There are also highly processed ("hydrolyzed") protein diets you can try your dog on, with your vet's approval.
These diets are pretty awful, though - only would use this as a last resort, if my dog truly were proven to be intolerant of all natural animal proteins. But I would prefer this option to going all veg, because there are amino acids in animal source proteins that cannot be found in plant sources, and these are vital for good health.
Usually when food allergies are suspected, your best course is a novel foods, elimination diet.
You would choose a novel animal protein. Something really far out like kangaroo or eel would be my preference to eliminate the chance that my dog's been exposed to it somewhere. You would also select a novel carb because grains and starchy veggies also have proteins that can be suspect in an allergy case. Again, green peas, sweet potatoes, tapioca - all these are excellent choices unless your dog has had them.
Then comes the important part. Your dog can't have anything but those two things, and micronutrients (vitamins and minerals), for two to three months. It takes that long to clear out residual allergens and histamines, though you may see almost immediate results if your dog's allergies are severe and definitely food-related.
If your dog has never had duck or potato, or venison and sweet potato, you can use the Natural Balance formulas to do an informal elimination diet. It won't be the real deal because there's flax seed in there, and there's also the chance of cross-contamination in the manufacturing plant. But it might give you some other meats to try besides the usual chicken, lamb, and beef that is commonly available.
I have a dog with multiple allergies, some of them pretty severe. He can't have chicken or eggs, but he can have duck and other poultry with no problem. He can't have cooked beef for a long time, but he can have raw beef with no problem - ditto venison. He is much, much less sensitive to beef and venison than chicken - I can even use a kibble with beef or venison for a few weeks, all else being equal.
He has immediate and severe reactions to corn, wheat, and rice. Barley, soy, and oatmeal not so bad, but still I'll see some itching and hot spots.
So food allergies are really complicated and frustrating, but it's worth it to go to the trouble of figuring it out and feeding the most nutritious diet possible while taking your dog's limitations into account.
Becca Shouse
Maggie and Zhi (queen bees), Ben, Gus, Cord, Rocky, Ted (BCs), Lu and Tully (Maremma flock guardians)
"Protect, Herd, Love" Irena Farm, Semora, NC
"We didn't have a phone, and we couldn't call you!" Helen, AIDS orphan. Don't let them die of ignorance - WorldHelp.net
"I-Dog Biggest Loser" Official Cheerleader/Mascot
Start 09/15/08: 186.4 lbs Current 185.5 lbs