Cystic endometrial hyperplasia/pyometra in dogs: a review of the causes and pathogenesis.

In this review the roles of endogenous and exogenous steroid hormones, steroid receptors, spontaneously occurring and experimentally induced genital tract infection, and endometrial trauma in the cause and pathogenesis of cystic endometrial hyperplasia/pyometra in bitches are considered. It is postulated that intrauterine bacteria, which ascend from the vagina during pro-oestrus and oestrus, induce the disease during metoestrus by acting on the progesterone-primed endometrium directly via toxin production, or indirectly by the release of inflammatory mediators. The lesion can be produced by endometrial trauma in a sterile uterus without the systemic effects associated with the disease. There is no association with premature or increased progesterone concentrations at the time that the disease is normally diagnosed in mid- to late metoestrus; however, such aberrant endocrine changes cannot be excluded from involvement in the pathogenesis of this disease, as there are no data from bitches preceding the onset of clinical signs. Expression of oestrogen and progesterone receptors are modified by endogenous steroid hormones but there is no clear evidence that changes in these receptors are involved in the pathogenesis of the disease; receptor expression can be modified by exogenous hormones.