Meet Our Veterinarians
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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 clinical medicine in horses, cattle and small ruminants. 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.
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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.
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.
Sydney Weil, BVetMed (Hons) MRCVS
Dr. Sydney Weil is a Sonoma County native, having grown up in Santa Rosa. She started riding horses at a young age and competed in dressage as a teenager. She also has a deep love of wild animals and spent many Saturday nights working with sick and injured seals and sea lions at The Marine Mammal Center. She received her Bachelor's degree in Biology from Amherst College, where she was also captain of the IHSA team.
She then went on to vet school at the Royal Veterinary College in London, graduating with honors. After vet school, Sydney completed a year-long internship with BW Furlong & Associates in New Jersey, where she expanded her skills at a busy referral hospital and ambulatory practice. Her professional interests include ambulatory/preventative care, internal medicine, and reproduction.
Outside of work, Sydney enjoys traveling to new remote destinations, reading, and baking. She is very excited to return to her roots and care for the Sonoma County large animal community!

Meet our Client Service Representatives

Julie
Julie has been a part of the Sonoma Marin Vet team since January 2005. She was born in Petaluma and grew up loving animals, country life, and nature. Julie spent a lot of time on her grandparent's chicken ranch as a young child. She now has her own little mini-ranch with her 4 dogs. Her two favorite past times are photography and taking her dogs for long walks.
Marisa
Marisa has been a part of Sonoma Marin Veterinary Service since 2016. She grew up in Marin County riding hunter jumpers and spent all her free time at the barn. When not at work, she enjoys hiking and being outdoors with her bully dogs and cuddling with her cats.
Morgan
Morgan has been a part of the Sonoma Marin Vet team since September of 2022 after moving to Sonoma County in 2020. She has been involved in horse husbandry since she was young and horses have been a part of her life ever since. When she is not at the office, she loves to trail ride her horse, Wiz, and spend as much time outdoors or at concerts as possible. Additionally, Morgan is attending Sonoma State University where she is majoring in Psychology. 
Lizzie
Lizzie has been working in veterinary medicine since June of 2015. She worked at Sonoma Marin Vet and then in various small animal hospitals before reuniting with Sonoma Marin full time in April of 2024. Growing up, Lizzie participated in both 4-H and FFA. When she is not at work, you can find Lizzie baking new recipes, taking photos, plant shopping for the office or kayaking with her boyfriend.