# Effect of surgical antimicrobial prophylaxis duration for colic surgery on complications and resistome

## TL;DR

The study found that extending antimicrobial prophylaxis beyond 24 hours in horses undergoing colic surgery does not reduce complications and may increase antibiotic resistance.

## Contribution

This is the first study to evaluate the impact of SAP duration on the resistome in horses undergoing colic surgery.

## Key findings

- No significant difference in complications between 24-h and 72-h SAP groups.
- Time, not SAP duration, was the main driver of microbiome and resistome changes.
- Discharge beta-lactam resistance genes were higher in the 72-h SAP group.

## Abstract

Based on human studies, surgical antimicrobial (AMD) prophylaxis (SAP) beyond 24 h is unnecessary and potentially detrimental.

To compare clinical and microbiological outcomes in patients receiving 24‐ or 72‐h of SAP for colic surgery.

Prospective randomised clinical trial.

Horses that recovered from colic surgery were considered. Exclusion criteria were (1) age <2 years; (2) Miniature Horses, pony, and draught breeds; (3) azotaemia; (4) recent hospitalisation, colic surgery, or AMDs; (5) local AMD administration. Eligible horses were randomly assigned to receive SAP with potassium penicillin and gentamicin for 24‐ or 72‐h. Clinical data and complications were compared between SAP groups. Admission and discharge faecal samples from a subset of horses (N = 49) underwent shotgun metagenomic sequencing on an Illumina platform. Host reads were filtered by aligning to reference genomes using the Burrows–Wheeler Aligner, and taxonomic classification was performed with kraken2. Sequencing reads were aligned to the Comprehensive Antimicrobial Resistance Database (CARD)5 and characterised using the AMR++ pipeline. The microbiome/resistome was characterised and compared between SAP groups over time.

One hundred and forty horses completed the study (24‐h N = 71 and 72‐h N = 69). The only clinical variable that was different between SAP groups was age (24‐h median age 16 [IQR 9, 20] and 72‐h 12 [6, 18] years, p = 0.03). There was no significant difference between groups for any complications including incisional infection (24‐h 17 [95% CI 10–27]% and 72‐h 16 [9–26]%, p = 0.9). Time was the main driver of changes in the microbiome/resistome: alpha diversity decreased while AMD resistance genes associated with administered AMD increased between admission and discharge. Discharge beta‐lactam resistance genes were significantly higher in the 72‐h than the 24‐h group.

Single hospital, small numbers for complications, clinicians not blinded to SAP group.

SAP for 24‐h is recommended for horses undergoing colic surgery.