The present study aimed to investigate whether Escherichia coli virulence affects the roles of sex hormone receptors in female dogs with simulated pyometra. A total of 33 healthy, nulliparous, crossbred female dogs were divided into four groups, with 10 dogs in each of the three experimental groups and 3 dogs in the control group. Estradiol was administrated to female dogs in group 1 continuously at 0.6‑4.8 mg/kg twice daily for 12 days (the dose doubled every three days), followed by intramuscular injection of 0.2‑1.8 mg/kg progesterone. The progesterone was administrated with an initial dose of 0.2 µg/kg and increased 0.2 mg/kg every three days, twice daily until the maximum of 1.8 mg/kg for 24 days and maintained at 1.8 mg/kg for 19 days. Progesterone only was administrated at 1.8 mg/kg in group 2 (twice daily) for 55 continuous days and only estradiol was administered with an initial dose of 0.6 µg/kg (dose doubled every 3 days for 12 days) in group 3 twice daily and maintained at 4.8 mg/kg for the following 43 days. A strongly virulent E. coli strain, nau‑b, and a weakly virulent strain, nau‑i, were screened. On the 12th day of diestrus, 5 female dogs in each of the experimental groups were inoculated with E. coli nau‑i strain, while the other five in each group were inoculated with nau‑b strain. Histopathological changes of uterine tissues were microscopically observed 50 days after E. coli inoculation and hormone receptor expression levels were detected by quantitative polymerase chain reaction. Simulated pyometra was observed in dogs administrated with progesterone alone or progesterone combined with estradiol. The clinical symptoms and histopathological observation demonstrated that inoculation with strongly virulent E. coli strain, nau‑b, caused earlier onset of pyometra symptoms and more severe pyometra symptoms compared with the weakly virulent E. coli strain, nau‑i. Furthermore, estrogen and progesterone receptor levels in dogs with pyometra inoculated with E. coli strain nau‑i was higher than those in dogs with pyometra inoculated with E. coli strain nau‑b. These results indicated that E. coli affects the roles of sex hormone receptors in female dogs with simulated pyometra.

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