Antibacterial use in Norwegian horses: a nationwide registry-based cross-sectional study.

Researchers analyzed 2022–2024 national prescription records to see which antibiotics Norwegian horses received and for which conditions, with extra attention to reproductive problems. Most use was older, lower-risk antibiotics—mainly trimethoprim–sulfonamide pastes—and registry completeness improved over time, but limited diagnostic detail hampered stewardship evaluation.

What the study asked and why it matters

• Question: How, how much, and for which diagnoses are antibacterials used in Norwegian horses, and how fit is Norway’s registry data for upcoming EU reporting?
• Why it matters: Antibacterial use in animals drives antimicrobial resistance (AMR) affecting animal and human health. Horses have been understudied nationally in Norway and in most countries, leaving gaps in stewardship policy and EU-required monitoring.
• Special focus: Gynecological and obstetrical (reproductive) conditions, where evidence for optimal systemic antibiotic choices and routes is evolving and stewardship is critical.

Data sources, design, and population

• Design: Nationwide, registry-based cross-sectional analysis covering 2022–2024.
• Source: Norwegian Veterinary Prescription Registry (VetReg), capturing antibacterial prescriptions/dispenses for horses.
• Sample size: 30,362 records.
• Reporters: 42.5% submitted by veterinarians and 57.5% by pharmacies, reflecting mixed channels of prescribing and dispensing.
• Comparative completeness check: VetReg records were benchmarked against national sales data to estimate coverage of total use.

How antibacterial use was measured

• Primary metric: Kilograms of active substance dispensed/used.
• Implication: Mass-based metrics can make high-mass, broad-use formulations (e.g., oral pastes) appear dominant; they do not capture dosing frequency, duration, or treated horse counts.
• Classification: Antibacterials grouped by AMEG (EMA’s Antimicrobial Advice ad hoc Expert Group) categories:
  • Category D (prudent use; lowest risk for resistance selection relative to other classes)
  • Higher categories (C, B, A) imply increasing restriction/avoidance

Key findings on indications and patterns of use

• Most common treated conditions:
  • Skin and hoof disorders
  • Musculoskeletal conditions
  • Together, these two groups accounted for about half of all registry records.
• Gynecological and obstetrical conditions:
  • Represented 4% of records.
  • Showed similar antibacterial class patterns to the overall dataset.
• Dominant substances and routes (by kg active substance):
  • Oral pastes with trimethoprim–sulfonamide combinations exceeded 90% of total use in each year (2022–2024).
  • Parenteral beta-lactamase–sensitive penicillins were the next most used group.
• Stewardship profile:
  • AMEG category D agents accounted for >99% of use across all years.
  • Use of higher-priority agents (categories C/B/A) was minimal.

Data quality and readiness for EU reporting

• Completeness (VetReg vs. sales data):
  • Improved from 75.5% in 2022 to 84.7% in 2024.
  • Comparable to, or better than, species already submitted by Norway to the European Medicines Agency.
• Limitations affecting stewardship analytics:
  • Diagnostic coding often captured broad organ-system categories, not specific infectious diagnoses or severity.
  • Absence of reliable horse population denominators constrained per-animal or per-population rate calculations and cross-year/country comparisons.
  • Mass-based metrics do not reflect course length, dosing intensity, or outcomes.

Interpretation: what the results mean

• Encouraging stewardship signal:
  • Very high reliance on AMEG category D (>99%) suggests widespread adherence to using lower-risk, older antibiotics when appropriate.
  • Compared with Spain (85% category D), Norway’s profile indicates lower use of higher-priority agents.
• Areas of concern:
  • Despite prudent class selection, the extensive use of trimethoprim–sulfonamide (TMP–sulfa) combinations may exert selection pressure; rising carriage of resistant E. coli in horses underscores the need to re-evaluate default reliance on these pastes.
  • Broad diagnostic codes limit assessment of whether antibiotics were indicated, optimally chosen, and appropriately dosed.
• Clinical nuance:
  • Penicillins remain an appropriate parenteral backbone for many equine bacterial infections; their presence as the second most used group aligns with narrow-spectrum stewardship principles.
  • However, without culture/susceptibility data linkage, it is difficult to judge appropriateness at the case level.

Gynecological and obstetrical focus

• Use mirrored overall patterns:
  • Predominant use of TMP–sulfa and penicillins also characterized reproductive cases.
• Stewardship implications:
  • Reproductive tract infections often benefit from diagnostics (e.g., culture/cytology for endometritis) to enable targeted therapy and reduce unnecessary systemic exposure.
  • Given the high share of oral TMP–sulfa by mass, reviewing indications where local treatments, non-antibiotic interventions, or narrow-spectrum alternatives suffice could reduce selection pressure.
• Data gaps:
  • General organ-related codes prevent distinguishing prophylaxis from therapy and hinder evaluation of route (systemic vs. local), duration, and timing relative to procedures or foaling.

Strengths

• Nationwide scope with a large sample (30,362 records) across three years.
• Inclusion of both veterinary and pharmacy reports, improving completeness.
• Direct assessment of registry completeness against sales data, showing year-on-year improvement.
• Alignment with EU/EMA categorization (AMEG) facilitates international comparisons.

Limitations

• Cross-sectional, descriptive design; cannot assess causality, appropriateness, or outcomes.
• Broad diagnostic coding impedes indication-specific stewardship evaluation.
• Lack of reliable denominator data (horse population size/structure) limits rate-based comparisons over time and across countries.
• Mass-based consumption metrics may over-represent high-mass formulations and under-inform on treatment intensity and duration.
• Potential under-reporting remains (completeness <100%), although improving.

How this compares internationally

• Norway’s >99% category D usage exceeds the proportion reported for Spain (85%), indicating comparatively limited use of higher-priority agents.
• Few countries publish nationwide, horse-specific, registry-based analyses, making Norway’s dataset a valuable reference point for EU harmonization.

Policy and practice implications

• Maintain low use of higher-priority antibiotics by reinforcing first-line, narrow-spectrum choices where clinically appropriate.
• Reassess routine reliance on TMP–sulfa, especially for conditions where culture-guided therapy or non-antibiotic management may suffice.
• Enhance diagnostic specificity in VetReg (e.g., standardized infection codes, linkage to lab results) to enable robust stewardship metrics.
• Develop better denominators (e.g., national horse census, unique animal IDs) to calculate use per horse or per biomass and to support fair cross-country comparisons.
• Prepare for EU reporting by continuing data quality improvements and aligning metrics with EMA guidance.

Priority next steps for research and surveillance

• Link prescriptions to microbiology and clinical outcomes to evaluate appropriateness and effectiveness.
• Differentiate prophylactic, peri-procedural, and therapeutic use, especially in reproductive and surgical contexts.
• Adopt multiple metrics (e.g., number of treated horses, courses, or defined daily doses for animals) alongside kg to balance interpretation.
• Monitor resistance trends in commensal and pathogenic equine bacteria to detect impacts of current prescribing patterns, particularly for TMP–sulfa.
• Assess route-specific strategies (systemic vs. local treatments) to minimize overall antibiotic exposure without compromising outcomes.