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.
It has been suggested that pyometra is a condition of middle aged and older dogs.22-25 However, pyometra has been reported in dogs as young as 4 months old and as old as 16 years.4,12,22 A study from Japan of 165 colony-raised beagles revealed a prevalence of 15.2% in nonspayed female dogs over the age of 4.23 In this study, the average age of onset was about 9 years. A Swedish study reported the prevalence of pyometra at about 25% in intact, publicly owned female dogs under the age of 10 and about 2% in bitches more than 10 years old.24 In yet another survey, the mean age of dogs presenting with pyometra was 2.4 years.12 The younger age was attributed to the frequent administration of estrogens for mismating in this population.
Pyometra can occur in any breed of dog. Breeds thought to be predisposed to pyometra include rough-coated collies, rottweilers, miniature schnauzers, Cavalier King Charles spaniels, golden retrievers, Bernese mountain dogs, and English springer spaniels.16,24,26 Breeds thought to have a decreased predisposition to pyometra include Drevers, German shepherds, dachshunds, and Swedish hounds.24 Not all studies have confirmed a breed disposition.9 Meanwhile, previous pregnancy has been reported to be protective in some dog breeds but not in all.22,26
The clinical signs associated with pyometra typically appear one to three months after the completion of estrus.6,7 Most of the clinical signs are not specific for pyometra, including lethargy, depression, pyrexia, anorexia, vomiting, diarrhea, polydipsia, and polyuria. A serosanguineous to mucopurulent vaginal discharge can be seen if the cervix is open. Vaginal discharge may be the only clinical finding in some patients.
In patients with closed-cervix pyometra, vaginal discharge may not be present. Dogs with closed-cervix pyometra are often more seriously ill at the time of diagnosis than those with open-cervix pyometra.16 A closed cervix inhibits the elimination of the infectious uterine material, delaying the recognition of illness and increasing the likelihood of complications such as septicemia, endotoxemia (most commonly associated with E. coli infection), and septic peritonitis.5 Bacterial contamination in cases of septic peritonitis may be the result of uterine rupture, translocation of bacteria across the diseased uterine wall, or, less likely, oviduct leakage. More severely afflicted closed-cervix patients may present with signs of shock, dehydration, or collapse.5
A history of recent estrus and typical clinical signs, especially vaginal discharge, should raise your suspicion for pyometra. In patients lacking vaginal discharge, the diagnosis can be more challenging. An enlarged uterus may be palpated within the abdomen; however, aggressive attempts at palpation should be avoided to prevent inadvertent uterine rupture. Other potential causes of vaginal discharge or uterine enlargement, including pregnancy, estrus, hydrometra, mucometra, vaginitis, neoplasia, and uterine torsion, need to be considered as differential diagnoses.
Laboratory findings in patients with pyometra are not specific for pyometra and frequently reflect the presence of inflammation and secondary metabolic disturbances. The results of a complete blood count will often demonstrate a neutrophilia with a regenerative left shift; however, a normal white blood cell count or even a neutropenia with a degenerative left shift may be identified in animals with endotoxemia.7,12 Chemotactic chemicals released from the inflamed uterus attract neutrophils to the uterine lumen where they become sequestered.7 Eventually, the bone marrow may not be able to keep up with the demand for mature neutrophils, leading to a degenerative left shift. A high white blood cell count may be seen after surgical removal of the uterus, as neutrophils are no longer sequestered. Mild normocytic, normochromic anemia has been documented in dogs, possibly secondary to bone marrow suppression associated with the uterine inflammation and its associated toxemia.7
The most commonly encountered biochemical abnormalities reported include elevated alkaline phosphatase and alanine transaminase activities, and blood urea nitrogen (BUN), creatinine, globulin, and total protein concentrations.12 Bacterial endotoxemia and reduced liver perfusion are thought to contribute to the elevated liver enzyme activities. Dehydration will frequently contribute to elevated BUN and creatinine concentrations, although concurrent kidney insufficiency in an older dog may also be responsible for the azotemia.4,27 Kidney damage secondary to immune complex deposition has not been substantiated in recent studies.28,29 Hyperglobulinemia and elevated total protein concentrations can occur secondary to chronic inflammation and dehydration. Although less commonly encountered, hypoglycemia can be attributed to septicemia.
Urinalysis findings can be variable. Many patients will have a high urine specific gravity secondary to dehydration. In other patients, the urinalysis will reveal isosthenuria, hyposthenuria, proteinuria, or bacteriuria.7,10 The loss of urine-concentrating ability by the kidneys reported in some patients may be secondary to bacterial endotoxin impairment of renal tubule function.10,12,28 Meanwhile, the presence of severe proteinuria at the time of treatment has been associated with the future development of renal failure in some dogs.29 To retrieve urine for evaluation, a free-catch sample is preferred. Collecting urine by cystocentesis—even if performed with ultrasound guidance—can lead to inadvertent uterine puncture and subsequent leakage of infectious material into the peritoneal cavity.
However, radiography can be insensitive for detecting pyometra since uterine distention typically cannot be detected until the diameter of the uterus is larger than that of the adjacent small bowels. In addition, radiography cannot distinguish pyometra from other causes of uterine distension, such as mucometra or early pregnancy (prior to fetal skeletal mineralization).
For guidance on how to treat canine pyometra—whether medically or surgically—see the article Surgical and medical treatment of pyoderma .
Brain Lucas Hamm, DVM
Jeff Dennis, DVM, DACVIM
BluePoint Specialty & Emergency Medicine for Pets
11950 W. 110th St., Suite B
Overland Park, KS 66210
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