Equine Metritis Study Guide

Overview and Clinical Importance

Metritis in mares encompasses inflammation of all uterine layers (endometrium, myometrium, and perimetrium) and represents a significant cause of reproductive failure. This study guide covers two primary forms: contagious equine metritis (CEM) and postpartum metritis, both critical topics for the NAVLE examination.

Contagious Equine Metritis (CEM)

Definition and Etiology

CEM is an acute, highly contagious venereal disease caused by Taylorella equigenitalis, a gram-negative, microaerophilic coccobacillus. First identified in 1977 in the United Kingdom, CEM has since been reported worldwide but is considered eradicated from the United States.

Organism Characteristics

Taylorella equigenitalis is fastidious and slow-growing, requiring 3-7 days on specialized chocolate blood agar under microaerophilic conditions. Two biotypes exist: streptomycin-sensitive and streptomycin-resistant. The organism is cytochrome oxidase and catalase positive but otherwise biochemically unreactive.

Transmission and Epidemiology

CEM is transmitted primarily through natural breeding, artificial insemination with infected semen, and contaminated fomites. Stallions are the primary reservoirs, harboring the organism in the smegma of the prepuce, urethral fossa, and penile surface without showing clinical signs. Infected stallions can transmit T. equigenitalis to virtually every mare bred.

Clinical Signs

In Mares

After an incubation period of 2-13 days, affected mares may develop a grayish-white, mucopurulent vulvar discharge of uterine origin. The discharge is typically odorless and can vary from minimal to copious amounts. Clinical signs include:

• Endometritis, cervicitis, and vaginitis of variable severity
• Short estrous cycles (failure to maintain pregnancy)
• Temporary infertility lasting several weeks
• Return to estrus after shortened cycles

In Stallions

Stallions remain completely asymptomatic and show no clinical signs of infection despite harboring the organism for months to years. This makes identification of infected stallions extremely challenging without routine testing.

Pathological Findings

Gross lesions are most severe in the uterus and include swollen, edematous endometrial folds with mucopurulent exudate. The cervix may show edema, hyperemia, and mucopurulent exudate. Microscopically, lesions include neutrophilic infiltration during acute stages, followed by lymphocyte, macrophage, and plasma cell infiltration in chronic cases.

Diagnosis of CEM

Sample Collection

Laboratory Testing

Bacterial culture remains the gold standard for diagnosis. T. equigenitalis requires chocolate blood agar with 5-10% CO2 and microaerophilic conditions. Colony identification is based on:

• Small, gray, translucent colonies after 3-7 days
• Catalase and oxidase positive
• Nonfermentative and nonproteolytic
• PCR testing available for rapid identification

Serology

Complement fixation test available for mares only (not reliable for stallions). Antibodies develop 10+ days post-infection but only indicate past exposure, not current carrier status.

Treatment of CEM

Most mares with acute endometritis recover spontaneously, but the carrier state in both mares and stallions can be difficult to eliminate and requires aggressive treatment protocols.

Postpartum Metritis

Definition and Etiology

Postpartum metritis is inflammation of all uterine layers occurring within 10 days after parturition, most commonly 2-4 days postpartum. It is typically associated with dystocia, retained fetal membranes, or excessive contamination during foaling.

Primary Bacterial Pathogens

The most common pathogen is Streptococcus zooepidemicus, but other important bacteria include:

• Escherichia coli
• Klebsiella pneumoniae
• Pseudomonas aeruginosa
• Crossiella equi

Predisposing Factors

Clinical Signs

Clinical signs typically appear as early as 12 hours postpartum and may include:

Systemic Signs

• Fever (often greater than 101.5°F)
• Depression and anorexia
• Dehydration
• Muddy mucous membranes (endotoxemia)
• Laminitis (secondary to endotoxemia)

Local Signs

• Fetid, purulent vulvar discharge
• Uterine enlargement and pain on palpation
• Delayed uterine involution
• Cervical inflammation

Diagnosis of Postpartum Metritis

Clinical Examination

Treatment of Postpartum Metritis

Treatment Goals

• Eliminate bacterial infection
• Remove inflammatory debris and toxins
• Control endotoxemia and systemic effects
• Prevent laminitis
• Restore normal uterine function

Prognosis

Prognosis depends on early recognition and aggressive treatment. With prompt intervention, most mares recover completely. However, severe cases can lead to septicemia, laminitis, and death. Future reproductive performance is generally good if treatment is successful and no permanent uterine damage occurs.

Prevention Strategies

CEM Prevention

• Mandatory testing and certification of breeding animals
• Quarantine and testing of imported horses
• Regular surveillance testing in breeding populations
• Proper hygiene and biosecurity at breeding facilities

Postpartum Metritis Prevention

• Proper foaling environment and hygiene
• Prompt management of dystocia
• Immediate treatment of retained fetal membranes
• Postpartum monitoring and early intervention

Regulatory Considerations

CEM is classified as a foreign animal disease in the United States and is immediately reportable to USDA APHIS and State Animal Health Officials. International horse movement requires CEM testing and certification. The United States maintains CEM-free status through strict import requirements and surveillance.

High-Yield Facts for NAVLE

• Stallions are asymptomatic carriers but transmit to virtually all mares
• CEM causes temporary infertility with grayish-white, odorless discharge
• Postpartum metritis requires aggressive treatment to prevent laminitis and death
• Retained fetal membranes are the most important predisposing factor