Getting Under Canine Pyoderma's Skin

Vet Times The website for the veterinary profession https://www.vettimes.co.uk Getting under canine pyoderma’s skin Author : DAVID H SHEARER Categories : Vets Date : June 23, 2008 DAVID H SHEARER explains various procedures that can help to treat this common but persistent problem CANINE bacterial pyoderma is a common problem in small animal practice, most often caused by the facultative pathogen Staphylococcus intermedius. It is often secondary to some other concurrent and underlying skin disease that leads to microclimate changes that are either locally, or more generally, favourable to colonisation and infection by Staphylococci. Although an underlying disease may be identified, some dogs appear to have an idiopathic recurrent pyoderma that entirely responds to antibacterial therapy, but recurs at a variable time after cessation of treatment. Dermatologists do not agree on the exact pathogenesis of these idiopathic pyoderma cases. Aetiology It is generally accepted that S intermedius is resident on the surface of the mucous membranes (nares, oropharynx and anal ring) and the associated cutaneous margins and is, therefore, considered to be a facultative pathogen. Under some circumstances, the organism spreads from carriage sites to other areas, where it colonises the skin and adnexae. To do this, the organism must be able to adhere to corneocytes and then proliferate at these sites. The proliferation of the organism leads to the production of a 1 / 16 variety of extracellular and intracellular antigens. The extracellular antigens are produced by the living bacteria and may be involved in adhesion to keratinocytes and protection from the host immune system. These include toxins and enzymes, some of which can act as superantigens. They are cell components that are released when the bacteria die. The cellular antigens are known to produce a humoral immune response involving immunoglobulin G, but whether this is a protective response is not known. Super ant I gens can produce an inappropriate or aberrant immune response, and may be involved in the promotion of allergy or the development of autoimmune skin disease. In this way, recurrent pyoderma in an atopic dog may act as a significant promoter of the allergic response in the skin immune system. It has also been proposed that chronic exposure to Staphylococci antigens may promote the development of autoimmune skin disease, but this remains unproven. For these reasons alone, it is vital to treat or control any recurrent pyoderma in dogs. It is important to note that a staphylococcal pyoderma is considered to be a flare factor in atopic dogs, but there is suspicion that it may promote allergy and even contribute to the development of autoimmune disease. Clinical classification Pyoderma is usually clinically classified as surface, superficial and deep. However, it can also be classified as a localised (see Table 1) or generalised disease. • Surface pyoderma The most common example of a surface pyoderma is acute moist dermatitis, which is an ulcerative inflammatory process caused by trauma, typically following a flea bite. Acute moist dermatitis is a common superficial form of pyotraumatic dermatitis. The other form is a folliculitis, which is a deeper process. Intertrigo is a surface pyoderma occurring as the result of friction. • Superficial pyoderma Superficial pyodermas are defined as those affecting the infundibular portion of the hair follicle. - Impetigo Impetigo is a superficial pyoderma seen in young dogs and presents as a non-pruritic, non- follicular, pustular disease with collarettes, especially on the non-haired axillary and inguinal skin. Cases may spontaneously resolve, but should respond to topical antibacterial shampooing. 2 / 16 - Cutaneous bacterial overgrowth This is a syndrome that is presented as a pruritic dermatitis that responds completely to antibacterial therapy. It is characterised by the presence of large numbers of bacteria on examination of acetate strip preparations that are not necessarily within phagocytes. - Mucocutaneous pyoderma This is a relatively common localised pyoderma that affects mucous membranes and/or cutaneous junctions, such as the lips and nares. The main differential diagnoses for these lesions is cutaneous lupus (discoid lupus). However, diagnosis can prove difficult because the histological features are indistinguishable from one to the other (Wiemelt et al, 2004). Topical antibacterial therapy and systemic antibiotics should resolve mucocutaneous pyoderma, but recurrence will occur if predisposing factors remain (such as nasal discharges). - Bacterial folliculitis This is probably the most common manifestation of canine pyoderma. The lesions seen are papules, pustules, collarettes and alopecia. Underlying allergies, ectoparasitic or endocrine diseases should always be considered and appropriate diagnostic tests performed. - Superficial spreading pyoderma This is another form of superficial pyoderma, characterised by large collarettes with an erythematous expanding margin, most often affecting the trunk. These cases lack pustules. • Deep pyoderma A deep pyoderma is one that affects the entire hair follicle and is usually identified histologically as a furunculosis. Deep pyoderma is most often seen in interdigital pyogranulomas, lick granulomas and chin acne. An uncommon form of deep pyoderma is idiopathic German shepherd dog deep pyoderma, which produces ulcers and sinus tracts on the trunk, ventrum and thighs (Figure 1). This is most likely a multifactorial disease leading to deep bacteria pyoderma, but the exact aetiopathogenesis remains unproven. Working up cases 3 / 16 As with all dermatology cases, a thorough history is essential and, apart from a detailed examination of the skin, a general physical examination should be performed, and the lesions carefully documented. The type of lesions and their distribution will often indicate the likely type of pyoderma present and the differential diagnoses (see Table 2 for signs associated with pyoderma). • Diagnostic techniques - Cytology Because of the thin nature of the stratum corneum in the dog, compared to species such as humans, pustules rapidly rupture and form crusts. Hence, we rarely find intact pustules to sample in most cases of bacterial pyoderma in the dog. Cytological examination of impression smears of pustule contents (if you manage to find any) and the contents of sinus tracts or nodules and acetate strip preparations (as used to detect yeasts) are useful in the evaluation of the various forms of canine pyoderma. If you easily find numerous coccoid bacteria on microscopic examination in these samples, whether on squames or within neutrophils, then they should be considered potentially significant and appropriate antibacterial therapy prescribed. For most cases of canine pyoderma, cytological examination of pustules or acetate strips from the lesion surface should be part of the initial diagnostic investigation. - Biopsy Tissue biopsies may be collected for both histopathology and microbiology. Both techniques have their place in the investigation of pyoderma. In particular, deep pyodermas producing nodules and sinus tracts are best investigated using both cytology of sinus contents and biopsy for histopathology and microbiology. Histopathology can be used in an attempt to rule out neoplasia and identify the important differential diagnoses of dermatophytosis and demodicosis. Treatment Cases of canine pyoderma are treated by identifying and treating a concurrent or underlying cause (see Table 3), systemic antibiotics and topical antibacterial agents. The most important underlying diseases to consider in all cases of pyoderma are those formed from allergies and parasites. This is especially true in cases presenting with pruritus as a major sign, and where pruritus remains after antibacterial therapy has resolved crusts, scales and alopecia. Parasites and allergies should always be considered high on the list of differential diagnoses in 4 / 16 cases of pyoderma with pruritus (Table 4). • Antibiotics used in treatment of canine pyoderma There are numerous antibiotics available for the treatment of canine pyoderma and the reader should always check the data sheet for directions carefully. See Table 5 for the possible causes of antibacterial treatment failure. The antibiotics suggested for pyoderma at first presentation (first-line) are trimethoprimsulphonamide (15mg/kg q12hr to 30mg/kg q12hr), erythromycin (15mg/kg q8hr), lincomycin (20mg/kg q12hr to 30mg/kg q12hr) and clindamycin (5mg/kg q12hr to 10mg/kg q12hr). Antibiotics suggested for recurrent or deep pyodermas (second-line) are cephalosporins (for example, cephalexin 20mg/ kg q12hr to 30mg/kg q12hr), amoxicillin and clavulanic acid (20mg/kg q12hr to 25mg/ kg q12hr) or fluoroquinolones (enrofloxacin at 5mg/kg q24hr to 10mg/kg q24hr and marbofloxacin 2mg/kg q24hr to 5mg/kg q24hr). • Topical antibacterial preparations A number of shampoos are available for the treatment of pyoderma. The instructions on the bottles should be followed carefully, but treatment two to three times weekly is usually advised initially. In all cases of bacterial folliculitis, both systemic and topical antibacterial therapy should be used and continued for several weeks after clinical cure. Bacterial autogenous vaccines (bacterins) made from a sample of the patient’s own staphylococci have been used to treat recurrent bacterial pyoderma. The mechanism of action is not known. However, it is suspected to be via stimulation of a cell mediated

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