In any dog, including Chihuahuas, the most common symptom associated with allergies is itching of the skin, either localized (in one area) or generalized (all over the body). In some cases, the symptoms involve the respiratory system, with coughing, sneezing, and/or wheezing. Sometimes, there may be runny discharge from the eyes or nose. 

Unfortunately, allergies are quite common in dogs of all breeds and backgrounds. Most allergies appear after the pet is six months of age, with the majority of affected dogs over age one or two.

Chihuahuas can have seasonal allergies (usually spring & fall), food allergies, and environmental allergies. That is why it is so difficult to treat allergies; you first have to figure out which type of allergy your Chihuahua has.