Pyometra is an infection of the uterus that may occur in dogs and cats making the pet very ill. The uterus is generally filled with pus. Although the disease has been recognized for decades, the true disease process has still not been completely understood. It is generally recognized that progesterone and estrogen and their receptors have a role in the development of pyometra; however, the infection is triggered by bacterial involvement.
The cyclical hormonal influences of the female dog and cat allow the uterus to go through changes that will be acceptable for fertilization of an embryo. If bacteria are introduced into the uterus at a certain time during the cycle, hormonal regulation of the uterus allows the infection to start and become life-threatening. The bacteria typically cultured from the uterus are bacteria that would be found in the areas of the intestines and vagina (E. coli is the most common). Therefore, many of the infections are considered either from an ascending infection from the vagina, a concurrent urinary tract infection, or fecal contamination.
Most dogs and cats that are spayed early in life will not develop pyometra. However, a uterine stump pyometra may occur after incomplete ovariohysterectomy which allows a segment of the uterine body or horn to become infected. Typically, either a portion of the ovarian tissue is still present or the animal has been subjected to progestational hormones to allow this situation to develop.
Source: ACVS American College of Veterinary Surgeons