Antimicrobial Stewardship in Oncology: What Vets Need to Know

Published on VetOnIt CE Blog

How Antibiotics During Cancer Treatment Affect the Microbiome

A February 2026 study from Texas A&M University examined how antibiotic use during intensive chemotherapy for acute myeloid leukemia (AML) affects the gut microbiome and antibiotic-resistant infection risk.¹

“Because doctors would rather be safe than sorry, they start treating their patients with antibiotics when they get a fever, even if they don’t have a bacteria-positive culture,” explains Dr. Jessica Galloway-Peña, the study’s principal investigator. "However, our study shows that antibiotic use during treatment can be refined in ways that protect patients while also limiting unintended effects on the microbiome."¹

The research addresses a common clinical scenario: cancer patients routinely receive multiple antibiotics to prevent infection, but this practice may select for antibiotic-resistant organisms already present in the gut.

California, Oregon, Kentucky, and Nova Scotia now mandate antimicrobial stewardship CE for veterinarians, making this research relevant for practitioners managing oncology patients in those states.

The Perfect Storm: Cancer, Chemotherapy, and Compromised Immunity

Veterinary oncology patients face a unique constellation of infection risks that make antimicrobial stewardship particularly complex:

Immunosuppression from Multiple Sources

Cancer itself often compromises immune function through several mechanisms:

• Bone marrow infiltration in hematologic malignancies reduces neutrophil production
• Malnutrition and cachexia impair cellular immunity
• Tumor-associated immunosuppression creates localized immune dysfunction
• Barrier dysfunction from mucosal tumors increases translocation risk

Chemotherapy-Induced Vulnerabilities

The Texas A&M study demonstrates that chemotherapy compounds these issues by:

• Directly damaging gut epithelial barriers through cytotoxic effects
• Disrupting protective microbial communities that normally prevent pathogen overgrowth
• Reducing neutrophil counts below critical thresholds (typically <0.75-1.0 × 10⁹/L)
• Altering local immune responses in ways we’re only beginning to understand

Surprising Findings About Antibiotic Diversity

The Texas A&M research revealed unexpected patterns in antibiotic resistance. Patients treated with a wider variety of antibiotics actually ended up with fewer kinds of antibiotic-resistance genes than expected. The team initially expected that exposure to more antibiotics would lead to greater resistance, but reducing the diversity of bacteria in the gut also reduces “the number of possible combinations,” potentially limiting how easily antibiotic resistance genes are transferred.¹

However, not all antibiotics are equal. Linezolid—a powerful antibiotic used to treat serious gram-positive infections including MRSA—showed a particularly complex pattern. Increased linezolid exposure was associated with greater loss of microbiome diversity, but longer linezolid use was also associated with reduced risk of colonization by antibiotic-resistant organisms.¹

“Even though the loss of diversity is associated with infection, it doesn’t necessarily mean that it’s associated with antibiotic-resistant infection,” Dr. Galloway-Peña explains. "You have to keep those two concepts separate."¹

The Dysbiosis-Infection Connection

When chemotherapy-induced dysbiosis occurs, several concerning changes follow:

• Loss of colonization resistance allows pathogenic bacteria to establish
• Increased intestinal permeability promotes bacterial translocation
• Reduced production of beneficial metabolites like short-chain fatty acids
• Compromised local immunity in mucosal tissues

Evidence-Based Antimicrobial Use in Veterinary Oncology

Recent research has provided clearer guidance on when antimicrobial prophylaxis is truly indicated in cancer patients, moving us away from reflexive prescribing toward evidence-based stewardship.

Neutropenia Thresholds: Moving Beyond Arbitrary Cutoffs

A 2020 study published in Veterinary and Comparative Oncology evaluated the commonly used absolute neutrophil count (ANC) cutoff of 0.75 × 10⁹/L for antimicrobial prophylaxis in canine chemotherapy patients.² The findings were illuminating:

• Only 8.8% of chemotherapy administrations resulted in prophylactic antimicrobial prescribing using the 0.75 × 10⁹/L cutoff
• Using higher cutoffs (1.0 or 1.5 × 10⁹/L) would have increased prescribing to 15.3% and 25.8% respectively
• No correlation was found between these arbitrary cutoffs and actual infection rates

This research suggests many institutions may be over-prescribing antimicrobials based on neutrophil counts alone, without considering other risk factors or clinical signs of infection.

Risk Stratification: Beyond Numbers

Evidence-based antimicrobial prophylaxis in oncology requires comprehensive risk assessment:

High-Risk Scenarios (Consider Prophylaxis):

• ANC <0.75 × 10⁹/L with clinical signs of infection
• Recent invasive procedures (surgery, bone marrow aspirates)
• Concurrent corticosteroid therapy (especially high-dose protocols)
• Previous episodes of neutropenic sepsis
• Compromised barrier function (mucositis, diarrhea)

Moderate-Risk Scenarios (Monitor Closely):

• ANC 0.75-1.0 × 10⁹/L with no clinical signs
• Known microbiome disruption (recent broad-spectrum antimicrobial use)
• Environmental risk factors (hospitalization, recent boarding)

Low-Risk Scenarios (No Prophylaxis):

• ANC >1.0 × 10⁹/L with no clinical signs
• Stable outpatients with normal appetite and energy
• First-cycle chemotherapy in previously healthy patients

Practical Stewardship Strategies for Oncology Practice

1. Implement Evidence-Based Protocols

Develop standardized protocols that move beyond neutrophil counts alone:

Neutropenic Fever Protocol (Example Framework):
- Temperature >103°F (39.4°C) OR
- ANC <1.0 × 10⁹/L WITH clinical signs
- Obtain blood culture before antimicrobials
- Consider urine culture if urinary signs present
- Start empirical therapy only if high-risk features present

2. Optimize Diagnostic Workup

Before prescribing antimicrobials:

• Obtain cultures when possible (blood, urine, wound)
• Document clinical signs thoroughly
• Assess hydration status and barrier function
• Review concurrent medications and recent antimicrobial history

3. Choose Targeted Therapy

When antimicrobials are indicated:

• Narrow-spectrum agents for documented infections
• Avoid fluoroquinolones unless culture-guided (WHO critically important)
• Consider local resistance patterns in hospital environments
• Shortest effective duration (typically 3-7 days for prophylaxis)

4. Monitor and Adjust

Implement systematic monitoring:

• Daily clinical assessment during antimicrobial therapy
• Culture results to guide therapy adjustment
• Duration reassessment at 48-72 hours
• Adverse effect monitoring (especially GI effects)

The Microbiome Consideration: Protecting What Protects Our Patients

The Texas A&M research highlights the need to consider microbiome preservation in our antimicrobial decisions. Practical approaches include:

Microbiome-Sparing Strategies

• Probiotic supplementation during and after chemotherapy
• Narrow-spectrum antimicrobials when therapy is needed
• Shortest effective duration to minimize collateral damage
• Consider IV over oral routes to reduce gut microbiome disruption

Alternative Approaches

• Enhanced monitoring instead of prophylaxis in moderate-risk patients
• Environmental management to reduce pathogen exposure
• Nutritional support to maintain immune function
• Stress reduction to preserve natural resistance

Regulatory Landscape: Meeting Mandatory CE Requirements

The regulatory environment for antimicrobial stewardship continues to evolve, with several jurisdictions now mandating specific continuing education:

California (SB 361)

• 1 hour every 4 years on judicious use of medically important antimicrobials
• All licensed veterinarians regardless of practice type
• Effective since January 1, 2018

Oregon (OAR 875-010-0090)

• 1 hour per renewal cycle (every 2 years) in judicious antibiotic use
• Applies to all licensed veterinarians

Kentucky (KBVE Rules)

• 2 hours in antimicrobials, controlled substances, OR laws (veterinarian’s choice)
• Effective September 2024

Nova Scotia

• Mandatory AMR course for all veterinarians
• Only Canadian province with mandatory AMR continuing education

These requirements reflect growing recognition that antimicrobial stewardship is a core competency for all veterinarians, not just those in specific practice types.³

Clinical Case Example: Applying Stewardship Principles

Signalment: 8-year-old Golden Retriever, Day 7 post-vincristine/prednisolone for lymphoma

Presentation:

• Temperature: 102.8°F (39.3°C)
• ANC: 0.9 × 10⁹/L
• Mild lethargy, normal appetite
• Soft stool × 2 days (consistent with Texas A&M findings)

Traditional Approach: Reflexive antimicrobial prophylaxis based on low ANC

Stewardship Approach:

1. Risk assessment: Moderate risk (mild fever, borderline ANC, GI signs suggesting dysbiosis)
2. Diagnostic workup: Blood culture, urinalysis, fecal examination
3. Initial management: Close monitoring, supportive care, probiotic consideration
4. Antimicrobial decision: Hold unless temperature reaches >103°F or clinical deterioration
5. Follow-up: Recheck in 12-24 hours

Outcome: Patient improved with supportive care alone, avoiding unnecessary antimicrobial exposure during vulnerable microbiome recovery period.

Future Directions: Personalized Antimicrobial Stewardship

Emerging research suggests we may soon have tools for personalized stewardship decisions:

Microbiome-Based Risk Assessment

• Dysbiosis indices to quantify infection risk
• Pathogen-specific monitoring using molecular diagnostics
• Personalized probiotic therapy based on individual microbiome profiles

Biomarker-Guided Therapy

• Procalcitonin levels to distinguish bacterial from non-bacterial causes
• Cytokine profiles to assess infection severity
• Microbiome metabolites as indicators of colonization resistance

Key Takeaways for Veterinary Oncologists

1. Neutrophil counts alone are insufficient for antimicrobial prescribing decisions
2. Chemotherapy-induced dysbiosis is a real phenomenon that affects infection risk
3. Evidence-based protocols should guide prophylaxis decisions
4. Microbiome preservation should be considered in treatment planning
5. Regulatory requirements for antimicrobial CE are expanding
6. Diagnostic workup should precede empirical therapy when possible
7. Shortest effective duration minimizes resistance development and microbiome disruption

Conclusion: Balancing Act in the Age of Resistance

Managing infection risk in oncology patients requires balancing cancer treatment needs with microbiome preservation. The Texas A&M research shows that antibiotic protocols affect the microbial communities that protect patients from resistant infections.

As veterinarians in jurisdictions with mandatory antimicrobial CE requirements know, stewardship isn’t just good practice—it’s a professional obligation. The challenge lies in applying these principles thoughtfully in cancer patients, where the stakes are particularly high and the margin for error particularly narrow.

The future of veterinary oncology will likely include microbiome-informed treatment protocols, personalized infection risk assessment, and targeted therapies that preserve beneficial microbial communities while effectively managing cancer. Until then, evidence-based stewardship provides our best path forward.

For comprehensive information on state-specific antimicrobial CE requirements and resources, visit the VetOnIt Antimicrobial Resistance Resource Center.

Ready to deepen your antimicrobial stewardship knowledge? Join Dr. Lauren Forsythe on February 26th for VetOnIt CE’s comprehensive antimicrobial stewardship course. This RACE-approved program meets mandatory CE requirements for California, Oregon, Kentucky, and Nova Scotia while providing practical, evidence-based strategies for optimizing antimicrobial use in your practice.

Register now at VetOnIt CE →

External Source Links (Compliance)

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References

1. Texas A&M College of Veterinary Medicine & Biomedical Sciences. (2026, February 10). Texas A&M Study Reveals How Antibiotic Use During Leukemia Treatment Reshapes the Gut Microbiome. Retrieved from https://vetmed.tamu.edu/news/press-releases/antibiotic-use-chemotherapy-gut-microbiome-aml/

2. Bisson, J.L., et al. (2020). Evaluation of a 0.75 × 10⁹/L absolute neutrophil count cut‐off for antimicrobial prophylaxis in canine cancer chemotherapy patients. Veterinary and Comparative Oncology, 18(1), 83-89.

3. Guardabassi, L., et al. (2018). Antibiotic prophylaxis in veterinary cancer chemotherapy: A review and recommendations. Veterinary and Comparative Oncology, 16(3), 397-407.

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