Pyometra is an infection of the uterus in dogs and cats. It is relatively common, affecting approximately 25% of unspayed female dogs and cats. It is a serious condition which results in a variety of clinical and pathological signs requiring emergency surgery to remove the infected uterus. While medical treatment is sometimes attempted for this condition, it is often ineffective, and can be dangerous. Although the disease has been recognized for decades, the true pathogenesis 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 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 fulminate. If bacteria enter the uterus at the times when the protective physical barriers are breached, such as estrus, parturition, or immediately after parturition, the normal uterine defense mechanisms are likely to eliminate these bacteria. However, the hormonal influences may not allow the body to clear the bacteria. The bacteria typically cultured from the pyometra 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. Certain bacteria are more virulent than others and therefore allow a bacterium that is normally found on the dog to develop into an infection. Pyometra is most commonly seen in intact dogs 4-8 weeks after estrus (mean time of 5.4 weeks); however, it can be seen 4 months post estrus as well. Although seen less commonly, cats generally develop pyometra between 1-4 weeks after estrous. Pyometra generally occurs in older (7 to 8 years) intact bitches and queens; however, it may occur in younger animals that have been given estrogen (mis-mating shots) or progestins for estrus suppression.

If pyometra is allowed to continue untreated for a significant period of time, it can affect the entire body, leading to critical disease, shock and death. Because the infection can be so overwhelming, the reasons for presentation are not limited to the genital tract. The animal can become so overwhelmed by the inflammation associated with the infection that the animal may die from its own uncontrolled inflammatory process. A dog with an open pyometra (the cervix is open) will often have vaginal discharge, which can look like blood, pus or mucus. Many dogs and cats will have a closed cervix and therefore the obvious sign of a bloody to mucopurulent, hemorrhagic vaginal discharge may not be present. Furthermore, many dogs will clean themselves, removing any trace of vaginal discharge before it is visible to an owner which makes detection more difficult. The most common clinical signs that are present in greater than 50% of cases include lethargy, depression, anorexia, fever, excessive water intake and excessive urination. Pale mucous membranes, vomiting, diarrhea, weight loss, abdominal distension, and inflamed eyes have been reported although much less frequently. Up to 16% of patients may have no clinical signs other than purulent vaginal discharge. It is the bacterial infection of the uterus which causes increasing inflammation within the organ and leads to the systemic effects observed in the majority of patients. The severity of the resulting illness is greatly influenced by the degree of drainage from the uterus. If the cervix is closed, then fluids and toxins accumulate, with potential for toxic effects. Rupture or slow leakage from one of the uterine horns can release inflammatory products into the abdominal cavity, causing peritonitis. If the cervix is patent, or open, then drainage limits the accumulation of inflammatory products and bacterial toxins, and increases the likelihood of early recognition of the problem. The clinical signs of increased thirst and urination have been linked to the direct influence of bacterial toxins on the kidneys’ urine concentrating mechanisms. Bacterial infection and toxins may cause secondary damage to the liver as well. Endotoxic shock alters blood supply to all tissues and can disrupt normal blood clotting mechanisms. Microscopic blood clots or clumps of circulating bacteria can further impact upon the blood supply to vital organs such as the heart and brain, permitting seizures, cardiac rhythm disturbances and other grave consequences. The most life threatening complications associated with pyometra are sepsis and systemic inflammatory response syndrome (SIRS) /multiple organ dysfunction syndrome (MODS). If an unspayed female dog or cat is exhibiting any of these symptoms, they should be evaluated immediately.

Diagnosis is based in part on the history, reproductive status, and clinical signs. Physical examination of the pyometra patient reveals abdominal distention, an enlarged, palpable uterus, vaginal discharge if the cervix is open, and lethargy. A closed-cervix pyometra more likely will result in the animal showing signs of septicemia, including shock, hypothermia, dehydration, vomiting, and collapse. Laboratory testing and imaging are frequently used to aid in the diagnosis. A complete general chemistry profile, complete blood count, urinalysis, abdominal radiographs, abdominal ultrasound and perhaps vaginal cytology analysis are performed in suspected pyometra cases. The dog’s complete blood count, or CBC, is influenced by the degree of drainage from the uterus. Patients with a closed cervix and limited uterine drainage are more likely to show significant elevations of or reductions in, the white blood cell count. The white blood cells are also more likely to appear immature or unhealthy in those patients. Red blood cell counts are often reduced; patients with chronic disease frequently have low-grade anemia. Dehydration can mask this feature by reducing the amount of water in the bloodstream; consequently, the red blood count appears higher than it really is. Blood urea nitrogen, or BUN, and creatinine reflect blood flow to the kidneys. The level of these nitrogenous waste products in the blood will frequently rise with dehydration and kidney dysfunction, which are common in patients with pyometra. Elevated blood protein levels and disturbed electrolytes will often reflect the state of dehydration. The urine may be very dilute, reflecting toxic influences on the kidneys, or well concentrated as an appropriate response to dehydration. The urine may contain bacteria or inflammatory cells, if collected after voiding, due to contamination by the vaginal discharge. If pyometra is suspected, urine samples are rarely collected directly from the urinary bladder, via needle aspiration, because of worries about perforation of the distended, fluid-filled uterus. Urinary protein levels may be elevated if the kidneys have been damaged by the presence of chronic infection The vaginal discharge can be examined microscopically for the presence of white blood cells and bacteria. Diagnostic x-rays of the abdominal cavity may demonstrate a fluid-dense tubular structure. A ground-glass appearance on the x-ray may suggest fluid accumulation around the diseased uterus if leakage has contributed to peritonitis. Ultrasound imaging will help to detect or verify the uterine enlargement and to define uterine size and wall thickness.

Pyometra necessitates immediate medical and surgical therapy. Those patients with a closed cervix may be more ill at the time of diagnosis. Intravenous fluids and antibiotics are routinely administered to patients that are severely ill, irrespective of the patency of the cervix. Potent antibiotics are given by injection, in combinations to target the most common bacterial pathogens. Supportive measures are customized for individual patient needs, according to the levels of shock, dehydration, electrolyte imbalance, organ dysfunction or cardiac arrhythmia. The patient is stabilized medically, if possible, to prepare for emergency ovariohysterectomy, or spay, to remove the infected uterus and the ovaries from the abdominal cavity. The prognosis with ovariohysterectomy can be as high as 90-100% if abdominal contamination is avoided during surgical intervention and shock/sepsis is managed appropriately perioperatively. It should be mentioned that a pyometra spay is considerably more challenging than a routine spay. Special care has to be taken so that the infected, dilated, friable and easily breakable uterus does not rupture and spill its toxic contents into the sterile abdomen. If severe sepsis and organ failure develops, the prognosis can be grave. Some patients may remain PU/PD (increased urination and water intake) and in a state of permanent kidney damage. Although surgery is considered the therapy of choice, very special case selection meeting certain criteria may allow valuable breeding bitches to be treated medically. Stable patients may be given prostaglandin f2-alpha by injection for several consecutive days to dilate the cervix, stimulate uterine contractions and to decrease the blood progesterone level. The dog must remain hospitalized for observation, monitoring for side effects of the prostaglandin or for worsening condition, and for continued antibiotic administration. Clinical improvement may be expected within the first 48-96 hours of medical treatment. Surgery should be considered for patients that deteriorate. If purulent vaginal discharge persists seven days after conclusion of treatment, or if other parameters indicate ongoing infection or uterine enlargement, then repeating the treatment may be advised, provided that the patient remains physiologically stable. Dogs are susceptible to developing pyometra again after medical treatment; the recurrence rate is as high as 80%. In addition, the chance of successful subsequent breeding after medical management of pyometra is approximately 50:50. Because of the high rate of recurrence and diminished breeding capacity, even those dogs that have been successfully managed medically should receive an ovariohysterectomy when their breeding purposes are finished.

The majority of patients are released two to three days following an uncomplicated surgical procedure. Antibiotic therapy and pain management are continued for seven to 10 days after most procedures. It should be emphasized that pyometra is extremely easy to prevent. An appropriately performed spay procedure between 6 months and one year of age will prevent the development of pyometra. However, an inappropriately performed spay in which a portion of the ovarian tissue, uterine body or horn is not removed may result in what is called uterine stump pyometra. Ultrasound imaging is especially helpful in detecting stump pyometra. Surgical removal of the infected remnant is usually curative. In conclusion, an elective spay procedure of the young dog or cat will virtually eliminate the possibility of pyometra from ever developing in the overwhelming majority of pets. Clearly, hormone administration for mismating events and estrus suppression should be avoided except for the absolute necessities, as avoidance of estrogen or progesterone administration will decrease the risk of pyometra in both young and mature pets. When pyometra does occur, the combination of aggressive and prompt medical and surgical intervention is successful in almost all cases.