by Judy Evans

Created on: December 14, 2010   Last Updated: April 21, 2012

Flea allergy dermatitis, often known as FAD, is the most common of a variety of allergic skin disorders that can affect dogs. It is estimated that half of all dogs are allergic to flea bites. Often the allergy is slight and not sufficient to cause the animal much discomfort. Cats are also affected by the disease.

A pet that has strong allergic reaction to flea bites can have a very miserable life. Only one flea bit is needed to send the animal into a vicious circle of excessive licking, scratching and biting with some being driven almost frantic by their symptoms. Such self-induced trauma can result in secondary infections.

There are two types of allergic response. Within 15 to 20 minutes of being bitten the cat or dog will show intense biting or scratching at the site.  Then, within the next 24 hours, a more generalised inflammation and itching will manifest over many areas of the body. Small bumps or papules burst to form crusty scabs.

FAD occurs when an animal has a hypersensitivity to the saliva of the flea. When a flea bites, its saliva passes into the skin. The saliva contains a number of different antigens which act to prevent coagulation of the blood. Following a flea bite the immune system of a susceptible dog over-reacts. This response results in severe inflammation and intense, unremitting itching.

While many animals have little or no reaction to flea bites, some react very badly. Dogs are reasonably adept to getting rid of their own fleas, however a dog with FAD, once bitten, is sure to come down with the severe itching which is so debilitating.

Other conditions can lead to similar symptoms and the only way to reliably diagnose FAD in a dog is by visiting a veterinarian.

Symptoms
The most immediate symptom is pruritus (itching), redness, irritation and swelling. The itching causes the dog to scratch, bite and lick excessively. This leads to:
*  alopecia (hair loss)
*  short, prickly patches of hair caused by rubbing
*  pimpled skin
*  crusted abrasions
*  ‘hot spots’ of red, painful sores. These circular sores are most common along the dorsal lumbar area and near the base of the tail but they can occur anywhere on the dog.

Cats may develop hairless patches along the back.

In temperate climates, symptoms are generally worse during summer and autumn when heat and humidity are highest. In milder regions, there may be signs of FAD throughout the year.

Over time, chronic changes may occur with the

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