Canine pyometra: Early recognition and diagnosis
In this article, we review the pathophysiology, signalment, clinical signs, and diagnosis of canine pyometra. And in the next article, we summarize the surgical and medical management options available for treating this condition.
PATHOGENESISThe physiologic changes responsible for predisposing a uterus to pyometra are not completely understood. The vagina is not a sterile environment. Many bacteria types have been cultured from the normal vaginal vault, including Escherichia coli and Staphylococcus, Streptococcus, Klebsiella, Pasteurella, Pseudomonas, and Proteus species.1-3 These same bacteria are commonly cultured from the uteri of patients with pyometra,4-10 which suggests that bacteria ascending from the dog's vaginal vault are the likely source of uterine infection in most patients developing pyometra.11 In a study of 10 dogs with pyometra, the bacteria isolated from the uterus were genetically similar to those found in the patients' gastrointestinal tracts, demonstrating that bacteria within a patient's own body—and not exogenous bacteria—are responsible for infection. Primary urinary tract infections and the hematogenous spread of bacteria from nongenitourinary sites have been suggested to be less frequent sources of infection than ascending infections from the vagina.11,12
However, bacterial contamination of the uterus does not appear to be solely responsible for the development of pyometra. Vaginal bacteria will normally cross the cervix into the uterus when the cervix is open (proestrus and estrus), yet pyometra does not routinely develop.2 Other uterine factors are thought to predispose the uterus to progressive infection.
Studies performed in the 1950s suggested that cystic endometrial hyperplasia is a prerequisite for the development of pyometra in a bitch.6,13 Cystic endometrial hyperplasia develops in most intact female dogs as they age. It is caused by chronic recurrent exposure of the endometrial lining to progesterone produced by the corpus luteum during diestrus. Binding to uterine receptors, progesterone induces endometrial gland proliferation, stimulates endometrial gland secretions, decreases myometrial contractility, and induces closure of the cervix.7 Progesterone has also been shown to interfere with immune function within the uterus, possibly increasing its susceptibility to bacterial infection.14,15 Progesterone's effect on the endometrium is cumulative from reproductive cycle to reproductive cycle.16
The studies suggested that accumulating uterine secretions, prominent endometrial gland crypts, and immunosuppression caused by progesterone stimulation during diestrus make the uterus an ideal environment for bacterial proliferation leading to pyometra.6 This condition has been subsequently termed cystic endometrial hyperplasia-pyometra complex.6,17
Not all dogs with pyometra have cystic endometrial hyperplasia. Other factors can play a role in the development of pyometra. Studies have demonstrated that irritants within the uterus, such as foreign material that has passed through the cervix or even a subclinical bacterial infection, may induce endometrial inflammation and hyperplasia.10,18 These endometrial changes contribute to a favorable environment for bacterial colonization or proliferation, leading to pyometra. The propensity of some pathogenic bacteria, such as E. coli, to attach to the endometrium may explain why some bitches without cystic endometrial hyperplasia develop pyometra.17 Exogenous hormones can also prime the uterus for infection. Pyometra has been noted to occur after the exogenous administration of estrogen used to inhibit pregnancy after a mismating.19-22 Exogenous estrogen enhances the uterus's sensitivity to endogenous progesterone.22
The sequence of pathophysiologic events leading to pyometra varies among bitches and continues to be studied. A consistent factor in the development of pyometra is the presence of a progesterone-primed uterus.