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advances in anaesthesia, surgical techniques and postoperative care, it continues to carry significant risks. Horses requiring surgery are often already critically ill, the procedures can be lengthy, and opening the intestine inevitably increases the risk of bacterial contamination. To minimise the chance of postoperative infection, surgeons routinely administer prophylactic antibiotics, known as surgical antimicrobial prophylaxis (SAP).
However, the widespread use of antibiotics raises another important concern: antimicrobial resistance. The more frequently antibiotics are used, and the longer they are administered, the greater the selective pressure encouraging bacteria to develop resistance. Antibiotics can also disrupt the normal community of beneficial microorganisms living within the horse's digestive tract – the gut microbiome – which plays an important role in digestion, immunity and overall health.
In human medicine, evidence has shown that extending antibiotic prophylaxis beyond 24 hours after abdominal surgery offers little additional benefit and may even be harmful. Current human guidelines therefore recommend limiting SAP to less than 24 hours whenever possible. Whether the same approach could safely be adopted in equine colic surgery has remained uncertain.
To investigate, researchers led by Louise L. Southwood at the Department of Clinical Studies, New Bolton Center, University of Pennsylvania, together with colleagues from the Children's Hospital of Philadelphia, carried out a prospective randomised clinical trial comparing 24 hours and 72 hours of antibiotic prophylaxis following colic surgery.
The study enrolled 140 horses recovering from colic surgery, with 71 receiving potassium penicillin and gentamicin for 24 hours and 69 receiving the same treatment for 72 hours. The researchers monitored the horses for postoperative complications, including surgical wound infections and gastrointestinal problems. They also examined how the different treatment durations affected the horses' gut microbiome and "resistome" - the collection of antimicrobial resistance genes carried by the bacteria living in the intestine.
To analyse these changes, faecal samples from a subset of 49 horses were collected when they entered hospital and again before discharge. Using advanced DNA sequencing techniques, the team identified both the bacterial species present and the antibiotic resistance genes they carried.
The clinical results were reassuring. There were no significant differences between the two treatment groups in any measured postoperative complication. Rates of incisional infection were almost identical, occurring in around 16–17% of horses regardless of whether antibiotics had been given for one day or three. Gastrointestinal complications were also similar.
The microbiological findings told a different story.
Simply being hospitalised and undergoing surgery significantly altered the composition of the horses' intestinal microbiome. Overall bacterial diversity declined during hospitalisation, while genes associated with resistance to the antibiotics used during treatment became more common. These changes were considerably more pronounced in horses that received antibiotics for 72 hours.
In particular, the longer treatment group showed significantly higher levels of beta-lactam resistance genes, including TEM beta-lactamases, by the time they left hospital. These genes enable bacteria to resist an important class of antibiotics, raising concerns that prolonged treatment may increase selective pressure for antimicrobial resistance without providing additional clinical benefit.
The researchers conclude that limiting surgical antimicrobial prophylaxis to 24 hours appears to be the preferable strategy for horses undergoing colic surgery. It provided postoperative outcomes comparable to a 72-hour course while causing less disruption to the gut microbiome and resulting in fewer increases in antimicrobial resistance genes.
The authors note several limitations, including the study being conducted at a single hospital, relatively small numbers for detecting rare complications, and the fact that clinicians were aware of which treatment each horse received. Nevertheless, the findings provide encouraging evidence that shorter antibiotic courses may safely reduce antimicrobial use in equine practice while helping to preserve both the gut microbiome and the long-term effectiveness of these vital medicines.
For more details, see:
Southwood, L. L., Long, A., Perez, J., Daniel, S., Bittinger, K., Aitken, M., & Redding, L.
Effect of surgical antimicrobial prophylaxis duration for colic surgery on complications and resistome.
Equine veterinary journal, (2026) 58(2), 390–403.
