Flea bite and flea allergic dermatitis is the most common cause of itching in dogs and cats. With the development of specific, safe, and highly effective flea control products, the incidence of both flea allergic dermatitis and flea bite dermatitis is decreasing. The difference between flea bite and flea allergic dermatitis is similar to, albeit on a less dramatic, the difference between a person who is not allergic to bee stings and a person who is allergic. Flea bites produced irritation in the non-allergic animal (or human being). The reaction is localized to the site of the bite. Flea allergic dermatitis, just like bee-sting allergy, involves the immune system, and can produce symptoms beyond the site of the actual bite.
Fleas require a blood meal in order to survive and reproduce. The lifecycle of the flea includes the classical stages of insect development: egg, larva, pupa, and adult. The egg, larva, and pupa occur in the environment – off the animal. Fleas can remain in a cocoon for extended periods of time before emerging to infest and feed on a host. The vibrations and carbon dioxide detected from a passing animal (or human being) can cause the flea to emerge. Once hatched, the adult flea must feed quickly or it will die. However, once fed, the lifecycle can rapidly complete and a large flea population can develop in a short period of time.
Flea bites look like small red bumps, or papules. They may or may not be that itchy. Most often, these papules can be found on the belly and around the base of the tail. The classical lesion of flea allergic dermatitis in the dog is known as the "hot spot," or pyotraumatic dermatitis. This lesion is a patch of skin that the dog can chew into a red, oozing, hairless sore within minutes of the flea bite. The overall pattern of rear-half of the body itching is typical of flea allergic dermatitis. Most of "hot spot" lesions can be treated with topical medications; injections and pills of corticosteroids (cortisone, steroids) are often not needed.
Lesions of flea allergic dermatitis in the cat are less specific. Cats can develop minute crusts or scabs on the body (miliary dermatitis), red and oozing, raised patches (eosinophilic plaques and granulomas), and even swollen lips. Some cats secretly groom all the hair off of the rear half of their body. Cats are particularly good at removing fleas; it is uncommon to actually find a flea on an allergic patient (and more likely to find them on the allergic patient's house companion who does not groom so vigorously).
Treatment of flea bite and flea allergic dermatitis relies upon good and consistent flea control. There is a plethora of good flea control products available at the veterinary office now. Many over the counter remedies are either not effective and/or potentially dangerous. The veterinary dermatologist can help guide you into the selection of the flea control products and treatment plan that is appropriate for your companion animal and your household.