The horse is 12 years old. We used invermicin,. The vet put on a long plastic glove and ran his arm up his rectum, pulling back a sample. He called us the next day. He is the only vet around here. I do not trust him, that is why I'm looking for other help. The oats change didnt go for weeks, more like 1 and a 1/2 weeks. But he had been getting better, poop was like cow patties before he started eating grass. Now nothing is helping him get solid. The vet also did a blood sample, found nothing.