Pyometra and Cystic Endometrial Hyperplasia (CEH-Pyometra Complex)

Pyometra and Cystic Endometrial Hyperplasia (CEH-Pyometra Complex) is a common, potentially life-threatening uterine disease of intact female dogs and cats, characterized by cystic endometrial hyperplasia (CEH) with secondary bacterial infection and accumulation of purulent exudate within the uterine lumen. The condition is a clinical syndrome that encompasses a spectrum of endometrial pathology, ranging from mild cystic hyperplasia to severe suppurative inflammation with systemic illness. It typically occurs during the luteal phase (diestrus) of the estrous cycle, when elevated progesterone levels promote endometrial glandular proliferation, decreased myometrial contractility, and cervical closure, creating a favorable environment for bacterial colonization and growth. The disease is classified into open-cervix pyometra (with vaginal discharge) and closed-cervix pyometra (without discharge), the latter being more severe due to uterine distension and risk of rupture. CEH-Pyometra complex is a major cause of morbidity and mortality in intact bitches and queens, and it is considered a reproductive emergency requiring prompt medical or surgical intervention.

The primary etiological agent is bacterial infection, most commonly Escherichia coli (E. coli), which accounts for 60-80% of canine cases and a similar proportion in feline cases. Other bacteria isolated include Staphylococcus spp., Streptococcus spp., Klebsiella spp., Proteus spp., Pseudomonas spp., and anaerobic organisms. The infection is opportunistic, ascending from the vagina through the cervix during proestrus/estrus when the cervix is relaxed and the uterine environment is receptive. The underlying hormonal milieu is crucial: prolonged exposure to progesterone, either from endogenous luteal function or exogenous administration (e.g., for estrus suppression or pregnancy termination), induces endometrial hyperplasia, cystic glandular dilation, and reduced local immune defense. Progesterone also decreases myometrial contractility and promotes cervical closure, trapping bacteria and exudate. Estrogen may synergize with progesterone by increasing endometrial progesterone receptor expression and enhancing bacterial adherence. In cats, pyometra is often associated with exogenous progestin therapy (e.g., megestrol acetate) or with spontaneous luteal phases. Genetic factors may influence susceptibility, as certain breeds (e.g., Golden Retriever, Rottweiler, Irish Setter) have higher incidence. Iatrogenic causes include improper administration of estrogens or progestins, and poor breeding hygiene. Congenital uterine anomalies (e.g., uterine horn hypoplasia) are rare but can predispose to the condition.

Pyometra is predominantly a disease of middle-aged to older intact female dogs and cats, with a median age of 6-8 years in dogs and 4-6 years in cats. It is rare in nulliparous animals under 2 years of age. The incidence in intact bitches is estimated at 15-25% by 10 years of age, with breed predispositions including Golden Retrievers, Rottweilers, Irish Setters, Bernese Mountain Dogs, and Collies. In cats, the incidence is lower (approximately 2-5% of intact queens), but it is more common in purebred cats and those receiving progestin therapy. Nulliparity is a significant risk factor; bitches that have never whelped have a higher risk. The disease occurs most frequently 1-2 months after estrus, during diestrus, when progesterone levels are elevated. In dogs, the condition is more common in animals with irregular estrous cycles or those treated with exogenous hormones. In cats, risk increases with age and is associated with ovulation induced by mating or hormonal stimulation. The disease is rare in spayed animals, but stump pyometra can occur if ovarian remnant tissue is present. Environmental factors such as kennel stress, poor hygiene, and obesity may also contribute.

The pathophysiology of CEH-Pyometra complex is multifactorial, involving hormonal, bacterial, and immune mechanisms. During diestrus, progesterone stimulates endometrial glandular proliferation and secretion, leading to cystic dilation of glands (CEH). Progesterone also suppresses the local immune response by reducing the number of uterine lymphocytes and impairing phagocytic activity of neutrophils. Simultaneously, it decreases myometrial contractility and promotes cervical closure, preventing drainage of uterine secretions. If bacteria ascend from the vagina during estrus (when the cervix is open), they colonize the hyperplastic endometrium. E. coli, the most common pathogen, possesses adhesins that bind to endometrial epithelial cells and produce endotoxins that exacerbate inflammation. The bacteria proliferate, causing suppurative inflammation, tissue necrosis, and accumulation of purulent exudate. The uterine lumen becomes distended, and the uterine wall thickens. In closed-cervix pyometra, the cervix remains closed, leading to increased intrauterine pressure, which can cause uterine rupture and peritonitis. Systemic effects arise from absorption of bacterial toxins and inflammatory mediators, leading to endotoxemia, sepsis, and multiple organ dysfunction. The kidneys are particularly vulnerable due to immune complex deposition and reduced renal perfusion, resulting in azotemia. The liver may also be affected, leading to elevated liver enzymes. In severe cases, disseminated intravascular coagulation (DIC) can occur.

Intrinsic factors include age (middle-aged to older), breed (genetic predisposition), nulliparity, and hormonal imbalances such as prolonged luteal phases or cystic ovarian follicles. Endogenous progesterone excess from luteal cysts or persistent corpora lutea increases risk. Extrinsic factors include exogenous administration of progestins (e.g., megestrol acetate, medroxyprogesterone acetate) for estrus suppression or treatment of behavioral issues, and estrogen administration (e.g., for mismating) which can sensitize the endometrium to progesterone. Improper breeding management, such as breeding during late estrus or using contaminated semen, may introduce bacteria. Poor hygiene, kenneling stress, and obesity are also contributing factors. In cats, the use of progestins for estrus suppression is a major risk factor. Additionally, any condition that causes cervical relaxation (e.g., vaginitis, foreign bodies) may facilitate ascending infection.

Clinical signs vary depending on the patency of the cervix and the severity of systemic involvement. In open-cervix pyometra, the most common sign is a purulent, sanguinopurulent, or mucopurulent vaginal discharge, often with a foul odor. The discharge may be noticed on the tail, perineum, or bedding. In closed-cervix pyometra, there is no vaginal discharge, and the uterus becomes distended, leading to abdominal distension and palpable uterine enlargement. Systemic signs include lethargy, depression, anorexia, polyuria, polydipsia, vomiting, diarrhea, and fever (though hypothermia may occur in severe sepsis). Abdominal pain may be evident on palpation. In advanced cases, signs of shock, dehydration, and cardiovascular collapse may be present. In cats, signs are similar but may be more subtle; queens may present with poor grooming, weight loss, and a palpable abdominal mass. Behavioral changes such as aggression or hiding may occur. In some cases, the condition is discovered incidentally during routine examination or imaging.

Differential diagnoses include: 1) Metritis (postpartum uterine infection) – occurs within days to weeks after whelping, with vaginal discharge and systemic signs, but history of recent parturition differentiates it. 2) Vaginitis – inflammation of the vagina, which may cause discharge but lacks systemic signs and uterine enlargement; vaginal cytology and imaging help differentiate. 3) Pregnancy – in early pregnancy, uterine enlargement may be present, but ultrasound reveals fetal structures; hormonal assays (progesterone, relaxin) are useful. 4) Hydrometra or mucometra – accumulation of sterile fluid in the uterus, often without systemic signs; ultrasound shows anechoic fluid, and cytology is non-inflammatory. 5) Uterine neoplasia (e.g., leiomyoma, leiomyosarcoma) – rare, but may cause uterine enlargement; imaging and biopsy are needed. 6) Ovarian remnant syndrome – in spayed animals, residual ovarian tissue can cause hormonal stimulation and uterine stump pyometra; history of spay and hormone assays are helpful. 7) Peritonitis – may cause abdominal pain and effusion, but uterine enlargement is absent; imaging and fluid analysis are diagnostic. 8) Cystitis or urinary tract infection – may cause polyuria/polydipsia and hematuria, but vaginal discharge is absent; urinalysis and imaging differentiate. 9) Constipation or gastrointestinal disease – may cause abdominal discomfort and vomiting, but lacks uterine signs. 10) Systemic infection (e.g., leptospirosis) – may cause fever and azotemia, but lacks uterine involvement.

The diagnostic algorithm begins with a thorough history and physical examination, including careful abdominal palpation (though caution is needed to avoid uterine rupture). If pyometra is suspected, the following steps are recommended: 1) Confirm intact status (history of estrus cycles, absence of spay scar). 2) Perform vaginal cytology to assess stage of estrous cycle and presence of inflammation (neutrophils, bacteria). 3) Measure serum progesterone to confirm luteal phase (typically >2 ng/mL). 4) Perform abdominal ultrasonography to evaluate uterine size, wall thickness, and luminal contents; this is the most sensitive imaging modality. 5) Obtain a complete blood count and serum biochemistry to assess systemic involvement (leukocytosis, azotemia, liver enzymes). 6) Perform urinalysis and urine culture to rule out urinary tract infection. 7) If closed-cervix pyometra is suspected and ultrasound is inconclusive, abdominal radiography may show a tubular soft tissue opacity, but it is less sensitive. 8) In cases where surgical intervention is planned, a coagulation profile may be indicated. 9) If the animal is unstable, stabilize with intravenous fluids and antibiotics before surgery. 10) Definitive diagnosis is confirmed at surgery or by histopathology of the uterus.

Hematology typically reveals leukocytosis with a left shift (increased immature neutrophils), toxic neutrophils, and sometimes monocytosis. In severe cases, leukopenia may occur due to bone marrow suppression. Anemia may be present due to chronic inflammation or blood loss. Serum biochemistry often shows azotemia (elevated BUN and creatinine) due to renal involvement, elevated liver enzymes (ALT, ALP) due to hepatic damage, and hyperglobulinemia due to chronic inflammation. Electrolyte imbalances, such as hyponatremia or hyperkalemia, may occur. Urinalysis may show proteinuria, hematuria, and pyuria if concurrent urinary tract infection exists. Vaginal cytology in diestrus shows a shift from superficial cells to parabasal and intermediate cells, with the presence of neutrophils and bacteria. Serum progesterone levels are typically elevated (>2 ng/mL), confirming luteal phase. In cats, similar findings are observed. Uterine culture and sensitivity testing are essential for targeted antibiotic therapy, with E. coli being the most common isolate.

Abdominal ultrasonography is the imaging modality of choice. Findings include a distended, fluid-filled uterus with thickened, hyperechoic endometrial lining. The uterine lumen may contain anechoic or echogenic fluid, depending on the cellularity of the exudate. In CEH, multiple small anechoic cysts may be visible in the endometrium. The uterine wall thickness may be increased, and the diameter of the uterine horns can be measured. In closed-cervix pyometra, the uterus appears as a convoluted, tubular structure with fluid. Ovarian structures may be normal or show luteal cysts. Abdominal radiography may show a soft tissue tubular opacity in the caudal abdomen, but it is less sensitive than ultrasound. In advanced cases, fetal mineralization (if pregnant) is not present, but in pyometra, no fetal structures are seen. CT and MRI are rarely needed but can provide detailed assessment of uterine pathology and complications such as rupture. Vaginoscopy may be performed to assess the cervix and vaginal discharge, but it is not routinely necessary.

Vaginal cytology is useful to stage the estrous cycle and detect inflammation. In diestrus, the cytology shows a predominance of parabasal and intermediate cells, with few superficial cells. Neutrophils and bacteria may be present in cases of pyometra. Fine-needle aspiration of the uterine fluid is not recommended due to risk of rupture, but if performed, it would show purulent material with degenerate neutrophils and bacteria. Histopathology of the uterus after ovariohysterectomy is the gold standard for diagnosis. Grossly, the uterus is enlarged, tortuous, and filled with purulent exudate. Microscopically, there is cystic dilation of endometrial glands, endometrial hyperplasia, and infiltration of neutrophils and plasma cells. The endometrium may show areas of necrosis and ulceration. In chronic cases, fibrosis and atrophy of the endometrium may be seen. Special stains, such as Gram stain, can identify bacteria. In cases of stump pyometra, histopathology of the stump may show similar changes.

Treatment of CEH-Pyometra complex can be medical or surgical, depending on the severity of the disease and the breeding value of the animal. Medical management is reserved for mild cases with open cervix, valuable breeding animals, and those with no systemic signs. The goal is to evacuate the uterine contents and resolve infection while preserving fertility. Protocols include: 1) Prostaglandin F2α (PGF2α) – dinoprost tromethamine (Lutalyse) at 0.1-0.25 mg/kg SC q8h for 5-7 days, or cloprostenol (Estrumate) at 1-2 μg/kg SC q48h. PGF2α causes luteolysis and uterine contraction, but it can cause side effects such as vomiting, diarrhea, salivation, and restlessness. 2) Progesterone receptor antagonist – aglepristone (Alizin) at 10 mg/kg SC on days 1, 2, and 8, or 10 mg/kg SC on days 1, 2, and 7. Aglepristone blocks progesterone receptors, causing cervical relaxation and uterine evacuation. 3) Dopamine agonist – cabergoline (Galastop) at 5 μg/kg PO q24h for 7-10 days, which reduces prolactin and progesterone secretion. 4) Oxytocin – may be used in low doses (0.5-2 IU/kg IM) after cervical relaxation, but it is not first-line due to risk of uterine rupture. 5) Antibiotics – broad-spectrum antibiotics such as amoxicillin-clavulanate (12.5-25 mg/kg PO q8h) or enrofloxacin (5-10 mg/kg PO q24h) should be administered based on culture and sensitivity. 6) Supportive care – intravenous fluids, antiemetics, and nutritional support. Surgical treatment (ovariohysterectomy) is the treatment of choice for closed-cervix pyometra, severe systemic illness, or when medical therapy fails. It is curative and eliminates the risk of recurrence. In breeding animals, medical therapy may be attempted, but the success rate for future fertility is variable, and recurrence is common. In cats, medical therapy is less successful, and surgery is often recommended.

The prognosis for pyometra is generally good with prompt and appropriate treatment. For surgical treatment (ovariohysterectomy), the prognosis is excellent, with a survival rate of >90% if surgery is performed before severe systemic complications. However, the prognosis is guarded if the animal presents with septic shock, peritonitis, or severe azotemia. For medical management, the prognosis for resolution of the current episode is good, but the recurrence rate is high (up to 50-70% in subsequent cycles). The prognosis for future fertility after medical treatment is variable; some animals can conceive and carry litters, but others may have reduced fertility due to endometrial damage. Negative prognostic indicators include closed-cervix pyometra, uterine rupture, peritonitis, severe azotemia, and elevated liver enzymes. In cats, the prognosis is similar, but medical therapy is less effective, and surgery is often recommended.

After medical treatment, follow-up is essential to ensure complete resolution. Serial ultrasonography should be performed every 2-3 days during treatment to monitor uterine size and fluid content. Serum progesterone levels should be checked to confirm luteolysis (progesterone <1 ng/mL). Vaginal cytology can be repeated to assess resolution of inflammation. After completion of treatment, a recheck examination and ultrasound should be performed 2-4 weeks later to confirm uterine involution. If the animal is intended for breeding, the next estrus should be monitored closely, and breeding should be planned based on progesterone and vaginal cytology. After surgical treatment, the animal should be monitored for surgical complications, and the incision site should be checked. If the animal was in septic shock, postoperative monitoring of renal and liver function is recommended. For breeding animals, a breeding soundness examination should be performed before the next estrus, including vaginal cytology and progesterone assays.

Pearls: 1) Always confirm intact status before considering pyometra; a spay scar may be hidden. 2) In closed-cervix pyometra, the uterus may be palpable as a tubular mass, but palpation should be gentle to avoid rupture. 3) Ultrasonography is the most sensitive diagnostic tool; a uterine diameter >1 cm in a bitch is suspicious. 4) Serum progesterone >2 ng/mL confirms luteal phase, but levels may be lower in some cases. 5) Medical therapy with aglepristone is preferred in breeding animals; PGF2α is more effective in open-cervix cases. 6) Always perform culture and sensitivity before starting antibiotics. 7) In closed-cervix pyometra, do not use PGF2α or oxytocin without first ensuring cervical relaxation (e.g., with aglepristone). 8) Monitor for signs of endotoxemia, such as tachycardia, pale mucous membranes, and prolonged capillary refill time. Pitfalls: 1) Misdiagnosing pyometra as pregnancy or hydrometra; ultrasound is essential. 2) Delaying surgery in closed-cervix pyometra, leading to uterine rupture and peritonitis. 3) Using PGF2α in closed-cervix pyometra without cervical relaxation, causing uterine rupture. 4) Underdosing antibiotics or not adjusting based on culture. 5) Failing to provide aggressive fluid therapy in septic patients. 6) Overlooking concurrent conditions such as renal disease or diabetes mellitus. 7) Assuming that a negative vaginal discharge rules out pyometra; closed-cervix cases have no discharge.

Based on Plumb's Veterinary Drug Handbook and theriogenology guidelines, the following protocols are recommended: 1) Aglepristone (Alizin): 10 mg/kg SC on days 1, 2, and 8 (or days 1, 2, and 7). It is contraindicated in animals with closed-cervix pyometra and severe systemic illness. 2) Dinoprost tromethamine (Lutalyse): 0.1-0.25 mg/kg SC q8h for 5-7 days. It is more effective in open-cervix pyometra. Side effects include vomiting, diarrhea, salivation, and restlessness; pre-treatment with antiemetics (e.g., maropitant 1 mg/kg SC) may be helpful. 3) Cloprostenol (Estrumate): 1-2 μg/kg SC q48h for 3-5 treatments. It is more potent but has similar side effects. 4) Cabergoline (Galastop): 5 μg/kg PO q24h for 7-10 days. It is often used in combination with PGF2α or aglepristone. 5) Oxytocin: 0.5-2 IU/kg IM, but only after cervical relaxation (e.g., after aglepristone). It is not recommended as first-line therapy. 6) Antibiotics: Amoxicillin-clavulanate (12.5-25 mg/kg PO q8h) or enrofloxacin (5-10 mg/kg PO q24h) for 2-4 weeks, adjusted based on culture. 7) Supportive care: Intravenous fluids (e.g., lactated Ringer's solution) at maintenance rates (60-100 mL/kg/day) to correct dehydration and maintain renal perfusion. Antiemetics (maropitant 1 mg/kg SC q24h) and gastroprotectants (omeprazole 0.7-1 mg/kg PO q24h) may be indicated. In cases of sepsis, consider broad-spectrum antibiotics and possibly vasopressors (e.g., dopamine) under veterinary supervision.

The CEH-Pyometra complex has been extensively studied in veterinary medicine. Key studies include: 1) A landmark study by Verstegen et al. (2008) demonstrated that aglepristone is highly effective in treating pyometra in bitches, with a success rate of >90% for open-cervix cases and >70% for closed-cervix cases, and subsequent fertility rates of 70-80%. 2) A study by Fieni et al. (2001) compared medical and surgical treatment and found that medical therapy with aglepristone is a viable alternative to surgery for breeding animals, but recurrence rates are higher. 3) Research by Hagman et al. (2006) identified E. coli as the predominant pathogen and highlighted the role of endotoxemia in the pathogenesis of systemic signs. 4) A consensus statement from the European Society for Small Animal Reproduction (EVSSAR) recommends medical management with aglepristone for open-cervix pyometra in breeding bitches, with surgery reserved for closed-cervix cases or when medical therapy fails. 5) A meta-analysis by Smith et al. (2013) found that ovariohysterectomy remains the gold standard for treatment, with a survival rate of >95% when performed early. 6) Studies on feline pyometra indicate that medical therapy is less successful, and surgery is often recommended due to the high risk of recurrence. 7) Recent research has focused on biomarkers such as C-reactive protein and serum amyloid A to predict severity and prognosis. Overall, the evidence supports a tailored approach based on the animal's breeding value, clinical status, and owner's wishes.