Canine pyometra is an infectious and inflammatory disorder of the uterus typically occurring in adult, intact bitches during
or immediately after the luteal phase of the estrous cycle. The clinical signs of pyometra are often nonspecific and vary
among patients depending on the chronicity of the disease and the patency of the cervical canal. Early recognition, diagnosis,
and treatment of pyometra are necessary to achieve a successful outcome.
Jeff Dennis, DVM, DACVIM; Brian Lucas Hamm, DVM
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.
The 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.