Dr. Nelson has been in private veterinary practice for nearly four decades, and he has served as an executive board member of the American Heartworm Society (AHS) since 2001. He was elected AHS president in 2004. He is the co-author of the AHS guidelines for the prevention, diagnosis, and management of heartworm infection. He has also served on the board of Companion Animal Parasite Council. Dr. Nelson has written or co-written several papers and contributed to several textbooks on the subject of heartworm disease. He is considered a pioneer for his clinical work in the study of heartworm disease in cats.