Dr. Holtz has been an integral part of MAH since July 2010 and is experienced with treating both large and small animals. Dr. Joanna Holtz was born and raised in a large and supportive family in Southern California. From the time she was a young girl she had aspirations to become of veterinarian. Her first love was horses and she acquired her horse, Splinter, when she was 16 years old. After graduating from UC Davis in 2003 with a degree in Animal Science and Management, she entered veterinary school at Oregon State University. Initially, she had her sites set on large animal only medicine. However, after various experiences with multiple species in veterinary school, she elected to pursue mixed animal practice. After obtaining her Doctorate of Veterinary Medicine, she moved to the small town of Brookings, Oregon to practice in a large private mixed animal practice. Over the three years there she solidified her love of practicing medicine on a variety of species. Dr. Holtz was happy to return to her home state of California where she can spend more time with friends and family. 

Dr. Shackelton to our practice November 2014.  Dr. Dana Shackelton graduated from UC Davis Veterinary School where her track was small and large animals.  She then completed an internship for a year at Pioneer Equine Hospital in Oakdale, CA.  Growing up, her father worked for the National Park Service and his career took her to rural parts of Hawaii, California, and Alaska.  Her interest in veterinary medicine was sparked at a young age in Eagle River, Alaska where she dreamed of becoming a veterinarian on the Iditarod Trail.  During college she decided that she wanted to be in the back country of Yosemite National Park and pack mules full time.  However, when she graduated from college she realized that she wanted to continue learning and that her passion remained in veterinary medicine.  She moved from Mariposa, CA near Yosemite National Park to Hidden Valley Lake.  She shares her life with a rescue dog named Kiko and a mule named Josephine.  In her spare time Dana enjoys hiking, riding her mule, and hunting for the perfect mocha!  Dr. Shackelton's greatest asset is a positive attitude with exceptional attention to client and patient care.   We are thrilled to have her as part of our team at Middletown Animal Hospital.  

Dr. Knowles grew up in the suburbs of the SF Bay Area and currently lives a double life between Martinez and Sacramento - and now Middletown! Dr. Shackelton and Dr. Knowles were roommates in their undergraduate degrees at UC Davis and have been friends ever since. Dr. Knowles went to veterinary school in New Zealand and Massey University, graduating in 2012, and practiced exclusively in equine until 2017 when she jumped the fence and started doing cats and dogs at a practice in her hometown Martinez.

Dr. Knowles has an Australian Shepherd named Evie (Evelyn for when she's in trouble) who is 98% the perfect dog. She also enjoys CrossFit (or did enjoy CrossFit, thanks COVID!), yoga, camping and hiking(as long as it's not too cold), speaks French and almost became a winemaker instead of veterinarian. She does NOT enjoy headaches, traffic, IPA's or things that don't match. 

Dr. Penney is a Los Angeles native who has recently moved to the area. She majored in Animal Science at UC Davis before continuing on to complete veterinary school in Australia at University of Sydney. Outside of the hospital, Dr. Penney enjoys painting, crochet, and spending time with her Australian cat Maze.

Dr. Isabel Vaishampayan, earned dual bachelor's degrees in Pre-Veterinary Science and Public Health from the University of Massachusetts – Amherst, and graduated from UC Davis School of Veterinary Medicine in 2025. With a lifelong passion for animals, Dr. Vaishampayan was drawn to veterinary medicine by the challenge of understanding and helping those who can’t speak for themselves. She finds the greatest reward in watching her patients recover and return to their happy, healthy selves. Dr. Vaishampayan has a special interest in livestock medicine, where she enjoys the hands-on, hardworking nature of the field. Outside of the hospital, she enjoys running, cooking, baking, reading, and relaxing with her two senior dogs, Manfred and Tara, and her lizard, George. She also served as president of her school’s chapter of the Student American Veterinary Medical Association and looks forward to continuing to give back to the profession.