My dogs started getting sick last year. I actually had to put down my old one. My Vet said it was a biological bacteria. It wiped out the good bacteria in their stomachs. Found this out when, second do got sick. Gave him a shot and said I should see improvement within a few hours, if not we were to late. Thankfully he got better. This spring my new pup and dog and cat got it. Again I asked my Vet. He said just something that happens some never get it some get once and others continually. With out expensive testing I don't know what's doing it. I have a clean yard but live in the trees with,lots if wildlife. Also a lot of feral cats and neighbors dogs loose, who knows if the have any shots.