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Small Animal Section: No. 12ALLERGIC SKIN DISEASE IN CATS AND DOGSDiagnosis Hypersensitivities in pets have classically been divided into the four main groups of allergens : parasitic, atopic (inhalant), dietary intolerance (food allergy) and contact. In addition, hormonal, bacterial, drug and urticarial hypersensitivities may be encountered. The basis for diagnosis is a bedrock work-up – taking a careful history viz (a) the distribution of skin lesions (b) time of year the symptoms occur (c) the pet’s environment (d) searching for clues on a physical and microscopical examination, and weeks to months of trial by the owner. Histopathology gives additional information of an allergic reaction and should not be used as the sole diagnostic procedure. Biopsies should be taken early in the disease process. The initial stages of allergic dermatitis are more specific, but as the reaction becomes chronic, the histopathology is less specific with secondary changes predominating. It therefore becomes a final common pathway for different chronic and allergic skin conditions! Histopathology can guide the practitioner towards a diagnosis and, although it may take repeat biopsies over time, indicate the group of allergens to be on the lookout for. Pets can inherit the predisposition to develop allergies. The manifestation of clinical symptoms in an allergic patient requires a minimum level of exposure to allergens and is termed the allergic threshold which is unique to each patient. Management of the allergic patient depends therefore, on establishing and then eliminating at least some of the precipitating allergens. PARASITIC (flea, various species of mite, biting lice, biting flies, mosquito and internal parasites). Flea allergy presents on the back, usually from the tailbase forward, less frequently on the ventral abdomen, but, in cats often as crusting around the neck. Flea allergy differs markedly in severity from flea dermatitis. In an allergic patient, one flea bite may breach the threshold and cause an acute allergic reaction. Involvement of the ear pinnae, hocks and elbows indicates Sarcoptic mite hypersensitivity. House mite is inhaled and may cause symptoms as for atopic allergies. ATOPY (aeroallergens include pollens, house dust, house dust mite and mould spores). Paw licking, face rubbing, ocular discharge and reverse sneezing are some of the symptoms. If it is seasonal, search for pollinating plants and seasonal moulds. Non-seasonal itch may be house dust or house dust mites. Enzyme linked immunosorbant allergy (ELISA) testing and Radioallergosorbant (RAST) testing on serum samples assist in identifying some allergens. DIETARY INTOLERANCE can be to the tiniest food particle of any foodstuff. The symptoms are generally all over the body, but especially the head. Any molecule in the food can be a cause of dietary allergy. To test for dietary allergens, use ingredients to which the pet has not been previously exposed. Use a “homecook” diet and do a carefully monitored trial. Do not allow anything else to be fed for at least 3 months (no titbits). If pruritis ceases, introduce one new ingredient every 14 days. Since cats can wander, it is best to hospitalise / kennel them for the dietary trial. In CONTACT allergies, the parts affected will be those that make contact with the environment – often the glabrous areas (belly, paws, chin). Dogs can be restricted to firstly the inside of the house, or a specific area of the house (however, it is extremely difficult to restrict pets to one type of environment for a 6 week trial!). Cats especially need to be placed in a cattery or hospital and once the allergy has subsided, introduced to objects (e.g. carpets) one at a time to see if they produce any reaction. CHRONIC “COMPLEX” DERMATITIS : A number of allergens are usually involved in the chronic hypersensitive patient. Increases in one or more of these allergens can result in the allergic threshold being breached. This precipitates acute symptoms of allergic disease. Co-factors involved in the pathogenesis of chronic allergic dermatitis may include secondary bacterial and yeast infections. Identification of the allergens becomes more elusive in chronic complex inflammatory dermatitis. Dermatohistopathology plays an important role in identifying these co-factors. TREATMENT AVOIDANCE : Utilising the concepts of the allergic threshold and summation of effects is crucial to managing allergic patients. Removal of one or more of the inciting causes will often bring the patient below the allergic threshold and return it to the asymptomatic state. Removal of a single allergen such as flea eradication is a classic demonstration of this method of allergy management. Feathers (pillows, birds), dander (cats), house dust and house mite, newsprint (newspaper) and tobacco smoke are all examples of allergens which can be removed from the immediate environment of the patient. House dust and house mites can be reduced by damp mopping and vacuuming rooms, avoiding overstuffed bedding and covering bedding with impervious material. If pollens are incriminated, avoid fields, keep grass short and bring pets inside during the evening and early mornings when pollen counts are high. Moulds can be decreased by using dehumidifiers, avoiding damp areas (basements, barns) and keeping less plants in the house. IMMUNOTHERAPY : Avoidance is not always possible and hyposensitation has evolved as the mainstay therapy for atopic disease. Minute quantities of the allergens are injected over time to “desensitise” the patient. Successful hyposensitation involves the correct identification of allergens contributing to the hypersensitivity state. Hyposensitation will fail of the wrong allergen extracts are used. Injection of non-involved allergen extracts are also undesirable and blanket hyposensitation against, for example, all “indoor” allergens is not advised. In vitro testing of serum samples by laboratories in the USA is available to us here. Selection of allergen extracts should be based on trusted test results in conjunction with the history, clinical symptoms and the pets environment. For instance, if no one smokes, do not include tobacco extract. Concurrent low dose alternate day prednisolone administration, which may be necessary in severely allergic pets, has not been shown to decrease the success rate. Patient follow-up and owner compliance are critical to the successful outcome of hyposensitation. In general, atopic dogs require lifelong administration, and the older the pet, the less successful is the outcome. SYSTEMIC THERAPY : Corticosteroids are the most effective medications for the systemic therapy of allergic dermatitis. They are, however, the most likely to result in unwanted side-effects. Oral prednisolone or prednisone can be administered at 1mg/kg daily in the morning until the symptoms are controlled and then only on alternate days at a reducing dose. Controlling concurrent precipitating and secondary skin disease (acariosis, pyoderma, Malassezia dermatitis, flea infestation) are essential. The use of antihistamines either on their own or with glucocorticoids is recommended – if only to lower the dose of the glucocorticoids. H1 blockers such as chlorpheniramine, clemastine, hydroxyzine, diphenhydramine and trimeprazine have been found to allow for a reduce corticosteroid dosage and may be effective on their own. Dosages vary and chlorpheniramine and clemastine have been used at 0.5 – 1mg/kg twice to three times daily. TOPICAL THERAPY : Shampoos can alleviate dryness and pruritis, and assist in the control of of Malassezia and pyoderma. Shampoos containing urea and glycerin in a hypoallergenic base have a moisturising and anti-pruritic effect. Colloidal oatmeal has superior demulcent properties and is cleansing and soothing. Betadine and chlorhexidine containing shampoos are available for pyoderma. Shampoos containing chlorhexidine and ketoconasole are useful in the control of Malassezia dermatitis. ADJUNCTIVE THERAPY : Essential fatty acids (EFA), especially the omega-3 and omega-6 series, have been the subject of many studies. However, dosage and the optimal ratio between omega-3 and omega-6 fatty acids have not been finalised. The response of allergic pets to fatty acids varies on an individual basis and no supplementation exists which suits all patients. As is done for antihistamines, several formulations should be tried sequentially until one is found which has effect. A multi-disciplinary approach to diagnosing and treating allergic skin disease is necessary. Owner participation, regular follow-ups and minimising predisposing and secondary involvement can result in successful relief from allergies. Information supplied by : Dr Martin
Briggs FRCVS In conjunction with: VetPath: Veterinary Pathologists For further information contact: VetPath Veterinary Pathologists
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