Overview

• Failure or delay of normal postpartum uterine involution (normally requires 12–15 weeks to complete).
• Failure of eosinophilic masses of collagen at placental sites to slough at 3–4 weeks postpartum.
• Failure of fetal trophoblastic cells to regress (normally occurs within 2 weeks); instead, they invade the maternal deep glandular endometrium and myometrium.
• Cause-unknown; hormonal or uterine basis not suspected based on coexistence of unaffected and subinvoluted placental sites in the same uterus.

Signalment

• Dog only
• Bitch <3 years most common
• Higher incidence in first litter
• No breed predilections

Signs

Causes & Risk Factors

• Unknown.
• Hormonal-unlikely, because only some of the placental sites may be involved.
• Uterine disease-unlikely, because of high first litter prevalence.

Differential Diagnosis

• Metritis-differentiated by vaginal cytology and physical examination.
• Vaginitis-differentiated by vaginal cytology.
• Vaginal neoplasia-differentiated by vaginal cytology and vaginal endoscopy.
• Uterine neoplasia-differentiated by ultrasonography or exploratory laparotomy.
• Cystitis-differentiated by vaginal cytology and urinalysis obtained by cystocentesis.
• Coagulopathy-differentiated by clotting times.
• Trauma.
• Endogenous estrogen stimulation-bitch with an extremely shortened interestrous interval.
• Exogenous estrogen stimulation-oral medication or contact with topical hormone replacement product on human skin, bedding, or clothing.

CBC/Biochemistry/Urinalysis

Usually normal

Other Laboratory Tests

• Serology for Brucella canis negative
• Progesterone <2 ng/mL

Imaging

Uterine ultrasonography-focal uterine wall thickening; possible echogenic fluid in the lumen.

Diagnostic Procedures

• Vaginal cytologic examination-key for diagnosis; reveals erythrocytes and parabasal epithelial cells; may note pathognomonic trophoblastic cells (polynucleated, heavily vacuolated).
• Guarded anterior vaginal or transcervical uterine culture-if vaginal cytologic examination or hemogram supports a diagnosis of secondary metritis.

Pathologic Findings

• Gross-sites characterized by a thickened, hemorrhagic area that may be nodular.
• Histopathologic-definitive diagnosis; eosinophilic collagen masses with trophoblasts extending into the myometrium.

Drug(s)

• Oxytocin generally not successful.
• Ergonovine 0.2 mg/15 kg IM (10–30 µg/kg) once; do not use if uterus friable.
• Small study showed response to megestrol acetate 0.1 mg/kg q24h for 1 week, then 0.05 mg/kg q24h daily for 1 week; 5 of 6 treated bitches were successfully bred, with normal parturition and puerperal periods.

Contraindications/Possible Interactions

• Ecbolics-may cause uterine rupture.
• Progestational drugs-increase the risk of metritis, which may mimic pyometra or induce cystic endometrial hyperplasia.

Patient Monitoring

• Mucous membrane color and amount of discharge.
• Packed cell volume-if anemia is a concern.
• Changes in discharge color or odor and vaginal cytologic examination and culture-diagnose secondary infection.

Possible Complications

Infection, blood-loss anemia, or uterine rupture-rare

Expected Course and Prognosis

• Spontaneous resolution-the usual outcome in the majority of cases.
• Recurrence-not expected, occurs rarely.
• Prognosis for future reproduction-excellent with spontaneous resolution.