Flea allergy dermatitis
On average, 20% of all cats worldwide are infested with fleas, and of those, 8% have flea allergy dermatitis (FAD) .
Veterinarians have historically believed that one flea is all that is necessary to produce flea allergy dermatitis, but newer adulticides, such as fipronil, imidacloprid, nitenpyram and selamectin, have had a positive clinical effect on cats with FAD, throwing in question the once perceived dogma of the single flea bite concept. Current data would indicate that the primary role of these insecticides in managing FAD is in rapidly reducing flea numbers and reducing flea feeding rather than preventing flea bites.
Flea allergy dermatitis is one of the most common causes of skin disease in cats that cause itching. It is a result of infestation by the cat flea parasite Ctenocephalides felis. The skin damage which occurs secondary to flea infestation is a result of self-trauma rather than the parasite.
It is also one of the major causes of feline miliary dermatitis. FAD is most prevalent in the summer, although in warm climates flea infestations may persist throughout the year. In north temperate regions, the close association of pets and their fleas with human dwellings creates conditions that permit a year-round problem. Temperature extremes and low humidity tend to inhibit flea development.
When feeding, fleas inject saliva that contains a variety of histamine-like compounds, enzymes, polypeptides, and amino acids that span a wide range of sizes (40-60 kD) and induce Type I, Type IV, and basophil hypersensitivity. Flea-naive cats exposed intermittently to flea bites develop either immediate (15 min) or delayed (24-48 hr) reactions, or both, and detectable levels of both circulating IgE and IgG antiflea antibodies. Cats exposed continuously to flea bites have low levels of these circulating antibodies and either do not develop skin reactions or develop them later and to a considerably reduced degree. This could indicate that immunologic tolerance may develop naturally when continuously exposed to flea bites.
The age of the cat at the time of first exposure to fleas does not appear, at this stage, to play a role in the development of allergic dermatitis in cats, and there appears no breed or sex predilection.
Clinical signs associated with FAD are variable and depend on frequency of flea exposure, duration of disease, presence of secondary or other concurrent skin disease, degree of hypersensitivity, and effects of previous or current treatment. Nonallergic animals may have few clinical signs other than occasional scratching due to annoyance of flea bites. Those that are allergic will typically have a dermatitis that is characterized by pruritus.
Alopecia and overgrooming are relatively common clinical signs in adult cats, but this infestation can progress to severe anemia and skin disease in kittens, necessitating emergency treatment or euthanasia.
Miliary dermatitis is typically found on the back, neck, and face. The miliary lesions are not actual flea bites but a manifestation of a systemic allergic reaction that leads to generalized pruritus and an eczematous rash. Pruritus may be severe, evidenced by repeated licking, scratching, and chewing. Cats with FAD can have alopecia, facial dermatitis, exfoliative dermatitis, and “racing stripe” or dorsal dermatitis.
A number of factors must be considered in the diagnosis of FAD, including history, clinical signs, presence of fleas or flea excrement, results of intradermal testing, and ruling out other causes of dermatological disease.
Most cases occur in the late summer, corresponding to the peak of flea populations. In these cases, history can be highly suggestive. Age of onset is also important because FAD does not ordinarily occur before 1 yr of age. Usually, diagnosis is made by visual observation of fleas on the infested pet. Demonstration to the owner of the presence of fleas or flea excrement is helpful. Slowly parting the hair against the normal lay often reveals flea excrement or the rapidly moving fleas. Flea excrement is reddish black, cylindrical, and pellet- or comma-shaped. Placed in water or on a damp paper towel and crushed, the excrement dissolves, producing a reddish brown color.
Extremely hypersensitive animals are likely to be virtually free of fleas due to excessive self-grooming. In these cases, it is usually difficult to find evidence of fleas, thus making it more difficult to convince owners of the problem. Use of a fine-toothed flea comb (32 teeth/in.) facilitates finding of fleas and their excrement. Examination of the pet’s bedding for eggs, larvae, and excrement is also useful.
Intradermal skin testing may be used to support a presumptive diagnosis of FAD. Positive immediate reactions are characterized by a wheal 3-5 mm larger in diameter than the negative control. Alternatively, a positive wheal measurement can be defined as a response that is at least equal to the halfway point between the size of positive and negative control reactions. Observations for an immediate reaction (15-20 min) and, if negative, a 24-hr delayed reaction are recommended. The delayed reaction may not occur as a discrete wheal but rather as a diffuse erythematous reaction. A positive reaction does not conclusively indicate that the clinical condition is FAD—it indicates only that the animal is allergic to the flea antigen, either from present or past exposure. The reliability of intradermal skin testing in cats to diagnose FAD has been variable.
Serologic testing of IgE directed against flea-specific salivary antigens can be used to aid in the diagnosis of FAD.
FAD must be differentiated from other causes of dermatologic disease. The presence of fleas or a positive reaction to an intradermal test does not rule out the presence of another dermatologic disease responsible for the clinical signs. Differential diagnoses include allergic inhalant dermatitis (atopy), food allergy dermatitis, Sarcoptic or demodectic mange, other ectoparasites, and bacterial folliculitis. In cats, other conditions that can result in miliary dermatitis include external parasites (Cheyletiella spp, Trombicula spp, Notoedres spp and pediculosis), dermatophytosis, drug hypersensitivity, food allergy dermatitis, atopy, bacterial folliculitis, and idiopathic miliary dermatitis.
Several currently available insecticides provide excellent elimination of established flea infestations on both dogs and cats; these include fipronil, imidacloprid, nitenpyram, selamectin, and pyrethroids. Evidence also suggests that ivermectins have a reductive effect on clinically-overt flea allergy dermatitis.
Orally administered nitenpyram will eliminate fleas within 3-4 hr, while the topically applied residual spot-on formulations containing fipronil, imidacloprid, or selamectin take 12-42 hr.
Topical application of residual insecticides and administration of topical, injectable, or oral IGR have become the preferred methods of eliminating flea infestations. Several of these new insecticides and IGR have been shown to be extremely effective in controlling fleas on pets living in infested premises. Field studies have shown that fipronil (with or without the addition of (S)-methoprene), imidacloprid, lufenuron (with pyrethroid spray or nitenpyram tablets), and selamectin may be effective in controlling flea infestations, without the need for premise treatment. Flea infestations can be eliminated via chronic use of topical and systemic approaches because most fleas are killed prior to and/or directly inhibited from reproducing.
Despite the efforts of pet owners, the total elimination of fleas may not be feasible in some situations or may not occur rapidly enough to control clinical signs of FAD. Supportive medical therapy must be instituted to control pruritus and secondary skin disease in hypersensitive animals. Systemic glucocorticoids are often needed to control inflammation and associated pruritus. Short-acting prednisone or prednisolone can be administered initially at a dosage of 0.5-1.0 mg/kg, sid, tapering the dosage and using alternate-day therapy until the lowest dose possible that still controls the pruritus is given. As soon as flea control is accomplished, the glucocorticoid can be discontinued. Anti-inflammatory therapy should never be used as a substitute for flea control.
Secondary bacterial skin infection can be associated with FAD. Systemic antibiotics are commonly used to control the pyoderma and thus reduce the associated inflammation and pruritus. Selection of an appropriate antibiotic should be based on bacterial cultures and results of antibiotic sensitivity tests.
Hyposensitization consists of administering allergens to a hypersensitive animal on a regular basis in an attempt to obtain a state of clinical nonreactivity to flea bites. The effectiveness of currently available whole flea extracts is controversial.
- Dr Eric Barchas, 2009
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