I have never heard of that. Please PM me your phone number. I have some ideas.
z/d is a predigest protein. For lack of better description, it tricks the body - so the system doesn't recognize it as a protein, therefore, very unlikely he's allergic to it.
Here is an email from the HEad of nutrition @ UPENN about the food options for dogs like ours and the differences.
"I have sent a detailed summary of our conversation to Dr. Longo and let her
know my thoughts about potentially trying the Royal Canin HP and
Nestle-Purina HA diets as an alternative to the z/d ultra (I have
summarized some of the nutritional information about these diets below
for you as well).
1. HP seems to be the most palatable but it is the highest in
fat, 328
kcal/cup, 41% kcal from fat
2. HA has the least fat and 25% of the fat is MCT oil, 311
kcal/cup, 24%
kcal from fat
3. z/d ultra has been know to cause loose stools in dogs
with no known GI disease and our dermatologists worry that some chicken
allergic dogs react to it, also it is the lowest kcal food but not the lowest
in fat, 254 kcal/cup, 30% kcal from fat"
I will call you Friday. Don't dispair. We will get this. Been there. I know how scary, frustrating and angry you must be feeling. Totally helpless. Please, let me try and help.