Dog allergies to food
Just like with humans, allergens normally go hand in hand with other factors such as sinus and allergies and with what we come in contact with. Food allergies don't pinpoint the type of dog it wants to bother. These allergies can sometimes show up in a dog's life around 2 years of age and sometimes much later than that.
Some of the most common allergies of food for dogs have been found to be fish, wheat, soy, dairy, lamb, chicken, beef, and chicken eggs. This can be a surprise to many as there are a few ingredients found in foods that market it being sensitive for dogs that have allergies. Food allergy symptoms usually produce hair loss, biting on skin, scratching, and ear and skin infections.
Try to rule out the type of food your dog is eating by making them eat one thing for several weeks (choose one protein and one carbohydrate) and then figure out if the symptoms persist. Make sure that the dog is only eating what is specified as well as water up to about 12 weeks.
You want to make sure to rule out any dog allergy aggravators such as flea bit allergies or shampoos, etc. Once you rule out any aggravators you can begin testing the food that it eats. Pick one protein and one carbohydrate to test for the next 3 months with nothing but water and no treats. This is the only way, besides going to through expensive veterinarian testing. Once you narrow it down and are able to eliminate it, your dog will be much happier for it.