Superficial pyoderma is defined as a bacterial infection of the epidermis and of the superficial portion of the hair follicle. The main causative agent is Staphylococcus pseudintermedius, but other staphylococci such as S. schleiferi and other bacterial species may be involved.
AETIOLOGY, PATHOGENESIS AND EPIDEMIOLOGY
S. pseudintermedius is a coccoid coagulase positive bacterium belonging to the normal skin flora and is considered an opportunistic pathogen of the dog and cat. Experimental studies have shown the inability of staphylococci to cause disease in healthy skin. Any skin infection should therefore be considered as a sign of an underlying predisposing disorder.
The presenceof inflammation, obstruction or follicular degeneration are predisposing factors for bacterial infection. Allergic subjects are frequently affected by secondary bacterial infections in view of the damage to the skin barrier caused by pruritus-induced self-trauma as well as for the frequently used steroid treatments. In atopic dogs a greater adherence of S. pseudintermedius to keratinocytes has also been reported.
SIGNS AND SYMPTOMS
The clinical signs, as well as the severity of the pruritus, are highly variable and depending on the breed and type of hair coat various clinical presentations may be present. Superficial pyoderma is usually characterized by the presence of multifocal areas of alopecia (bacterial folliculitis, Fig. 1), papules and pustules, epidermal collarettes and widespread scabs on the trunk and on the animal’s ventral region (Figs. 2-9). In short-haired dog breeds, multifocal areas of alopecia that give the coat a "moth-eaten" appearance are the most frequent clinical manifestation. The diagnosis is based on the clinical signs and on the results of collateral tests.
DIAGNOSTIC TESTS
Microscopic hair analysis allows to detect the presence of keratin aggregates around the hairs and to exclude the presence of follicular parasites, such as mites of the genus Demodex and/or fungal hyphae or spores.
The cytologic examination of the sample harvested from pustules, from epidermal collarette margins and from the skin surface under the scabs allows to evaluate the presence of microorganisms, inflammatory cells or cells with abnormal characteristics (i.e. acantholytic keratinocytes). The cytologic pattern which is often present in the course of superficial pyoderma is characterized by neutrophilic inflammation with more or less degenerated neutrophil granulocytes and by intra- and extracellular coccoid bacteria; depending on the duration of the inflammatory process the presence of macrophages may also be encountered, while the finding of eosinophils may be suggestive of an underlying parasitic or allergic problem.
The bacteriological examination and the antibiotic sensitivity test are of fundamental importance for the selection of a correct antibiotic therapy, especially in chronic or relapsing cases or when there is a poor response to the ongoing treatment or when antibiotics have recently been administered to the patient.
The sample material to be sent to the laboratory should be taken from intact primary lesions, ideally pustules, and collected with a sterile needle (Tzanck Technique); the sample is then transferred to a sterile swab. Should this type of sampling technique not be possible, a bacteriological examination from the margin of an epidermal collarette or from the surface below a scab is indicated.
DIFFERENTIAL DIAGNOSES
Superficial bacterial folliculitis must be distinguished from other inflammatory conditions of the hair follicle. The main differential diagnoses are:
- Dermatophytosis, the exclusion or confirmation of which can be obtained by means of a culture test for dermatophytes, preceded by a Wood's lamp examination and by the microscopic examination of the hair.
- Demodicosis, the exclusion or confirmation of which can only derive from deep scrapings or from microscopic hair examination. In addition, demodicosis is a predisposing factor for bacterial infections of the hair follicle, which can therefore be concomitantly present.
Other less common conditions with pustules and scabs included among the differential diagnoses are sterile pustular diseases, such as pemphigus foliaceus, characterized by the presence of numerous acantholytic cells (the diagnosis of which is confirmed by biopsy and dermatopathologic examination) and sterile neutrophilic or eosinophilic pustulosis, characterized by the absence of bacteria on cytology and also confirmed by a sterile bacteriological examination.
TREATMENT
The treatment for superficial pyoderma may consist of local, systemic or a combination of the two therapies. Recently, in view of the increased number of resistant bacteria to one or more antibiotics both in human and in veterinary medicine, a more careful and targeted use of antimicrobial agents has become necessary. Staphylococci, in particular, have the ability to acquire resistance against β-lactam antibiotics, which are typically used as first line drugs for systemic therapy (methicillin-resistance).
For an appropriate therapeutic choice several factors are to be considered, such as the severity and extent of the lesions, the presence of concomitant diseases, the patient’s size, temperament and type of coat, as well as the owner’s dedication and desire to collaborate.
Topical therapy is considered useful in all patients with superficial pyoderma, as it allows the removal of microorganisms and debris from the skin surface. When used in conjunction with systemic therapy, it favours a quicker healing with a consequent reduction in the amount of antibiotics used.
Different formulations and types of products are available for topical application, such as shampoos, antiseptic sprays and lotions, antiseptic and antibiotic creams, gels and ointments. Topical treatment should be continued for a week after the complete clinical resolution; for products that require rinsing, the recommended skin contact time is of about 10 minutes. For the treatment of large areas, several studies have demonstrated the efficacy of antiseptic shampoos such as chlorhexidine, in combination or not with miconazole, and benzoyl peroxide applied with a frequency of 2-3 times a week. For small-size, focal lesions the use of antiseptic gels, wipes and creams is more indicated. The use of antibiotic creams with fusidic acid or mupirocin should be supported by the results of a bacteriological examination and antibiogram, as these molecules are used in human medicine for the treatment of infections caused by methicillin-resistant staphylococci.
Systemic antimicrobial therapy should be chosen on the basis of its effectiveness, availability, safety and cost, as well as to patient-related factors (prior adverse reactions, concomitant diseases, etc.) and should be administered at the doses indicated for the treatment of skin infections. The recommended administration times are of at least 7 days after the complete clinical resolution; the concomitant use of glucocorticoids is not recommended.
First-line systemic antibiotics for the treatment of superficial pyoderma in the dog are amoxicillin and clavulanic acid, first-generation cephalosporins and clindamycin (or lincomycin). Second-line molecules are reserved for those cases in which the use of first-line antibiotics is not possible or when indicated based on the bacteriological examination. These include third-generation cephalosporins, doxycycline, chloramphenicol, fluoroquinolones, rifampin and aminoglycosides. Third-line antimicrobials are also chosen based on the results of the bacteriological examination, should first- or second-line molecules not be indicated. Their use, however, is strongly discouraged as these molecules should be used only in human medicine for the treatment of severe methicillin-resistant staphylococcal infections (particularly for S. aureus).
PUBLIC HEALTH ISSUES
The transmission of bacteria, especially of staphylococci, from animals to humans and vice versa is possible; although the risk is low, especially in healthy subjects, cases of human staphylococcal infections transmitted by companion animals have been reported. The risk of transmission should therefore be minimized via appropriate hygiene measures and by appropriate communication to the owners and to all the people who may come into contact with the patient (particularly children, elderly persons or immunocompromised subjects).
Suggested reading
- Miller WH Jr., Griffin CE, Campbell KL. Muller and Kirk's Small Animal Dermatology, 7ed, Saunders, 2013.
- Devriese LA. Staphylococcus pseudintermedius sp. nov., a coagulase-positive species from animals International Journal of Systematic and Evolutionary Microbiology 2005;55:1569–1573.
- McEwan NA, Kalna G, Mellor D. A comparison of adherence by four strains of Staphylococcus intermedius and Staphylococcus hominis to canine corneocytes collected from normal dogs and dogs suffering from atopic dermatitis Research in Veterinary Science 2005;78:193–198.
- Bloom P. Canine superficial bacterial folliculitis: Current understanding of its etiology, diagnosis and treatment. The Veterinary Journal 2014;199:217–222.
- Frank LA, Kania SA, Hnilica KA. Isolation of Staphylococcus schleiferi from dogs with pyoderma. Journal of the American Veterinary Medical Association 2003;222:451-454.
- Hillier A, Lloyd DH, Weese JS, et al. Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases), Veterinary Dermatology 2014;25:163-e43.
- Weese JS, van Duijkeren E. Methicillin-resistant Staphylococcus aureus and Staphylococcus pseudintermedius in veterinary medicine Veterinary Microbiology 2010;140:418–429.