The increased popularity of using the COX-2 only inhibitor firocoxib in horses might have led to a false sense of security about mitigation of detrimental effects on intestinal health in some cases. A study at the University of Illinois examined ultrasonic indicators of colonic inflammation from firocoxib or flunixin meglumine administration [Bishop, RC.; Wilkins, PA.; Kemper, AM.; et al. Effect of Firocoxib and Flunixin Meglumine on Large Colon Mural Thickness of Health Horses. Journal of Equine Veterinary Science 2023; DOI: 10.1016/jevs.2023.104562].
The study used 12 healthy adult horses aged 6 to 22 years. Each was administered flunixin meglumine for five days and then allowed a six-month washout period followed by five days of administration with firocoxib. Treatment with NSAIDs and omeprazole was administered for five days in each protocol:
- Flunixin meglumine 1.1 mg/kg IV every 12 hours
- Firocoxib 0.34 mg/kg PO, followed by 0.1 mg/kg PO every 24 hours before morning hay
- Omeprazole 1 mg/kg PO every 24 hours given to all horses to mitigate gastric ulceration
Of note is that flunixin was given IV and firocoxib orally and is related to the fact that the IV firocoxib formulation was no longer available following the six-month washout period.
Serum chemistry profiles and complete blood counts were analyzed prior to and following each treatment course. Similarly, transabdominal ultrasonography was performed prior to and following each treatment course. Colonic edema was scored on scale of 0 to 2, with 2 being severe edema; colon wall thickness was also measured at its thickest point.
At the beginning of treatment, median colon wall thickness and appearance were within normal limits: 3.6 mm before firocoxib and 3.1 mm prior to flunixin treatment. After treatment, median maximum wall thickness for firocoxib was 5.8 mm and for flunixin 3.0 mm. Colonic edema was also considerably more present in 11 of 12 firocoxib horses with a score greater than zero compared to one of 12 flunixin horses.
Most hematological parameters remained within normal reference ranges for all horses. No horses experienced clinical colitis signs. However, firocoxib elicited a significant increase in colon wall thickness, which did not occur with intravenous flunixin meglumine. The low dose (1 mg/kg) of omeprazole did not elicit intestinal complications in this study as reported in a study of increased risk of intestinal complications when 4 mg/kg omeprazole was administered with phenylbutazone.
The authors concluded: “The significant increase in colon wall thickness following treatment with oral firocoxib suggests that COX-2 selective NSAIDs may carry a risk of subclinical colitis in healthy horses.” The authors also noted that this study does not suggest steadfast conclusions but simply that “data was collected opportunistically as part of separate studies, during the course of which the development of large colon thickening was noted and considered to be an interesting finding deserving of further investigation.”