Flea Allergy Dermatitis

What are allergies, and how do they affect dogs?   
One of the most common conditions affecting dogs is allergy.  In the allergic state, the dog's immune system "overreacts" to foreign substances (allergens or antigens) to which it is exposed.  These overreactions are manifested in three ways.

The most common is itching of the skin, either localised (one area) or generalised (all over the dog).  

Another manifestation involves the respiratory system and may result in coughing, sneezing, and/or wheezing.  Sometimes, there may be an associated nasal or ocular (eye) discharge.  

The third manifestation involves the digestive system, resulting in vomiting or diarrhoea.  The specific response that occurs is related to the allergen and the individual animal's immune system.
What is meant by the term flea allergy?
In spite of common belief, a normal dog experiences only minor skin irritation in response to flea bites. Even in the presence of dozens of fleas, there will be very little itching.  On the other hand, the flea allergic dog has a severe, itch-producing reaction to flea bites.  This occurs because the dog develops an allergic response to the flea's saliva.  When the dog is bitten, flea saliva is deposited in the skin.  Just one bite causes intense itching and this is of a long lasting nature.
What does this reaction do to the dog?
The dog's response to the intense itching is to chew, lick or scratch.  This causes hair loss and can lead to open sores or scabs on the skin, allowing a secondary bacterial infection to begin.  The area most commonly involved is over the rump (just in front of the tail).  This is probably because fleas find this part of the dog more desirable.  Many flea-allergic dogs also chew or lick the hair off their legs.
What is the proper treatment?
The most important treatment for flea allergy is to get the dog away from all fleas.  Therefore strict flea control is mandatory and this involves ensuring the dog is flea-free and also removing fleas from the environment.  There are many products available for flea control, and many work in entirely different manners.  Some are used on the dog and some on the dog's environment.  
Flea Control
Complete flea control is invariably difficult, particularly where dogs living outdoors in summertime when the weather is warm and humid, where a new population of fleas can hatch out every seven days.

Some dogs can be desensitised to the adverse effects of flea bites.  Flea saliva extract (flea antigen) is injected into the dog in tiny amounts over a prolonged period of time.  This is an attempt to reprogram the dog's immune system so it no longer over-reacts to flea bites.

If successful, itching no longer occurs or is less intense when the dog is bitten.  However, this approach is only successful in less than 30% of cases.

When strict flea control is not possible, corticosteroids can be used to block the allergic reaction and give relief.  This is often a necessary part of dealing with a flea allergy.  Dogs are more resistant to the side-effects of steroids than humans, but significant side-effects can occur.  For this reason, the goal is to administer the smallest amount of steroid needed to keep the dog comfortable.

Some dogs develop a secondary bacterial infection in the skin.  When this occurs, appropriate antibiotics must be used and steroid therapy reduced even further.