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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 immune response.

In the limited studies to date, there appears to be some benefit in this form of treatment (Curtis et al, 2006).

Sequelae to pyoderma

In most cases, effective treatment leads to complete resolution. In some cases of chronic recurrent pyoderma, especially deep pyoderma, the cycles of inflammation and repair by fibrosis can lead to the formation of masses termed “focal fibrous adnexal dysplasia”.

These are areas of fibrosis that contain distorted hair follicles and glands and occur most often on the distal limbs (see Figure 2). This is a common sequel to canine pyoderma. A similar process of repair by fibrosis can occur in some cases of chronic interdigital pyoderma, leading to deformation of the digits.

5 / 16 These lesions can prove very frustrating to manage, because they intermittently swell and ulcerate. But on each individual occasion, they may be either the recurrence of the bacterial pyoderma requiring antibacterial therapy, or due to a sterile foreign body reaction to hair growing from dysplastic hair follicles into the dermis and producing a sterile pyogranuloma (such as an “interdigital cyst”). Some dogs with severe chronic fibrosis between their feet may require podoplasty, but this must be approached with care, since postoperative ischaemia due to occlusion of blood supply or infection can prove a problem. I have seen some cases of podoplasty leading to multiple digit necrosis and loss because of poor surgical technique. These cases are best treated by referral to a soft tissue surgeon.

Mistaken for pyoderma

• Demodicosis

Demodicosis produces a folliculitis, with or without concurrent bacterial folliculitis. It is common in young and old dogs, but can occur at any age.

It is always important to check for demodicosis when presented with any form of alopecia, by using deep skin scrapes and hair plucks.

Occasionally, biopsy will reveal mites not found on skin scrapes. Folliculitis cases not responding to rational therapy should be scraped again for evidence of Demodex mites.

• Dermatophytosis

An important cause of alopecia and scaling in the dog is a dermatophytic folliculitis.

Apart from Microsporum canis, there are a number of other species of dermatophyte that can infect the surface and hair keratin, especially Trichophyton mentogrophytes.

Dermatophytosis is an important differential diagnosis in all cases of scaling and alopecia, and some cases can be pruritic. Some are mistakenly diagnosed as atopic because of the pruritus (see Figure 3). Any dog that habitually digs should be considered a candidate for dermatophytosis and, in particular, terrier breeds. I have seen several terriers with signs suggestive of atopy and positive intradermal skin tests, but have had dermatophytosis that responded entirely to prolonged therapy for that alone.

Pruritic Jack Russell terriers and other digging dogs may have dermatophytosis and not allergies. Don’t forget to look for dermatophytes by culture for histopathology, and state this differential diagnosis on your submission form.

6 / 16 • Pemphigus foliaceus

Although most cases of pemphigus foliaceus produce crusting and alopecia, with only mild to moderate pruritus, some can appear to have more severe pruritus. Pemphigus foliaceus can be mistaken for a superficial bacterial folliculitis.

However, the distribution and symmetry of the lesions in most, but not all, cases should raise the possibility of an autoimmune pathogenesis. This is another reason for biopsy and histopathology in cases not responding to antibacterial therapy.

The main differential diagnoses for folliculitis in the dog are staphylococcal infection and demodicosis and dermatophytosis, but don’t forget epitheliotropic lymphoma and pemphigus foliaceus.

• Malassezia dermatitis

This most often occurs concurrently with bacterial folliculitis, but without specific antifungal treatment, antibacterial therapy fails. As with demodicosis, cases of presumed bacterial folliculitis that fail to respond to rational therapy should be evaluated again for the presence of Malassezia, using acetatestrip examinations.

Some cases of atopic dermatitis with intermittent bacterial folliculitis that have been successfully controlled over a long period of time may become resistant to therapy because they have developed Malassezia dermatitis.

• Epitheliotropic lymphoma

Epitheliotropic lymphoma often affects the hair follicle’s outer root sheath, leading to hair loss and alopecia. The gross features in these cases are indistinguishable from a folliculitis (bacterial or dermatophytic), and this must be borne in mind with older dogs presenting with alopecia and scale.

If a case fails to respond to rational and appropriate antibacterial therapy, then biopsy for histopathology and sampling for dermatophyte cultures should be considered.

• Calcinosis cutis

Calcinosis cutis is associated with naturally occurring and iatrogenic hyperadrenocorticism (HAC), and can be mistaken for a pyoderma, because the mineralised collagen being extruded through the epidermis and hair follicles is interpreted as pus. In cases of HAC with areas of mineralised dermal collagen, but no clinical evidence, the lesions may develop once treatment has been initiated.

I often receive biopsies from cases of HAC that develop lesions following a few days of therapy

7 / 16 with the question: “pyoderma or drug reaction?” on the submission form. The answer is usually neither.

Calcinosis cutis may appear clinically following initiation of treatment for HAC, as the serum cortisol falls and the skin can mount a foreign body reaction.

Conclusion

In summary, canine staphylococcal pyoderma is usually secondary to an underlying disease process, but it can be an idiopathic and recur rent problem.

The underlying disease must be identified and treated. It is important to rule out differential diagnoses and treatment should be topical and, if necessary, parenteral. Use an appropriate systemic antibiotic, at the correct dose and for sufficiently long enough.

References and further reading

Wiemelt S P, Goldschmidt M H, Greek J S, Wiemelt A P and Mauldin E A (2004). A retrospective study comparing histopathological features and response to treatment in two canine nasal dermatoses, Vet Dermatology 15(6): 341-348. Curtis C F, Lamport A I and Lloyd D H (2006). Masked, controlled study to investigate the efficacy of Staphylococcus intermedius autogenous bacterin for the control of canine idiopathic recurrent pyoderma, Vet Dermatology 17(3): 163-168.

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Figure 1. German shepherd idiopathic deep pyoderma. Note the extensive ulceration, hair loss and sinus tracts across the lateral thigh. This dog’s flare factor appeared to be flea allergy.

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Figure 2. An eight-year-old Dobermann with a mass on its hock, which has developed slowly over a year. This could be a neoplasm, but it is actually an area of focal fibrous adnexal dysplasia at the site of a chronic deep pyoderma.

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Figure 3. A four-year-old Jack Russell terrier with facial and pedal pruritus, alopecia, scaling and erythema. This dog had positive intradermal skin test reactions, but was not clinically atopic. It had dermatophytosis, which responded entirely to antifungal therapy.

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TABLE 1. Localised forms of pyoderma

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TABLE 2. Clinical signs associated with pyoderma

TABLE 3. Underlying disease processes to consider

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TABLE 4. Differential diagnoses for important forms of pyoderma

15 / 16 TABLE 5. Possible reasons for a poor response to antibacterial therapy

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