CLINICAL USE OF LOW DOSE MEGESTROL ACETATE TREATMENT IN THE QUEEN FOR REPRODUCTION CONTROL: GENERAL HEALTH, EFFICACY AND INTERVAL FROM TREATMENT END TO RESUMPTION OF CYCLICITY

Megestrol acetate (MA) is a synthetic analog of progesterone. Originally formulated as an oral contraceptive for human use, its application has expanded to veterinary medicine. In bitches, it is used for estrus suppression, deferring the onset of heat. In queens, its administration comprises both short-term and extended suppression of estrus. EstropillTM (MSD, Italy) is a syrup MA formulation commercialized with the indication of heat prevention in queens. Our aim was to investigate the efficacy and safety of a low-dose MA protocol in suppressing cyclicity in queens and mating behavior and fertility of tomcats. We further set to estimate the interval between the end of treatment and the resumption of ovarian activity resumption in treated females. A total of 21 post-pubertal, healthy, privately-owned intact cats (18 queens and 3 tomcats) from 2 different facilities (Veterinary Teaching Hospitals of the universities of Padova, Italy and Ljubljana, Slovenia) were enrolled in the study and treated daily with an oral formulation of 11.5 μg/MA/kg (approximately 5 drops/kg/day PO). Animals were categorized according to treatment duration, which was decided based on owners’ request: 4 months (G4 4 females and 1 male), 5 months (G5 – 3 females and 2 males) and 6 months (G6 – 11 queens). A complete clinical and reproductive examination, hematology, biochemistry, urinalysis, and reproductive ultrasound were performed on all subjects before and after treatment. In addition, queens underwent vaginal cytology and progesterone assay while tomcats were checked for the presence of penile spikes before and after MA therapy. Subjects were monitored monthly repeating physical examination, reproductive ultrasound, and vaginal smears. Queens were reexamined upon demonstrating signs of heat after treatment. Estrus was confirmed cytologically. Time until ovarian activity resumption post-treatment, for queens, and weight gain, for all subjects, were statistically analyzed. Seventeen/18 queens exhibited consistent behavioral and cytological patterns of anestrus during treatment. The time until the resumption of ovarian activity post-treatment had an overall mean of 50.12 ± 17.08 days and was not significantly different between treatment groups (G4: 42.33 ± 30.08, G5: 49.3 ± 10.21, and G6: 52.45 ± 15.5 days). One queen was considered an outlier due to a prolonged delay in heat return. Two out of three tomcats arrested urinary marking and mating behaviors one month after treatment, with penile spikes disappearing within three months. The third male showed only a reduction in the size of the spikes and continued displaying mounting behavior, successfully impregnating a queen during treatment. Laboratory and ultrasonographic parameters during monthly checkups remained unchanged. However, all subjects exhibited increased appetite during treatment, resulting in weight gain. Weight gain was observed in all treatment groups, being statistically significant in G4 and G6. Most queens and all tomcats experienced non-significant weight loss following the end of treatment. The queen experiencing signs of heat during treatment showed substantial weight gain, which might have resulted in underdosing. This suggests that the dosage employed is close to the minimum effective dose of the drug, thus contributing to a greater safety of treatment. Conversely, this therapy does not seem to be fully effective for contraception in males. Further studies with an increasing number of tomcats and MA dosage could enhance outcomes, while monitoring the weight fluctuation could allow a better understanding of the impact of the drug in influencing appetite and weight gain.