Serum biochemistry changes in horses racing a multiday endurance event

Endurance competition frequently provokes serum biochemistry alterations in horses racing single day events. This study assessed the effect of consecutive days of endurance racing on commonly analysed serum biochemistry variables of horses. Blood was obtained once before 54 horses commenced racing 40 km or 80 km/day, and 4-6 h after horses finished each day of racing. Data were analysed via repeated measures ANOVA (P<0.05, mean ± standard error). Ten horses completed 40 km once, and 44, 18 and 9 horses completed one, two and three consecutive 80 km days, respectively, with valid results obtained for 41 of the 44 horses. Before racing, all variables were within reference intervals in both groups. After one day of racing, serum urea nitrogen and magnesium were higher in horses racing 80 km compared to 40 km, and total CO2 was lower. In both groups, total protein and globulin decreased after racing, and creatine kinase increased, exceeding the reference interval in the 80 km group on day one and two of racing. Within the 80 km group, serum urea nitrogen, creatinine, phosphorus and bilirubin were slightly higher on all race days than before racing, and serum glucose exceeded reference interval on day one of racing. Serum albumin was slightly lower on day two and three of racing, and aspartate transaminase was higher and exceeded reference interval on all race days. No significant changes occurred in sodium, potassium, chloride, calcium or magnesium concentrations in either category. Subset analysis of valid results from six horses that raced three consecutive 80 km days revealed similar changes in serum urea nitrogen, phosphorus, total protein, albumin, globulin, and muscle enzymes with racing. This study identified mild serum biochemistry changes in horses racing 80 km/day for up to three consecutive days, suggesting that non-elite multiday endurance competition is well tolerated.