Dr. Nielsen graduated with his DVM degree from the Royal Veterinary and Agricultural University, Denmark in 2001. He spent three years in equine veterinary practice before joining graduate school. He received his Ph.D. in equine parasitology at University of Copenhagen in 2007, and served as assistant professor there until 2011.

He then joined the M.H. Gluck Equine Research Center at University of Kentucky where he now serves as associate professor and holds the title of Schlaikjer Professor in Equine Infectious Disease.