5 Flea allergic dermatitis
In some parts of the world, flea allergic dermatitis (FAD) is the most common allergic disease and a major cause of pruritus in dogs and cats. In other parts it is a significant problem only at certain times of the year. Although allergic dermatitis is the main condition associated with fleas, a distinction between pruritus resulting from severe flea infestation and a hypersensitivity response should be made. In very young puppies and kittens, severe flea infestations provoke varying degree of pruritus, but more often patients exhibit signs of weakness, lethargy and anaemia. Fleas are also vectors of infectious organisms such as Bartonella, Rickettsia felis and Haemoplasma spp.
This varies between individuals but most pruritic dogs are presented with a history of pruritus and varying lesions affecting the lumbo-sacral region. As the flea life cycle is affected by environmental factors such as temperature and humidity, seasonal exacerbations may occur. Often flea control is only intermittently used and in-contact animals, especially cats, are inadequately treated. The history in this particular case was long and complex. The most relevant parts were:
A whole range of clinical signs, from primary lesions such as papules and pustules, to severe secondary hyperpigmentation, lichenification and fibropruritic nodules are seen, depending on the chronicity of the disease. Self-induced alopecia due to over-grooming and secondary bacterial infection is often seen in affected dogs. Atopic dogs are predisposed to flea bite hypersensitivity, even those that have been well managed. Some dogs will present with pyotraumatic dermatitis on the rump, or at other sites.
In this case, given the clinical signs and the distribution of the lesions, there was evidence for more than one type of hypersensitivity. The facial, pedal and ventral distribution suggested atopic dermatitis, and that affecting the dorsal aspect, flea allergy dermatitis. A concurrent adverse food reaction could also have been contributing to the pruritus. The other differentials included:
In addition to atopic dermatitis, the history, clinical signs and distribution of lesions were suggestive of flea allergic dermatitis. The diagnosis of flea allergy dermatitis is supported with additional tests and with response to aggressive flea control. The simplest test is the demonstration of fleas or flea faeces using a flea comb; however, about a third of animals fail to show any evidence of fleas, for various reasons: