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 micrograms 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.(ABSTRACT TRUNCATED AT 250 WORDS)