Flea bite dermatitis is caused by a hypersensitivity to the saliva of the common flea. Dogs that are allergic to the flea's saliva have incredibly severe reactions to the flea bites and can actually bite at their skin or scratch until they have open lesions and infections of the skin. Hair loss is also common when the dog becomes obsessed with licking and scratching at this incredibly itchy spots on their skin.
Flea bite dermatitis is typically most noted in the spring and fall when flea activity is at its peak. During these months even dogs that are treated with monthly flea applications will still be bitten by adult fleas, leading to the reaction. Unlike some allergens the 15 different possible compounds in the fleas saliva that a dog can react to do not seem to decrease in effect with exposure. Dogs that are sensitive will remain just as sensitive or become worse, not better, as they age.
Fleas tend to congregate in certain areas on the dog and these areas will be target of licking, scratching and biting. The worst area tends to be at the base of the tail, however around a collar, behind the ears and even under the arms are all prime locations for fleas to feast. Remember that the allergic reaction will spread outward from the original bite area so very sensitive dogs may seem to be scratching obsessively all over their bodies. Often due to the scratching the dog starts to lose hair in the worst areas and may also develop hot spots or moist, infected areas of skin. These secondary infections can become very serious and even result in permanent skin problems and hair loss if not treated.
Dog owners that routinely bath and flea treat their dogs are often puzzled by flea bite dermatitis as there are really no adult fleas to be seen on the dog's body. Remember that topical applications only work after the flea has bitten the dog, so at least one bite has to happen to kill the adult flea. Keeping the whole environment as free from fleas as possible is an important consideration, so treat the rugs, furniture and even the yard outside if at all possible. During the worse part of the year for fleas you may decide to limit your dog's access to environments that you know are not treated, instead provide lots of activities, exercise options and games inside your treated yard and house.
When using multiple types of flea repellants and treatments always consult with your vet. Some combinations of different types of chemicals and treatments can actually irritate your dog's skin, worsening the problem and making treatment much more difficult. In some cases where the dog is highly sensitive the vet may prescribe an antihistamine shot for long term relief from itching plus they may also prescribe steroids to immediately provide relief. Antibiotics will be used to treat any skin lesions or injuries from scratching and hot spots will be treated by clipping the area and using a drying agent such as hydrogen peroxide on a twice daily basis.