Efficacy, safety and interval from end of treatment to estrus in cats treated with an ultra-low dose megestrol acetate protocol for suppression of reproductive activity

Cat breeders need safe, predictable and fully reversible temporary control of reproduction in queens. Megestrol acetate (MA), a short-acting progestogen was investigated in this study designed to determine whether low-dose treatment is both effective and safe in cats for periods up to 6 months. Twenty-eight queens were treated orally with 11.5 μg/kg/day of MA for one to six months. A physical examination, vaginal cytology, and reproductive ultrasound were performed before, during and after treatment, whilst urinalysis and hematological/biochemical tests, including progesterone assay, were performed before and after treatment. MA suppressed reproductive function effectively in 27/28 queens. Transient mammary and uterine hyperplasia were detected in four (14 %) and three (11 %) queens, respectively, treated for more than four months, without associated clinical signs. Pyometra was observed in only one queen following her first estrus cycle post-treatment. Significant but reversible weight gain was observed in 85 % of the animals. The resumption of cyclicity occurred on average 6 weeks after the end of treatment but was influenced by the duration of treatment and seasonality. An ultra-low dose MA treatment was effective in suppressing estrus in queens treated up to 6 months. Close monitoring should be paid to queens treated for longer than 4 months as the incidence of side effects, albeit minor and manageable, increases thereafter. Mammary gland assessment and progesterone assay are indispensable before treatment. Fertility is preserved, making MA a valuable option for temporary control of reproductive activity in queens, who otherwise cycle continuously leading to both unwanted behavior and pregnancies.