The contraceptive effects of levonorgestrel in the domestic cat

The effects of subdermal implantation of levonorgestrel (LNG) on reproduction were studied in domestic cats (Felis domestica). Levonorgestrel was administered via a slow-release subdermal silastic implant to 10 queens. The implants contained 16 mg of LNG and were designed to release 60 microgram of the drug daily. Each treated queen received one implant. Five queens (control, group 1) received subdermal silastic implants containing no drug. Changes in body weight, mammary gland structure (determined by palpation), serum blood glucose concentrations, and reproductive factors (occurrence of estrous cycles, serum progesterone concentrations, and pregnancy) were monitored for 1 year. Four treated queens (treatment/recovery, group 2) were used to investigate reproductive function following 12 months of LNG treatment. To assess effects of treatment on macroscopic and microscopic anatomic features of reproductive and nonreproductive tissues, the remaining six cats (treatment/histology, group 3) were studied. Hemiovariohysterectomy was performed in two queens each at 0, 2, and 6 months of the study. Later, the remainder of the reproductive tract was harvested at necropsy (two after 2 months of treatment, two after 6 months, two after 12 months) to assess change in individual queens. Nonreproductive tissues were also examined at necropsy to determine effects of LNG in these six queens. All queens retained the implants during the period of study without detectable discomfort. Estrus was suppressed and no pregnancies were recorded in the four LNG-treated cats that were housed with a male. Treatment with LNG had no effect on body weight, physical mammary gland structure, or serum blood glucose concentrations. All untreated queens (group 1) displayed estrous behavior and nine pregnancies resulted. Serum progesterone concentrations in untreated queens characteristically increased during pregnancy and decreased at parturition. Serum progesterone concentrations in each treated queen (groups 2 and 3) remained at basal levels (<1.0 ng/ml) throughout the treatment period. This may have been due to anovulation or defective luteal function. Following removal of implant after 1 year three of four queens (group 2) exhibited estrus and conceived with in 54 days. Histologic examination revealed corpora lutea in the ovaries of two queens (group 3) at the end of 12 months of treatment. Changes in 9 of the 10 uterine horns examined after treatment was begun included glandular epithelial hypertrophy and hyperplasia in addition to glandular proliferation and branching, suggestive of a progestational response. Small numbers of endometrial cysts were present in one of these uterine horns harvested after 2 months of treatment. In the tenth uterine horn examined, harvested after 6 months of treatment, some characteristics of pyometra were detected. No macroscopic or microscopic lesions were found in mammary gland tissues (five of five subjects) examined. No other macroscopic or microscopic changes in tissues harvested at necropsy could be attributed to LNG administration. The results indicate that subcutaneous administration of LNG, delivered by means of a slow-release silastic implant, may constitute an effective, safe, and reversible method of contraception in the domestic cat.