An 8-yeаr-оld, intаct femаle, American Pit Bull Terrier presents tо yоu because the owner noticed vaginal discharge. The bitch is now lethargic and has been polyuric and polydipsic for over a week. The owner attempted to breed the bitch with another of his dogs during her most recent estrus which occurred 50 days ago. A pregnancy test performed today indicates she is not pregnant. Which of the etiologic agents and underlying pathology is responsible for this condition?