Antibiotic Use in Horse Reproduction: How to Treat Uterine Infections in Mares

By Dr Duncan Houston

Uterine infections are one of the most common reasons mares fail to conceive or maintain pregnancy. The problem is not just the infection itself. It is how often these cases are misdiagnosed or overtreated, leading to poor outcomes and unnecessary antibiotic use.

If you are managing a mare with suspected uterine infection, the key question is not just “what antibiotic should I use” but “is this truly an infection, and what is the underlying cause”.

Quick Answer

Not all bacteria found in a mare’s uterus indicate infection. Cytology is essential to confirm inflammation before treating. Effective management includes uterine flushing, targeted antibiotic therapy, and correcting underlying anatomical issues. Without proper diagnosis and correction, infections will often recur.

What Is a True Uterine Infection?

A positive culture alone does not mean infection.

In practice, this is one of the biggest mistakes made in equine reproduction.

Contamination vs infection

• Contamination: Bacteria present without inflammation

• True infection (endometritis): Bacteria + inflammatory cells

What actually matters

Cytology tells you whether the uterus is reacting.

• Neutrophils present = active inflammation

• No inflammatory cells = likely contamination

Clinical insight:
Many mares are treated unnecessarily because decisions are based on culture alone. This leads to overuse of antibiotics without improving fertility.

Why Mares Get Uterine Infections

Uterine infections are rarely just about bacteria. They are usually a failure of clearance.

Common causes include:

• Poor vulvar conformation

• Pneumovagina (air entering the reproductive tract)

• Fluid retention after breeding

• Cervical dysfunction

• Age related changes

What matters most:
If you do not fix the underlying issue, the infection will come back.

How Do You Diagnose It Properly?

A proper workup should always include:

1. Culture to identify bacteria

2. Cytology to confirm inflammation

3. Ultrasound to assess fluid and uterine environment

Decision checkpoint

• Culture positive + cytology positive → treat

• Culture positive + cytology negative → do not rush to treat

• Fluid on ultrasound → clearance problem likely present

Treatment Options: What Actually Works?

Intrauterine Antibiotic Infusion

This delivers high concentrations directly to the uterus.

Best used when:

• Confirmed infection

• Accessible uterus

• Short course preferred

Key principles:

• Flush thoroughly before treatment

• Maintain strict aseptic technique

• Treat for 3 to 7 days depending on response

Clinical insight:
Antibiotics work poorly if the uterus is full of debris or fluid. Flushing is not optional. It is essential.

Systemic Antibiotic Therapy

Oral or injectable antibiotics can still reach the uterus.

Best used when:

• Repeated catheterisation is not ideal

• Risk of contamination is high

• Handling is limited

Common options include:

• Penicillin with or without gentamicin

• Trimethoprim sulfa

Limitations:

• Slower to reach effective levels

• Requires longer treatment

Infusion vs Systemic: How Do You Choose?

Intrauterine infusion

• High local concentration

• Faster response

• Requires skill and sterile technique

Systemic therapy

• Less invasive

• Lower contamination risk

• Slower and less targeted

Real world approach:
The choice is rarely about preference. It is about what is safest, most practical, and most effective for that specific mare.

Severity Framework

Mild

• Minimal fluid

• Mild cytology changes

• No systemic signs

Often responds well to flushing and targeted therapy.

Moderate

• Clear inflammation

• Recurrent breeding failure

• Fluid accumulation

Requires structured treatment and re evaluation.

Severe

• Significant uterine fluid

• Chronic infection

• Poor reproductive history

Often linked to underlying anatomical defects.

Critical

• Persistent infection despite treatment

• Repeated infertility

• Structural abnormalities present

These cases require corrective procedures, not just antibiotics.

Structural Problems: The Most Overlooked Cause

This is where most cases fail.

Common issues:

• Poor perineal conformation

• Pneumovagina

• Cervical damage or fibrosis

Caslick’s procedure

Often used to improve vulvar seal and reduce contamination.

Ultrasound role

• Detect fluid retention

• Assess uterine clearance

Clinical insight:
If you keep treating the infection but ignore the anatomy, you are not solving the problem.

When Is This an Emergency?

Uterine infections are rarely emergencies, but urgent attention is needed if:

• There is significant fluid accumulation

• The mare is systemically unwell

• There is a history of repeated failed treatments

• Breeding timelines are critical

What Should You Do Next?

1. Confirm diagnosis with both culture and cytology

2. Do not treat based on culture alone

3. Address uterine clearance with flushing

4. Choose targeted antibiotics based on sensitivity

5. Re evaluate after treatment

6. Investigate and correct anatomical issues

Time based guidance:

• Recheck after treatment before breeding again

• If infection recurs, reassess underlying causes immediately

Antibiotic Stewardship: Why It Matters

Overtreatment is a real issue in equine reproduction.

• Repeated empirical antibiotic use drives resistance

• Treating contamination does not improve fertility

• Poor protocols reduce long term outcomes

Best practice:

• Always use culture and sensitivity

• Avoid unnecessary repeated courses

• Focus on diagnosis, not just treatment

Common Mistakes

• Treating based on culture alone

• Skipping cytology

• Not flushing before infusion

• Using antibiotics without addressing anatomy

• Repeating treatments without reassessment

• Ignoring uterine clearance issues

Can This Be Prevented?

In many cases, yes.

• Regular reproductive exams

• Early identification of conformation issues

• Managing breeding technique and hygiene

• Monitoring uterine clearance post breeding

• Acting early before infections become chronic

FAQs

Do all mares with bacteria need antibiotics?

No. Only mares with confirmed inflammation on cytology should be treated.

How long should treatment last?

Typically 3 to 7 days for intrauterine therapy, longer for systemic treatment depending on response.

Is flushing always necessary?

In most cases, yes. It improves antibiotic effectiveness and removes debris.

Can infections come back after treatment?

Yes. Recurrence is common if underlying anatomical issues are not corrected.

What is better, infusion or systemic treatment?

It depends on the mare. Infusion is more targeted, but systemic therapy is less invasive.

Final Thoughts

The biggest shift in managing uterine infections in mares is not the antibiotics themselves. It is how we decide when to use them.

Accurate diagnosis, proper uterine clearance, and correcting anatomical problems are what drive successful outcomes.

Antibiotics support the process. They do not fix the underlying problem on their own.

If you are unsure whether your mare truly needs treatment, or you want guidance on choosing the right protocol and monitoring response, ASK A VET™ can help you make clearer, more confident decisions throughout the reproductive process.