A Switch in Time — Anne Barton

Dr. Erica Merrill, a young vet trying to establish her veterinary practice in her hometown, a small Northern Idaho county seat, gets involved in a murder investigation when a drug she has dispensed for a family dog is used to poison a wealthy widow. A battle over inheritance means that everyone involved with the household has a motive. But only the nine-year-old granddaughter has both means and opportunity. Neither Erica nor her deputy sheriff boyfriend, Clay Caldwell, want to believe that a child would murder her grandmother, but they wonder if her father could have persuaded her to do it. Erica must also contend with her father, who wants to run her life, a disgruntled colleague who reports her to the licensing authority, and a new physician in town with amorous intentions. It is Erica's knowledge not of the affect of the drug on the victim, but on the dog, that allows her to solve the mystery