Dr. Callan is a professor at Colorado State University in the Department of Clinical Sciences, service coordinator for the Livestock Medicine and Surgery Service, and Chief of Staff for the Large Animal Hospital at the CSU James L. Voss Veterinary Teaching Hospital. He obtained his DVM degree at Oregon State University in 1986 and then obtained a MS in reproductive physiology at Utah State University in 1988. Dr. Callan completed a residency in large animal internal medicine at the University of Wisconsin–Madison where he also obtained a PhD in virology in 1996. He has been a faculty member at CSU since 1996 and provides clinical care for ruminant and camelid livestock at the CSU Veterinary Teaching Hospital. Dr. Callan is a member of the American College of Veterinary Internal Medicine. His research interests focus on infectious diseases and immunity in livestock.