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Genital Tract Smooth Muscle Tumors Are Common in Zoo Felids but Are Not Associated with Melengestrol Acetate Contraceptive Treatment

Department of Pathology, College of Veterinary Medicine, University of Tennessee, Knoxville, TN (LMC ¹); Departments of Medicine and Epidemiology (IAG) and Pathology, Microbiology, and Immunology (LM), School of Veterinary Medicine, University of California, Davis, CA; and Marshfield Medical Foundation, Marshfield, WI (EDP)

In a survey of gynecologic 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 tumors, determine their prevalence, and assess if MGA was a risk factor for their genesis. Genital tracts from 219 zoo felids representing 23 species were evaluated, and 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.

Key words: Contraceptive; felid; leiomyoma; leiomyosarcoma; melengestrol acetate; progestin.

Request reprints from Dr. L. Munson, Department of Veterinary Pathology, Microbiology, and Immunolgy, 1126 Haring Hall, 1 Shields Avenue, Davis, CA 95616 (USA). E-mail: lmunson{at}ucdavis.edu.