BT with a super sensitive stomach - allergic to chicken or brown rice perhaps?

by Anthony
(New York, NY)

I just adopted a well-cared for 2 1/2 year old BT named Tucker from a local family. he was eating a small brand of food called Health Extension chicken and rice and the original owners gave me a new bag of the same food to take with him. for the first week, he was very finnicky come mealtime but figured it was just because nervousness/new home. after that first week, he started with vomiting and diarrhea. brought him to vet as i thought it was a virus, ran tests and found nothing. brought him home, fed him the same food, and was sick on and off for another day. then he refused to eat the next day, tried feeding him boiled chicken and rice, wouldn't eat it. the next day violently ill again, so i brought him to the vet, for the second time and again everything checked out okay. so they suggested feeding Hill's prescription diet until he was better. he's been eating that for the past 2 weeks now and is doing well. thought it was maybe the old brand of food. i just tried feeding him a new brand of food, Blue Buffalo chicken and brown rice, following recommended 25% new/75% old. i've fed him this twice and he's thrown up hours later both times.

the previous owners had been feeding him that food for months before i got him, why did he get sick now? did the change of environment trigger this reaction? is it possible Tucker is allergic to chicken in the food? the vets diet has chicken in it but doesn't seem to bother him. or is it possible he's allergic to the brown rice in the formula?

i'm considering feeding him a fish based dog food formula, probably natural balance sweet potato and fish formula. anyone have ideas or recommendations for Tucker? testing out new foods will get expensive and stressful, desperate for ideas!