General anesthesia can be associated with postoperative fever in horses. In a recent study, researchers at the University of Florida examined whether prophylactic antimicrobial drug (AMD) therapy could reduce the incidence of post-anesthetic fever [Hoblick, S.; Denagamage, TN.; Morton, AJ. Antimicrobial prophylaxis is not indicated for horses undergoing general anesthesia for elective orthopedic MRI. Equine Veterinary Journal 2023; DOI: 10.1111/evj.13978].
Study on AMD Use in Horses Undergoing MRI
The retrospective study used data from June 2006 to March 2020 of 791 elective orthopedic MRIs in healthy horses. Invasive surgery was not part of the procedure, which obviates fever reasons such as surgical site infection or inflammation. Travel to the surgical facility was relatively short and not a likely factor in fever development in this study.
Horses were sedated with detomidine or xylazine +/- butorphanol or diazepam, then maintained on isoflurane via an orotracheal tube. Some horses received flunixin meglumine; 122 were treated with some form of penicillin plus gentamicin prior to anesthesia and for the subsequent 24 hours post-anesthesia.
Forty-four study horses (5.6%) developed a fever within 15 hours following anesthesia. (This is comparable to previous studies of arthroscopic surgery of healthy horses, which report a fever incidence of 2-5.1%.) Of the 44 horses with fever, 18% were diagnosed with a respiratory infection. The data in this study yielded interesting results:
- Horses administered prophylactic AMDs were more likely to develop post-anesthetic fever compared to horses that did not receive AMDs.
- Horses aged 1-4 years had a greater likelihood of fever compared to horses ≥5 years.
Risks Associated with AMDs in Horses
Anytime AMDs are administered to horses, there is a risk of intestinal dysbiosis and subsequent secondary concerns as well as increased risk of development of antibiotic resistance. Not all fevers are related to infection. The authors pointed out that several anesthetic medications affect normal thermoregulatory response thresholds that would normally limit responses to change in body temperature. There is also the possibility of MRI radiofrequency exposure-induced inflammation contributing to fever in horses; this requires more research.
Most of the horses that developed fever following elective MRI had no definitive associated cause. Fever either self-resolved within the first 24 hours or was treated with an NSAID (43%) prior to hospital discharge. Only nine horses that received AMDs had a definitive diagnosis of post-anesthetic infection, while eight of the horses that did not receive AMDs developed a diagnosable infection. The study authors stated, “Given that only 1% of horses that did not receive prophylactic antimicrobials were diagnosed with early post-anesthetic infection, the routine use of prophylactic antimicrobials cannot be justified.”
Other studies recommend restricting AMD use to surgical cases that have a greater than 5% incidence of infection without use of prophylactic AMDs. The authors concluded: “The use of prophylactic antimicrobials to reduce the risk of post-anesthetic fever for elective MRI is not supported by this study.” It is also important to encourage owners to continue to monitor their horses for signs of infection following hospital discharge.