Welcome! I am Dr. Altman, a licensed veterinarian and happy to answer your questions today.
Are they only eating canned food
at this time? Have you considered doing a homemade diet or are you looking for an alternative canned diet only?
The concern with going from the prescription diet to the over the counter formula is two fold. The prescription foods are handled in a certain way to minimize other proteins from being in the food and between batches the facilities are sterilized to reduce risk of cross contamination. In addition, there was a recent study that found that 80% of over the counter foods contained a protein source not named on the bag so if there is an allergy to the chicken there is risk that there could potentially be chicken in the formula.
With a chicken allergy typically I will recommend avoiding any winged animals, so no duck, turkey nor chicken as a precaution. Since you are doing well with bison you can try a limited ingredient over the counter bison formula or another hooved animal as an alternative. Below are a few examples by a company I trust:
If you are interested in cooking at home, you want to ensure it is a balanced diet. The book I refer to and used to use for my dog with food sensitivities is Dr. Pitcairn's Complete Guide for Natural Health to Dogs & Cats
Another option is to feed a home made diet in combination with the royal canin food that you are currently feeding
so that you need less of the canned in the meal on a regular basis. For example, I mix in the more expensive canned variety into my dog's food and they are perfectly happy eating the other dry formula that is not as pricey with it. But instead using the homemade with the canned in combination.
There are other companies that do make bison canned foods but unfortunately the ones that are listed were also found in that study to have other protein ingredients when tested so I would avoid those.
I hope this information is helpful but please let me know if I can be of any further assistance to you all.
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