On their farm nestled in upstate New York, veterinarian Austin McKenzie and his wife Madeline keep themselves busy looking after ailing farm animals and pets in poor health. But while they may be able to mend creatures great and small, they have yet to find a cure for murder.   Before milk inspector Melvin Staples was found dead in a 400-gallon milk vat, the ladies' man got the goat of many a Summersville resident. Possible suspects include his discus-champion, jealous wife; his paramours' vengeful husbands; and the ancient, cane-wielding owner of Tre Sorelle goat-cheese dairy. To crack this case, Dr. McKenzie and Madeline will have to learn all they can about goats, local politics, and how to get through to the crotchety Italian widow. And along the way, the vet might just save some bleating hearts.