Common Causes of Allergies in Dogs and Cats

http://vetmedicine.about.com/od/diseasesandconditions/a/CW-AllergyCauses.htm

Flea allergies are the most common type of allergy. A flea allergy occurs when a dog or cat becomes infested with fleas and the fleas begin to feed on your pet. As the flea feeds, it's saliva secretes a substance to which some animals develop an abnormal sensitivity. An allergy to the flea is the result.

For dogs and cats that are allergic to fleas, one flea bite can be enough to cause a reaction.
Many dogs and cats that are allergic to fleas groom themselves excessively and some dogs and cats are quite good at removing the fleas. It can sometimes be difficult to find evidence of fleas on your dog or cat even if your pet is suffering from the symptoms of a flea allergy.
The symptoms of flea allergies can be prevented by preventing your dog or cat from becoming infested with fleas.

Another great article from About.com in this series about skin disease in pets. And what comes up as the most common cause of allergies? Fleas of course! Just one bite can be enough to cause a reaction. Remember your flea control and help keep your pet allergy free.