Prevention of pregnancy in bitches following unwanted mating: a clinical trial using low dose oestradiol benzoate.

A field investigation was undertaken in mated bitches to assess the efficacy and safety of a low dose of oestradiol benzoate (0.01 mg kg-1), given by subcutaneous or intramuscular injection, on days 3, 5 and sometimes day 7 after mating. Twenty-five veterinary practices in the UK collected data from 358 cases in total. Bitches were treated with oestradiol and then examined at about 65 days after mating, at which time owners were also questioned about any possible adverse signs seen since treatment. Overall 16 bitches (4.5%) became pregnant, although the apparent duration of gestation in some cases suggested that a later mating may have taken place, thus giving a pregnancy rate of only 3.4%. As regards possible side effects of the treatment, 4.5% experienced signs of false pregnancy, 7.8% had evidence of vaginal discharge and 7.3% developed pyometra at some stage during the ten weeks after treatment. None of the bitches showed any clinical signs of bone marrow suppression. The efficacy and safety of this low dose regimen suggest that it is suitable for widespread use, under veterinary supervision. However, it is important for bitch owners to be informed that efficacy can never be guaranteed in any particular case and thus a follow up pregnancy examination may sometimes be advisable.