THE CONTRACEPTIVE EFFECTS OF LEVONORGESTREL IN THE DOMESTIC CAT

The effects of subdermal implantation of levonorgestrel (LNG) on repro duction were studied in domestic cats (Felis domestica). Levonorgestre l was administered via a slow-release subdermal silastic implant to 10 queens. The implants contained 16 mg of LNG and were designed to rele ase 60 mu g 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 (d etermined by palpation), serum blood glucose concentrations, and repro ductive factors (occurrence of estrous cycles, serum progesterone conc entrations, and pregnancy) were monitored for 1 year Four treated quee ns (treatment/recovery, group 2) were used to investigate reproductive function following 12 months of LNG treatment. Ib assess effects of t reatment on macroscopic and microscopic anatomic features of reproduct ive and nonreproductive tissues, the remaining six cats (treatment/his tology, group 3) were studied. Hemiovariohysterectomy was performed in two queens each at 0, 2, and 6 months of the study. Later, the remain der of the reproductive tract was harvested at necropsy (two after 2 m onths of treatment, two after 6 months, two after 12 months) to assess change in individual queens. Nonreproductive tissues were also examin ed at necropsy to determine effects of LNG: in these six queens. All q ueens retained the implants during the period of study without detecta ble 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 structur e, or serum blood glucose concentrations. All untreated queens (group 1) displayed estrous behavior and nine pregnancies resulted. Serum pro gesterone concentrations in untreated queens characteristically increa sed during pregnancy and decreased at parturition. Serum progesterone concentrations in each treated queen (groups 2 and 3) remained at basa l levels (<1.0 ng/ml) throughout the treatment period. This may have b een due to anovulation or defective luteal function. Following removal of the implant after 1 year three of four queens (group 2) exhibited estrus and conceived within 54 days. Histologic examination revealed c orpora lutea in the ovaries of two queens (group 3) at the end of 12 m onths of treatment. Changes in 9 of the 10 uterine horns examined afte r treatment was begun included glandular epithelial hypertrophy and hy perplasia in addition to glandular proliferation and branching, sugges tive 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 mont hs of treatment, some characteristics of pyometra were detected. No ma croscopic or microscopic lesions were found in mammary gland tissues ( five of five subjects) examined. No other macroscopic or microscopic c hanges in tissues harvested at necropsy could be attributed to LNG adm inistration. The results indicate that subcutaneous administration of LNG, delivered by means of a slow-release silastic implant, may consti tute an effective, safe, and reversible method of contraception in the domestic cat.