Risk of pyometra in bitches treated for mismating with low doses of oestradiol benzoate.

A review of the literature concerning the effect of oestrogens for treating mismating in bitches on the occurrence of pyometra indicated that low doses of oestradiol benzoate substantially increase the incidence of pyometra. A retrospective study of the clinical records of a UK general practice found that the incidence of pyometra in the four months after administration of low doses of oestradiol benzoate was 8.7 per cent, whereas the incidence in the practice's untreated at-risk population was estimated to be less than 1.32 per cent per interoestrus interval, suggesting that more than 85 per cent of pyometras occurring within four months of treatment are iatrogenic.