Friend, certainly the soft stool could be related to the diet change. However, your dog has a history of chronic loose stool. It is important to remember that there are parasites that are common and could play a role in chronic loose stool. For this reason, it is always a good idea to rule out parasites like coccidia, giardia, and whipworms by deworming as these parasites are often negative on routine fecal analysis. And, dewormer like Strongid which is commonly used does not treat them. I would strongly recommend you discuss with your vet about ruling out these parasites. Or, you can treat with Panacur (fenbendazole) given once a day for 3 days, repeat after 3 weeks. Panacur will treat against Giardia and Whipworms. It is a very safe dewormer, it will not hurt your dog. To treat against coccidia you will need a prescription from your vet of Albon. Panacur can be bought online (amazon) and some pet stores. It also goes by the name of Safeguard. In the meantime, this is what you can do:
1- Pepto Bismol 1/2 tablespoon every 8 hours for 1-2 days.
2- No food or water for 8-12 hours, need to give his gastrointestinal tract time to rest. Then, start him on again on the food that he did not have any diarrhea. Or, you can start him on rice (75%) and boiled boneless chicken or ground beef (25%) until the stool is back to normal then wean him back into the regular diet over a period of 1 week.
3- Continue on the probiotic.
4- If after doing the above you do not see improvement within 24 hours I would have him checked by your family veterinarian for a hands on examination.
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