THE MERCK VETERINARY MANUAL
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Pyometra in Small Animals

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Pyometra is a hormonally mediated diestrual disorder characterized by cystic endometrial hyperplasia with secondary bacterial infection. Pyometra is reported primarily in older bitches (>5 yr old), 4–6 wk after estrus.

Factors associated with occurrence of pyometra include administration of longlasting progestational compounds to delay or suppress estrus, administration of estrogens to mismated bitches, and postinsemination or postcopulation infections. Progesterone promotes endometrial growth and glandular secretion while decreasing myometrial activity. Cystic endometrial hyperplasia and accumulation of uterine secretions ultimately develop and provide an excellent environment for bacterial growth. Progesterone may also inhibit the WBC response to bacterial infection. Bacteria from the normal vaginal flora or subclinical urinary tract infections are the most likely sources of uterine contamination. Escherichia coli is the most common bacterium isolated in cases of pyometra, although Staphylococcus, Streptococcus, Pseudomonas, Proteus spp, and other bacteria also have been recovered.

Because queens require copulatory stimulation to ovulate and produce progesterone from corpora lutea, pyometra is less common in queens than in bitches. Administration of medroxyprogesterone and other progestational compounds has been associated with development of pyometra in bitches and queens. Pyometra can develop in uterine tissue left after ovariohysterectomy (stump pyometra). It can also occur secondary to postpartum metritis.

By itself, estrogen does not contribute to the development of cystic endometrial hyperplasia or pyometra. However, it does increase the stimulatory effects of progesterone on the uterus. Administration of exogenous estrogens to prevent pregnancy (ie, “mismate shots”) during diestrus greatly increases the risk of developing pyometra and should be discouraged.

Clinical signs are seen during diestrus (usually 4–8 wk after estrus) or after administration of exogenous progestins. The signs are variable and include lethargy, anorexia, polyuria, polydipsia, and vomiting. When the cervix is open, a purulent vulvar discharge, often containing blood, is present. When the cervix is closed, there is no discharge and the large uterus may cause abdominal distention. Signs can progress rapidly to shock and death.

Physical examination reveals lethargy, dehydration, uterine enlargement, and if the cervix is patent, a sanguineous to mucopurulent vaginal discharge. Only 20% of affected animals have a fever. Shock may be present.

The leukogram of animals with pyometra is variable and may be normal; however, leukocytosis characterized by a neutrophilia with a left shift is usual. Leukopenia may be found in animals with sepsis. A mild, normocytic, normochromic, nonregenerative anemia (PCV of 28%–35%) may also develop. Hyperproteinemia due to hyperglobulinemia may be found. Results of urinalysis are variable. With E coli uterine infection, isosthenuria due to endotoxin-induced impairment of renal tubular function or to insensitivity to antidiuretic hormone (or both) may develop. A glomerulonephropathy caused by immune-complex deposition may result in proteinuria. These renal lesions are potentially reversible once the pyometra is resolved.

Pyometra should be suspected in any ill, diestrual bitch or queen, especially if polydipsia, polyuria, or vomiting is present. The diagnosis can be established from the history, physical examination, abdominal radiography, and ultrasonography. Vaginal cytology often helps determine the nature of the vulvar discharge. A CBC, biochemical profile, and urinalysis help exclude other causes of polydipsia, polyuria, and vomiting; they also evaluate renal function, acid-base status, and septicemia. The uterine exudate should be cultured and sensitivity tests performed. Differential diagnoses include pregnancy and other causes of vulvar discharge, polyuria and polydipsia, and vomiting.

Ovariohysterectomy is the treatment of choice for pyometra. Medical management could be considered if preserving the reproductive potential of the bitch or queen is desired. Fluids (IV) and broad-spectrum, bactericidal antibiotics should be administered. Fluid, electrolyte, and acid-base imbalances should be corrected as quickly as possible, before ovariohysterectomy is performed. The bacterial infection is responsible for the illness and will not resolve until the uterine exudate is removed. Oral antibiotics (based on the results of the culture and sensitivity) should be continued for 7–10 days after surgery.

Medical therapy with prostaglandin F (PGF) can be used for animals to be bred in the future, although prostaglandins are not approved in the USA for use in cats or dogs. PGF causes luteolysis, contraction of the myometrium, relaxation of the cervix, and expulsion of the uterine exudate. They should probably not be used in animals >8 yr old or in those not intended for breeding. The delay before clinical improvement and the many adverse effects of PGF preclude its use in a severely ill animal. PGF also should be used with caution in bitches or queens with a closed-cervix pyometra because of increased risk of uterine rupture. Pregnancy must be excluded, because prostaglandins can induce abortion.

Naturally occurring PGF (0.25 mg/kg/day, SC, for 5 days) is commonly used. Synthetic analogs (eg, cloprostenol, fluprostenol, and prostalene) are much more potent than natural PGF and have been used to treat pyometra in dogs. Broad-spectrum, bactericidal antibiotics, chosen on the basis of culture and sensitivity tests, should be given for ≥2 wk.

The adverse effects of PGF include restlessness, anxiety, panting, hypersalivation, pacing, tachycardia, vomiting, urination, and defecation. In cats, vocalization and intense grooming behavior also may be seen. These reactions disappear within 2 hr of the injection. The LD50 of PGF in dogs is 5.13 mg/kg. Severe ataxia, respiratory distress, and muscle tremors may be seen in queens given 5 mg/kg. If adverse effects are severe, IV fluids at rates appropriate for treatment of shock are indicated. Uterine evacuation after an injection is variable.

Other antiprogestins (eg, aglepristone) are available in some European countries. Clinicians using aglepristone report virtually no adverse effects as compared with prostaglandins. Aglepristone is also used to treat bitches with closed cervix pyometra. In one study, a dosage of 10 mg/kg given on days 1, 2, and 8 in 15 bitches with closed pyometra led to opening of the cervix after 26±13 hr in all treated animals.

Animals should be reexamined 2 wk after completion of medical therapy. If a sanguineous or mucopurulent vulvar discharge or uterine enlargement is still present, PGF therapy, using the same protocol, may be repeated; however, the prognosis for recovery is much worse. After medical therapy, the prognosis for initial resolution of the pyometra is good if the cervix is open but guarded to poor if closed. Of those animals that respond, as many as 90% of bitches and 70% of queens with open-cervix pyometra may be fertile. Recurrence is likely; 70% of bitches treated medically for pyometra had recurrence within 2 yr. Therefore, the animal should be bred on the next and each subsequent cycle until the desired number of puppies or kittens has been produced, and then spayed. Prostaglandins should not be dispensed for client administration because of the narrow safety index and the potential to trigger asthmatic events and pregnancy loss in people.

Last full review/revision November 2013 by Mushtaq A. Memon, BVSc, PhD, DACT

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