Simple SummaryEndurance riding is a physiologically highly demanding equestrian sport for riders and athletes. The sport has strict vet controls; depending on the distance, between three and seven vet checks are performed to ensure that only healthy horses remain in the competition. Longer distances and higher physiological demands for the horse increase the risk of elimination due to lameness or metabolic reasons. Our study focuses on identifying a blood parameter—the enzyme superoxide dismutase—in order to maybe implement a blood test in the future to withdraw horses at risk of lameness before equestrian endurance events. The study population was still relatively small; therefore, further studies are necessary to validate the findings. We also measured a new parameter (ALAD) for the first time, although the results were not convincing.AbstractHigh elimination rates and concerns for horse welfare are important issues in endurance riding. Improved understanding of the causes of elimination could increase completion rates in this sport. We have identified pre-ride laboratory risk factors that enable an assessment of potential elimination before the ride. A longitudinal cohort study was performed among 49 healthy horses competing in the 160 km endurance ride at the 2016 World Championship of Endurance Riding in Samorin/Slovakia. Blood samples were taken before the event. For statistical evaluation, horses were categorized into three groups: finishers, lame horses, and metabolically eliminated horses. Risk factors were calculated for each group using multinominal logistic regression. δ-Aminolevulinic-dehydratase (ALAD), thiobarbituric acid reactive substances (TBARSs), iron, and serum amyloid A (SAA) were measured and did not show an impact on the race outcome, but elevated pre-ride superoxide dismutase (SOD) was shown to have an effect on lameness elimination (p= 0.011). It might serve as an indicator for withdrawing horses at risk of later elimination before endurance rides, ultimately resulting in lower elimination rates and an increase in overall horse welfare.