Pruritus in the cat
Andrea Cecilia Wolberg DVM and Alejandro Blanco DVM
Pruritus in cats is a real diagnostic challenge. In keeping with their behavioural traits, cats tend to scratch and/or to lick themselves when they are not seen by their owners, making it difficult to determine with certainty whether they have been scratching or licking themselves or not. It is also difficult to differentiate between intense grooming habits and excessive licking caused by itching. To complicate matters a little further, skin reactions in cats may be the result of affective states. In short, skin disorders can have a myriad of causes. The aim of this article is to present a diagnostic approach designed to help clinicians establish a definitive or near-definitive diagnosis of pruritus in cats and determine the most appropriate treatment for their patients.
Skin reaction patterns associated with pruritus in cats
There are 4 classical skin reaction patterns in pruritic cats (Table 1):
1. Feline eosinophilic granuloma complex
1. Feline eosinophilic granuloma complex
Feline eosinophilic granuloma complex is manifested in 3 ways:
The above manifestations are grouped under the heading of eosinophilic granuloma complex as they share certain characteristics, including a positive response to corticosteroid treatment and unknown aetiology (even though they tend to be associated with hypersensitivity reactions). Not all of them, however, are pruritic. Pruritus is a cardinal feature of eosinophilic plaques and mosquito bite hypersensitivity. Eosinophilic plaques are characterized by single or multiple granulomatous lesions that are red, round or oval, and often ulcerated. They occur on the skin surface of the abdomen, the inner thighs, the axillae, and the interdigital spaces. Affected areas tend to be wet due to intense licking as the lesions are highly pruritic. Differential diagnosis should include neoplastic skin diseases and granulomas of infectious origin (Figure 1). Histological findings are diagnostic (eosinophilic infiltrates in tissue). Mosquito bite hypersensitivity is characterized by crusted papules and varying degrees of swelling on the bridge of the nose, pinnae, and preauricular areas. Itching is moderate to severe, and symptoms disappear once the mosquitoes are successfully eliminated from the cat’s environment (Figure 2). An allergy study should be the first test performed to determine the aetiology of the lesions. The most common allergies in cats are flea allergy dermatitis, food allergy, and feline atopy.
Flea allergy dermatitis
Flea allergy dermatitis is a hypersensitivity reaction induced by antigens present in the saliva of the flea Ctenocephalides felis. The condition is classified as type 1 and type 4 hypersensitivity, although hypersensitivity reactions have not yet been fully defined in cats (1). Flea allergy can occur in cats of any age, sex, or breed (2,3) and the resulting skin lesions are variable. It is associated with moderate to severe pruritus. Diagnosis is made by association of clinical signs, the presence of either fleas or flea faeces on the cat’s body (although these can sometimes be difficult to spot as the itching sensation induces excessive grooming), and the disappearance of symptoms once flea control has been achieved. Response to corticosteroid therapy is good (Figure 3).
Food allergy is caused by an immunological reaction to certain proteins contained in food. The most important types of hypersensitivity reactions involved are type 1, type 3, and type 4 reactions. It is the second most common allergy in cats and can occur in animals of any age or sex, although Siamese and Burmese cats exhibit a certain predilection (4). The main clinical sign is mild to severe itching and in certain cases, dermatologic and gastroenterological reactions (diarrhoea and vomiting) may occur. It can present as any of the forms grouped under the heading eosinophilic granuloma complex, as pruritus of the face, head, and neck, self-induced symmetrical alopecia, or miliary dermatitis (Figure 4). Lesions appear as a result of scratching and, unlike in dogs, bacterial or yeast infections are rare. It is estimated that 20% to 30% of all food allergies in cats are associated with atopy or flea allergy. Response to treatment with corticosteroids is variable (5). Diagnosis is confirmed through food allergy testing, which consists of eliminating all the foodstuffs that the patient is known to have eaten in its lifetime and prescribing an elimination diet (home-made or manufactured) that should be followed strictly for a minimum of 8 weeks and sometimes up to 10 weeks. The diet should consist of 2 ingredients (one protein source and one carbohydrate source) that the patient has never been exposed to.
A commercial hydrolyzed protein diet is a diagnostic option if a home-made diet is not possible. Although commercial hydrolyzed protein diets are not a first-choice diagnostic tool because they do not permit the detection of adverse reactions to food additives, they are still an excellent option for treatment (6) (Editor’s note: In reality, an adverse reaction to food additives is an extremely rare occurrence). If the pruritus has resolved by the end of the elimination diet, a challenge diet should be followed until the offending allergen is identified. Intradermal skin tests and in vitro allergy tests are not diagnostic for food allergies. Histological findings can point towards a hypersensitivity reaction but do not confirm adverse reactions to food. The pattern observed is a superficial or deep perivascular dermatitis with a predominance of eosinophils and mast cells (7). Mural folliculitis has been observed in some cases (8).
Feline atopy is a hereditary condition that is characterized by predisposition to hypersensitivity reactions following exposure to environmental allergens (9). The most common allergens that affect cats are house dust mites, particularly Dermatophagoides farinae (10) and, to a lesser extent, pollen, skin flakes and mould. The immunological mechanisms involved in the pathogenesis of feline atopy are Langerhanns cells, eosinophils, mast cells, CD4+ T lymphocytes, and several types of immunoglobulin (Ig)E antibodies (11). The disease can occur in cats aged between 6 months and 3 years, and the main sign is moderate to severe pruritus (Figure 5). It can cause any of the skin lesions described above as well as swelling of the lip and chin. Non dermatological signs such as rhinitis, cough, and dyspnoea (asthma) may also be observed in certain patients. Diagnostic allergy tests are not of value in cats, and intradermal skin tests are difficult to interpret due to the nature of cat skin and the weak positive reactions observed. Serology tests have not been studied in depth in cats, and their results do not correlate with those of intradermal tests. The effectiveness of hyposensitisation therapy, however, albeit low, is similar when based on intradermal and in vitro test results. Specific IgE levels do not vary from healthy cats to atopic cats (12). Atopy is essentially diagnosed on the basis of clinical findings (history, signs, response to corticosteroids). Because feline atopy often coexists with other allergies, it is necessary to rule out all allergies before establishing a definitive diagnosis.
Procedure to follow when a cat has pruritus and a form of eosinophilic granuloma complex
If eosinophilic plaque is confirmed by histological findings and the skin lesions respond well to treatment with corticosteroids, the next step is to control the different predisposing factors to hypersensitivity reactions (contaminants (bacteria), ecto- and endoparasites).
If the underlying cause is flea allergy dermatitis, the lesions will be eliminated permanently if a successful flea control program (that treats the patient, its environment, and any other animals living in this environment) is implemented. If the symptoms return, a food allergy study will need to be performed to confirm or rule out food hypersensitivity. Accordingly, a restrictive diet needs to be followed for 8 to 10 weeks, after which there are 3 possible scenarios:
• The itching and the lesions have disappeared, confirming that the cat has a food allergy. In this case, a challenge diet must be prescribed. This involves reintroducing the foodstuffs that formed part of the cat’s normal diet one by one every 2 weeks until the offending allergen is identified (pruritus will return) and eliminated from the cat’s diet.
2. Bilaterally symmetrical alopecia
Cats with symmetrical alopecia will experience hair loss on both flanks. There are generally no underlying lesions and the main sign is pruritus. Differential diagnosis in this case should include psychogenic alopecia (Figure 6), neurodermatitis and (idiopathic) feline symmetrical alopecia.
Procedure to follow when a cat has pruritus and symmetrical alopecia
Once the presence of fleas has been ruled out and the cat’s sanitation and feeding habits have been reviewed, the same procedure as described above for eosinophilic granuloma complex should be followed to check for flea allergy dermatitis, food allergy, or atopy. This is necessary because symmetrical alopecia tends to be caused by hyper-sensitivity reactions. A diagnosis of psychogenic alopecia or neurodermatitis can be confirmed on the basis of history and response to treatment pruritus will persist despite treatment with corticosteroids).
3. Head and neck pruritus
Head and neck pruritus can present with varying degrees of severity, ranging from mild alopecia and erythema to erosive, ulcerated, or crusted lesions on the forehead, preauricular areas, pinnae, head, and neck. Depending on the cause, it may be the primary lesion. Possible causes include:
Notoedric mange is a contagious disease that causes intense pruritus. It is caused by the mite Notoedres cati, which mainly affects cats. The primary lesion is a crusted papule on the head and neck (Figure 7) but lesions can spread to distant parts of the body such as the front or back toes due to contact through scratching. Because cats tend to curl up while sleeping, lesions can also affect the perineum. Secondary manifestations include alopecia, crusts, and excoriations of varying degrees of severity. Information on the cat’s habits and possible contact with other cats is relevant. A definitive diagnosis can be made when mites and/or eggs are observed in skin scrapings.
Otodectic mange is a contagious, pruritic condition caused by Otodectes cynotis. It is not specific to cats. It commonly affects animals from breeders or that live in a community. It manifests itself as otitis externa, and produces a dark waxy secretion whose colour ranges from amber to black. The primary skin lesion is a crusted papule, but due to its location (inside the external ear canal) and scratching by the cat, there is often hair loss on the ear, neck, and head. There may also be lesions on other parts of the body to which the mites have migrated. Diagnosis is confirmed when a mite is seen (by the naked eye or using a magnifying glass or a low-power microscope) in the exudate or skin scrapings (if the cat has developed lesions).
Feline demodicosis can be caused by Demodex cati or Demodex gatoi. Demodex cati, which lives in pilosebaceous structures, causes localized skin lesions that may affect the face (preauricular areas, periocular areas, and chin) and the neck. The lesions may, however, also be generalised, and involve the trunk and limbs in addition to the head and neck (Figure 8). The generalised form of the disease tends to be associated with serious systemic disease. Lesions include alopecia, erythema, scaling, and crusts. Pruritus intensity is variable. The clinical manifestations caused by Demodex gatoi, a short-tailed mite that lives on the surface of the skin, are similar to those seen in notoedric mange or hypersensitivity disorders with severe pruritus and secondary lesions (alopecia, crusting, and scaling). The most commonly affected areas are the head, neck, and elbows. Symmetrical alopecia may also be observed in certain cases. The condition is contagious. The observation of mites in skin scrapings confirms demodicosis due to either Demodex cati or Demodex gatoi.
The most common cause of dermatophytosis in cats is Microsporum canis (13). Infection occurs as a result of direct contact with an affected animal or its environment (bed, cage, comb, etc.). Dermatophyte spores can survive in the environment for many months. The condition is highly contagious and can infect humans. Clinical manifestations are variable and pleomorphic (14), and though not common, pruritus may exist. Lesions may be localized or diffuse and are characterised by varying degrees of alopecia, erythema, and scaling. In cats, the disease normally affects the face, head, neck, and limbs, although the trunk can also be involved, with symmetrical alopecia or an eruption of small crusted papules (military dermatitis) (Figure 9). A definitive diagnosis can be made when a positive fungal culture result is obtained or when the lesions resolve following specific therapy. Histological examination of the skin using special stains (periodic acid-Schiff (PAS), Gomori methenamine silver) is sometimes required to see the dermatophyte spores.
Procedure to follow when a cat has pruritus and a lesion pattern affecting the head and neck
Bearing in mind the possible causes of dermatophytosis, once a thorough history focusing on clinical signs, environmental factors, habits, food, etc. has been obtained, it is mandatory to search for mites in superficial and deep skin scrapings and for fleas and flea faeces on the cat’s body. When dermatophytosis is suspected, hair samples and skin flakings should be obtained for direct examination and fungal culture. If the results are positive, the affected animal, its environment, and any other animals living in this environment should be treated with specific antifungal agents. If the results are negative, a flea control and prevention program should be implemented and the symptoms treated. If they exist, secondary infections should also be treated and antipruritic drugs (corticosteroids or antihistamines) administered until the symptoms disappear. If the lesions clear but the condition returns, the diagnostic procedure for identifying allergies is indicated (see the section: “Procedure to follow when a cat has pruritus and a form of eosinophilic granuloma complex” earlier in this article).
4. Feline miliary dermatitis
Feline miliary dermatitis (Figure 10) is a skin reaction pattern that has a range of very different causes (Figure 11). It presents as an eruption of papulocrustous lesions on the animal’s back, lumbar area, the back of hind legs, and neck. Secondary disorders include alopecia, excoriations, and crusts whose severity varies with the intensity of pruritus (which depends on the underlying cause).
Causes of feline miliary dermatitis:
Cheyletiella blakei is the most common mite responsible for feline cheyletiellosis. It lives in the keratin layer of the epidermis and does not penetrate hair follicles. The course of disease is generally slower in cats than in dogs as a large number of infected flakes are eliminated by grooming. Once contracted, the disease tends to be highly pruritic, causing alopecia and heavy scaling. Some cats develop diffuse crusted papules on their back (miliary dermatitis). Diagnosis is based on identification of mites in superficial skin scrapings or microscopic examination of hairs and scales obtained using a flea comb. Acetate tape impressions may also be used to detect adult mites or eggs. The most reliable identification method appears to be flea combing although this can fail in 58% of cats (15).
Superficial feline pyoderma
Superficial pyoderma is rare in cats. It is generally secondary to pruritic disorders (flea allergy dermatitis, food allergy, feline atopy, notoedric mange), systemic diseases (feline immunodeficiency virus), or immunosuppressive therapy (cancer therapy, corticosteroids, etc.) (16).
The most frequently isolated bacteria are Staphylococcus intermedius, S. simulans, and S. aureus. Characteristic pyoderma lesions range from localized alopecia (with or without erythema) to papules, pustules, erosions, ulcers, and crusts. Diagnosis is based on cytological examination of bacterial lesions. Samples should be obtained using impression smears in the case of erosive lesions and acetate tape in the case of crusted papules.
Procedure to follow when a cat has pruritus and a lesion pattern consistent with feline military dermatitis
As mentioned above, feline miliary dermatitis is multifactorial. It is therefore necessary to obtain a thorough history that includes information on clinical and systemic signs, the animal’s environment, contact with other animals, diet (quality), previous treatments and responses, health status, and control of endoparasites and ectoparasites. As the most common cause of miliary dermatitis is flea allergy dermatitis, the first diagnostic steps should include checking for the presence of fleas and observing response to flea control measures. Skin scrapings should also be taken during the first visit to check for mites. If necessary, the animal’s diet should be corrected. With certain lesions, cytological examination should be performed to determine if there is superficial pyoderma as this requires specific antibiotic therapy (as we have already mentioned, pyoderma is uncommon in cats). If dermatophytosis is suspected, a complete battery of diagnostic tests (direct observation of hair and scales, Wood’s lamp examination, fungal culture) should be performed. This is important even if itching does not seem to be particularly problematic. It is also important to bear in mind that dermatophytosis is a highly contagious condition that can affect humans. If the condition returns and the pruritus persists, and all the above diseases have been ruled out, the diagnostic procedure for identifying allergies is indicated (see the section: “Procedure to follow when a cat has pruritus and a form of eosinophilic granuloma complex” earlier in this article).
This article was kindly provided by Royal Canin, makers of Hypoallergenic diet for dogs and cats. For the full range please visit www.RoyalCanin.co.uk or speak to your Veterinary Business Manager:
1. Lewis DT, Ginn PE, Kunkle GA. Clinical and histological evaluation of immediate and delayed flea antigen intradermal skin test and flea bite sites in normal and flea-allergic cats. Vet Derm 1999; 10: 29-37.
This article was previously published in 2011 and but is still considered clinically relevant.
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