Chapter 45 Flea Allergy Dermatitis
Flea allergy dermatitis (FAD) is a hypersensitivity reaction to one or more components of fleas, especially allergens in flea saliva. Several types of hypersensitivities, such as cutaneous basophil hypersensitivity, immunoglobulin E (IgE)-mediated immediate hypersensitivity, late-onset IgE reactions, and delayed-type hypersensitivity, can occur alone or in combination. The hypersensitivity reactions cause inflammation leading to pruritus and the generation of most of the lesions.
ETIOLOGY
• Ctenocephalides felis is the species that usually infests both dogs and cats. Pulex irritans and less commonly Ctenocephalides canis may be responsible in some areas.
• Investigations with purified flea saliva led to the discovery of several antigens, and one named Cte f 1 has been cloned. Eighty percent of clinical flea-allergic dogs have an IgE response to Cte f 1, making this a major allergen involved in the pathogenesis. It is not the only allergen, and some flea-allergic dogs do not react to Cte f 1, nor do all have IgE-mediated disease.
Important Flea Life Stages
Pupal Stage
• This stage is most resistant to environmental changes, such as low humidity, and is also most resistant to environmentally applied insecticides, partly due to their location deep in the carpet. The pupa’s external sticky cocoon attracts a coating of environmental debris that also helps protect the developing adult inside the cocoon from insecticides.
• The pre-emerged adult flea inside the cocoon will wait for optimal environmental and local host factors before emerging. Emergence of the adult from the cocoon may be delayed up to 5 months and can be responsible for pet owners seeing young adult fleas even after environmental therapy. Delayed emergence and resurgence of fleas after effective use of environmental insecticides has been termed the pupal window.
Newly Emerged Adult Fleas
• The newly emerged adult flea will rapidly find its host in response to stimuli such as positive phototaxis, carbon dioxide, body heat, light changes from movement, and negative geotaxis.
• Even residual types of environmental insecticides and insect growth regulators (IGRs) are not rapid enough in killing fleas to prevent these newly emerged adults from finding their host. The time necessary for even effective topical or systemic therapies to eliminate all the developing stages from an enclosed environment has been termed the developmental window. As a result of these pupal and developmental windows, owners may see emerging young adult fleas and feel that the products are not beneficial, leading to poor compliance or premature termination of treatments that would eventually be effective.
Other Environmental Sources of Fleas
Other environmental sources for newly emerged or mature adult fleas must also be considered.
• C. felis commonly infest opossums, raccoons, skunks, coyotes, foxes, and some rodents. Areas outdoors frequented by these other hosts will keep flea populations present in locations that pets may frequent, exposing the pet to more fleas.
• In these circumstances, even the use of the newer topical or systemic flea-killing products may not prevent clinical allergy from occurring. As the products with the most rapid killing effect take 4 to 6 hours to kill newly emerged adult fleas, enough of these newly emerged fleas (or fleas from other environmental sources) may be present to keep significant disease occurring for days. Thus, active FAD may be present even though an infestation in the pet’s home environment may be prevented.
CLINICAL SIGNS
• Pruritus is the primary clinical sign, which the owner may observe as chewing (as if eating corn on the cob), rubbing, rolling, or scratching. Cats may groom excessively or pull out their hair. Severe chewing may lead to excessive wear of the incisor and canine teeth.
• Primary lesions are papules and erythematous macules in dogs and focal crusted erosions or papules (miliary eczema) in cats. Exudation and crusting may be seen.
• Secondary lesions result from the chronic inflammation and pruritus-induced trauma and include alopecia, excoriations, broken hairs, dry hair, scaling, hyperpigmentation, and lichenification.
Pattern of Involvement
• Involvement of the tail-base and dorsal lumbar region has been shown to be highly discriminant for the diagnosis of FAD. The caudal thighs, groin, and abdomen are frequently affected, although less severely than the dorsal lumbar region. In chronic and severe cases, there is extension of the lesions cranially on the trunk.
• Ear disease, otitis externa, perioral lesions, and pododermatitis (especially of the front paws) are usually not seen and are highly discriminant for a diagnosis other than FAD. When these signs are present in a dog suspected of FAD, they suggest either that flea allergy is not present or that flea allergy is present with a concurrent disease, such as atopic dermatitis or food adverse reaction.
Secondary Problems
• Acute moist dermatitis (hot spots), acral pruritic nodules, eosinophilic plaques, and eosinophilic granulomas may be seen in dogs and cats.
• Superficial or deep pyoderma may develop, especially in animals repeatedly treated with corticosteroids.
• Animals with superficial pyoderma will present with pustules, crusted papules, or circular spreading rings of crust over erythematous erosions, lichenified plaques, or papules.
DIAGNOSIS
History
• A history of otitis externa or paw licking suggests that other or concurrent allergies are present.
• Obtain information regarding the number and type of pets, housing, the type of floor covering, current pesticide use, and client concerns regarding the use of pesticides.
• Possible sources of exposure to fleas should also be investigated. A history of cats living in the same environment also has a positive correlation with a diagnosis of FAD. Investigate how much time is spent outside, the type of outdoor environments the pet has access to, and the possible presence of stray or feral cats as well as opossums and other wildlife.
• A history of prior favorable response to flea control also supports a diagnosis of FAD. Adverse food reaction is one of the few differential diagnoses for the lesions and pattern of involvement seen in FAD, and a gastrointestinal history (bowel movements, borborygmus, flatulence, etc.) may be helpful in this regard.
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