How Inflamed Is the Horse in Training? Insights into Exercise-Induced Acute Phase Response in Endurance Horses

The article aimed to review the current literature analysing the complexity of an exercise-induced acute phase response in athletic horses undergoing intense training and endurance competitions. Since the endurance discipline demands physical fitness, exceptional health and excellent adaptation to an increasing workload, diagnostic methods of assessing the factors mentioned above are highly required. Athletic horses in endurance training undergo numerous metabolic but also immune adaptations, including changes in pro-inflammatory and anti-inflammatory cytokine levels. The inflammatory reaction resembles typical inflammation only to some extent; therefore, the term exercise-induced acute phase response (APR) has been introduced. Among many biomarkers, acute phase proteins (APPs), like serum amyloid A (SAA) and different types of cytokines, especially interleukin 1 (IL-1), interleukin 6 (IL-6), and tumour necrosis factor α (TNF-α), appear to play a key role. These markers may be modulated by many factors; however, proper training seems to result in the occurrence of an "anti-inflammatory state", which is beneficial for the horse's health and highly required in high-performance equine athletes. Further understanding of the inflammatory reaction associated with extreme physical effort is crucial for ensuring the long-term career and welfare of endurance horses.