There are 3 main types of allergies a dog can have.
A Chihuahua may have an external contact allergy. This means that the dog may be allergic to an element outside of his or her body such as cleaning chemicals on the carpeting that he lies on, a reaction to detergent that bedding or clothing has been washed in, lawn care chemicals on grass that touch the body and more.
The most frequent allergy is from chemicals or pollen that have settled onto grass. These are often tracked back into the house after a Chihuahua has been outside and reactions to these elements can continue long after the puppy or dog has been outdoors.
The second type of dog allergy is an internal allergy. This will mean that when a dog ingests something, such as a certain food or very rarely a medication, it will trigger an allergic reaction.
Third are inhaled. Most common is grass, weed and tree pollen. Though, with year round symptoms, one must also look to possible dust mites as the culprit.