When the 61st Tevis Ride commences early on July 23, about 170 horse and rider teams will head down the historic Western States Trail across the rugged Sierra Nevada mountains in a challenging 100-mile journey. It will also allow equine researchers another opportunity to study elite endurance horses in a field setting.

Since 2012, and in addition to the required hands-on horse evaluations during the ride, researchers Greg Fellers, DVM; Langdon Fielding, DVM, Dipl. ACVECC; and Gary Magdesian, DVM, Dipl. ACVIM, ACVECC, ACVCP, have drawn and analyzed equine blood samples at Mile 36. They’ve been trying to uncover data that would objectively identify horses unlikely to finish the course within the allotted 24 hours.

Also since 2012, the researchers have reviewed the data after the ride and compared them with actual completion rates. They focused on potassium and chloride levels, which decrease as a horse sweats and serve as an indirect marker of hydration. In the first year, the team found that horses with specific potassium and chloride levels had a 25% event completion rate compared to a general completion rate of around 50%.

The blood-draw protocol continued in 2013 to gather more data, and in 2014 the decision was made to “red flag” horses whose blood test results indicated they could be at risk of not finishing. The researchers informed the riders and crew of the results and rechecked the horses before they left the one-hour hold at Mile 36. Unexpectedly, the team said, the 18 horses flagged at Mile 36 had an 80% completion rate