Polymyxin B’s Potential Side Effects in Horses 

Researchers evaluated the potential side effects of the antimicrobial agent polymyxin B in horses.
Sick horse might receive treatment with polymyxin B.
Polymyxin B is a common treatment for endotoxemia-associated sequelae in horses, but the antimicrobial agent can cause ataxia and weakness. | Adobe Stock

Veterinarians often use polymyxin B for its endotoxin-binding effects to manage endotoxemia in horses due to bacterial infections. This antimicrobial agent can help control endotoxemia-associated sequelae such as laminitis, coagulopathy, renal failure, and multi-organ failure. In humans, neuro- and nephrotoxicity are known adverse effects of polymyxin B. So researchers from the University Hospital at Zurich recently performed a study to assess these effects in horses, as well.  

Study on Polymyxin B

The team administered polymyxin B (PolyB at 6,000 IU/kg IV every 12 hours) to 20 horses of mixed breed populations for one to four days for treatment of surgical colic, peritonitis, typhlocolitis, pneumonia, and pyometra. They performed clinical and neurologic exams to track PolyB’s potential side effects on renal and urinary tract parameters and neurologic findings. They administered the horses various medications, including gentamicin or marbofloxacin and penicillin, metronidazole, flunixin, intravenous fluids, omeprazole, heparin, lidocaine, fenbendazole, pergolide, and meloxicam, as well as sedation with detomidine, xylazine, and butorphanol. Of the 20 horses, systemic anti-inflammatory response syndrome (SIRS) was present in 70% (14/20).

Research Findings

The researchers’ findings included: 

  • All horses developed ataxia of grades 1-3 (scale 0-5). 
  • Weakness was identified in 75% (15/20). 
  • Of 16 horses with measured creatinine and urea, all but one had values within the reference range. 
  • No urine measured from 14 horses showed proteinuria. 
  • In 50% of the horses, renal tubular damage was noted due to mild increases in the urinary GGT/creatinine ratio. 

Final Thoughts

In conclusion, all hospitalized horses treated with polymyxin B developed neurogenic side effects of ataxia and weakness. These signs subsided after the last dose of PolyB. Other medications administered along with PolyB, such as aminoglycosides and NSAIDs, can also contribute to neurotoxicity and nephrotoxicity. The authors noted that “the severity of neurological deficits depends on the dose and duration of polymyxin administration, and clinical signs are reversible after discontinuation of therapy in healthy horses.” They did not thoroughly evaluate renal function, especially in the face of ongoing intravenous fluid therapy given to horses in this study. 


van Spijk JN, Beckmann K, Wehrli Eser M, et al. Preliminary Investigation of Side Effects of Polymyxin B Administration in Hospitalized Horses. Antibiotics 2023, 12, 854; DOI: 10.3390/antibiotics12050854

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