determine if the widely used progestin contraceptive melengestrol acetate (MGA) had adverse effects, numerous leiomyomas and leiomyosarcomas were detected. This current study aimed to characterize the morphologic features of these tumours, determine their prevalence, and assess if MGA was a risk factor for their genesis. Genital tracts from 219 zoo felids representing 23 species kept in various zoological parks in US and received between 1988 and 1995 were evaluated. The median age of all study animals was 143 months. 98 felids were treated with MGA for at least 6 months and 121 felids had no history of exposure to progestins or other contraceptives (control). Leiomyomas were detected in 24% of the felids. Leiomyomas were often multiple and occurred in the myometrium, ovary or adjacent broad ligament. The risk of developing leiomyomas increased with age, but MGA treatment or parity had no effect. Five other felids had leiomyosarcomas. Leiomyosarcomas were distinguished from poorly demarcated leiomyomas by the presence of local invasion, metastasis and cellular atypia, but necrosis and mitotic rate were not distinguishing criteria. Four of five felids with leiomyosarcomas had been treated with MGA. These results indicate that leiomyomas are common spontaneous lesions in the genital tracts of zoo felids and their genesis is not linked to MGA exposure. Whether progression to malignancy is promoted by MGA warrants further investigation.