1 Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 112-114, NL-3584CM Utrecht, The Netherlands. Electronic address: R.vanWeeren@uu.nl.
2 Department of Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 112-114, NL-3584CM Utrecht, The Netherlands. Electronic address: W.Back@uu.nl.