Meet Our Veterinarians

Michael O'Connor, MS, DVM - Practice Owner
Mike O'Connor grew up outside of Pittsburgh, Pennsylvania. He attended the Pennsylvania State University, where he received his Bachelors and Masters degrees. Mike moved west and received his Doctorate of Veterinary Medicine degree from Washington State University in 1985. For the past 30 years, he has practiced in Sonoma and Marin counties and was instrumental in instituting modern dental equipment as a standard for the practice. Mike has a strong passion and commitment to sport horse medicine and dentistry.
Mike O'Connor grew up outside of Pittsburgh, Pennsylvania. He attended the Pennsylvania State University, where he received his Bachelors and Masters degrees. Mike moved west and received his Doctorate of Veterinary Medicine degree from Washington State University in 1985. For the past 30 years, he has practiced in Sonoma and Marin counties and was instrumental in instituting modern dental equipment as a standard for the practice. Mike has a strong passion and commitment to sport horse medicine and dentistry.

Nathan Keefer, MS, DVM- Practice Owner
Nathan Keefer has practiced veterinary medicine in Sonoma and Marin Counties since 2002. He was raised on his family’s dairy farm in Indiana and received his B.S. in biochemistry from Indiana University. After moving to the west coast he attended Oregon State University where he earned his M.S. in microbiology and his D.V.M. Nathan relishes the challenges of day to day practice, especially the variety of both common and rare emergency cases in horses, cattle, small ruminants, camelids and even occasionally swine. Nathan particularly enjoys practicing preventative medicine with a focus on dairy and beef herd health, bovine reproduction and strategies for transition into, and success in organic dairying. In his time away from veterinary medicine, he is devoted to spending time with his wife and 3 daughters, eating, cooking and enjoying the great outdoors.
Nathan Keefer has practiced veterinary medicine in Sonoma and Marin Counties since 2002. He was raised on his family’s dairy farm in Indiana and received his B.S. in biochemistry from Indiana University. After moving to the west coast he attended Oregon State University where he earned his M.S. in microbiology and his D.V.M. Nathan relishes the challenges of day to day practice, especially the variety of both common and rare emergency cases in horses, cattle, small ruminants, camelids and even occasionally swine. Nathan particularly enjoys practicing preventative medicine with a focus on dairy and beef herd health, bovine reproduction and strategies for transition into, and success in organic dairying. In his time away from veterinary medicine, he is devoted to spending time with his wife and 3 daughters, eating, cooking and enjoying the great outdoors.

Sean Hardcastle, DVM
Sean grew up in Bishop, Ca where he developed an interest in animal husbandry while raising pigs in 4-H and showing western horses. He completed all of his school at UC Davis and graduated with a Doctorate in Veterinary Medicine in 2014. The following year was spent as an intern at Steinbeck Country Equine Clinic. Sean joined SMVS in July 2015. His special interests include internal medicine, ophthalmology, and lameness. On his off time, Sean enjoys fly fishing, swimming, running, and a good cup of coffee.
Sean grew up in Bishop, Ca where he developed an interest in animal husbandry while raising pigs in 4-H and showing western horses. He completed all of his school at UC Davis and graduated with a Doctorate in Veterinary Medicine in 2014. The following year was spent as an intern at Steinbeck Country Equine Clinic. Sean joined SMVS in July 2015. His special interests include internal medicine, ophthalmology, and lameness. On his off time, Sean enjoys fly fishing, swimming, running, and a good cup of coffee.
Jill Thornton, DVM
Dr. Jill Thornton was born and raised in San Diego, California. She grew up riding dressage and earned her United States Dressage Federation Bronze and Silver Medals on her horse, Souvenir. Jill went on to attend both undergraduate and veterinary school at UC Davis, where she continued to be focused on equine sports medicine and rehabilitation. During her time in vet school, she worked with Dr. Sue Stover conducting research on the effects of synthetic footing arena on movements of dressage horses. Following vet school, Jill accepted a one-year internship position at Fairfield Equine Associates in Connecticut where she gained experience in advanced diagnostic imaging, lameness and the general care of performance horses. During her internship she also spent time in Wellington, FL with intensive exposure to sports medicine in the show circuit and FEI setting. After her internship, Jill and her beta fish Duke (RIP), migrated back to California to work at small sports medicine focused practice in Martinez, CA. Jill loves hiking, fly fishing, jigsaw puzzles, and watching movies in her spare time. |

Hannah Greene, M.A., D.V.M.
Hannah hails from the big skies of Western Montana. She grew up participating in 4-H and riding working cow horses. After moving around Canada and the U.S. to earn advanced training in Linguistics, she focused her training on veterinary medicine, earning her D.V.M. from Washington State University. She enjoys practicing preventive medicine for both bovids and equids. Equine dentistry is a particular passion along with ophthalmology and acupuncture. In her time off, Hannah enjoys riding dressage with her Quarter Horse mare, working out and exploring California with her husband and their two dogs.
Hannah hails from the big skies of Western Montana. She grew up participating in 4-H and riding working cow horses. After moving around Canada and the U.S. to earn advanced training in Linguistics, she focused her training on veterinary medicine, earning her D.V.M. from Washington State University. She enjoys practicing preventive medicine for both bovids and equids. Equine dentistry is a particular passion along with ophthalmology and acupuncture. In her time off, Hannah enjoys riding dressage with her Quarter Horse mare, working out and exploring California with her husband and their two dogs.

Elena de Morelos, D.V.M.
Dr. Elena de Morelos grew up in Marin County, California. She took years of horseback riding lessons from a family friend and has been hooked on horses ever since. She went to Cal Poly, San Luis Obispo, and completed her undergraduate degree in Animal Science with a minor in Equine Science. In the year following college, she moved to Lexington, Kentucky, to work in the medicine department at Hagyard Equine Medical Center. She then went on to get her veterinary degree from the University of California, Davis, and worked as a student-technician in the Large Animal ICU. She completed an equine-specific veterinary internship at Pilchuck Veterinary Hospital in Washington state, where she explored her interests in internal medicine, lameness, anesthesia, and emergency care. Despite her incredible time in the Pacific Northwest, she missed her home in California and moved to Petaluma to be with family and enjoy the sunshine. Outside of work, Elena enjoys hiking, reading, the beach, and doting on her two cats Miley and Figaro.
Dr. Elena de Morelos grew up in Marin County, California. She took years of horseback riding lessons from a family friend and has been hooked on horses ever since. She went to Cal Poly, San Luis Obispo, and completed her undergraduate degree in Animal Science with a minor in Equine Science. In the year following college, she moved to Lexington, Kentucky, to work in the medicine department at Hagyard Equine Medical Center. She then went on to get her veterinary degree from the University of California, Davis, and worked as a student-technician in the Large Animal ICU. She completed an equine-specific veterinary internship at Pilchuck Veterinary Hospital in Washington state, where she explored her interests in internal medicine, lameness, anesthesia, and emergency care. Despite her incredible time in the Pacific Northwest, she missed her home in California and moved to Petaluma to be with family and enjoy the sunshine. Outside of work, Elena enjoys hiking, reading, the beach, and doting on her two cats Miley and Figaro.