U.S. eventing team member Kim Severson had to withdraw from the Games Wednesday morning after her horse, Tipperary Liadhnan, fell ill.
"In recent days, the horse had an infection (cellulitis), which was improving daily," said Mark Phillips, the team's chef d'equipe, or manager. "It took a dramatic turn for the worse on Tuesday morning, which was 24 hours too late to make a substitution to the team.
"No one can determine why there was such a dramatic downturn," he said. "In the best interest of the horse's welfare, we made the difficult decision not to run him."
Phillips said the team veterinarian, Dr. Brendan Furlong, and local vets have been working with the horse.
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The late withdrawal means that none of the alternates can be named to the U.S. team, but one of the two individual riders can move up to give the U.S. four riders on the team.
Either Becky Holder, on Courageous Comet, or Karen O'Connor, on Mandiba, will join the U.S. team of Buck Davidson, riding BallyNoe Castle RM, Phillip Dutton, riding Woodburn, and Boyd Martin, riding Neville Bardos.
"The (team) selectors and the U.S. (Equestrian Federation) felt that having Tipperary Liadhnan on the squad was our very best chance to win a team medal and that was and still is our priority," Phillips said.
The fourth team member will be named this afternoon.