Allergies (also known as inhalant allergies or atopy) vary a great deal in both
their severity and age of onset. Among dogs, allergies can start as early
as 3 months and progress through to old age. Symptoms may be so mild at
first that they are not noticed but are usually progressive and become
clinically apparent before 3 years of age. Among humans, the nose is the
primary target of allergies - and hay fever symptoms predominate. Among
dogs and cats, the skin is the major target organ, so itching and scratching
are the main symptoms.