Introduction
Bacterial skin infections are common in small animal veterinary practice. These vary in severity from a transient involvement of the skin surface only to deep discharging infections which are non-responsive to therapy and which commonly relapse. The most frequent causal organism in bacterial skin infections in pets is Staphylococcus intermedius. S. aureus is the species usually isolated in man. Escherichia coli and Proteus species may also play a role in pyodermas. S. intermedius is a normal resident in the pet - nasal cavity, oropharynx, and the perianal region. It can be a transient resident in other sites especially if there is trauma to the area. The organism is probably transferred to these sites from the oral and anal mucosae during grooming. A dense hair coat has a protective effect, preventing the pathogenic bacteria from having access to the skin. This may explain why certain pyodermas are common in glabrous areas (e.g. impetigo in abdominal skin). Normally skin is highly resistant to colonisation by bacteria. Inflammation of the skin results in temperature changes and increased skin permeability. Colonisation is thus favoured which in turn results in the production of toxins and irritants and a cycle of further inflammation, infection, etc. In subcutaneous abscesses in cats which are usually from fighting, Pasteurella multocida is the principle bacterium found.

Pyodermas can be frustrating to deal with. They can be non-responsive to therapy and relapse repeatedly. Pursuing the underlying predisposing factors and using general principles of therapy, including antibacterials is necessary to successfully manage pyodermas. Gram negative bacteria are generally secondary invaders which are controlled by therapy effective against Staphylococcus. Pseudomonas, however, is a Gram negative bacterium which is difficult to eliminate and requires specific therapy.

Figure 1. Pustules on the ventrum of a 5 year old female Scottish terrier. The intense erythema in this case is indicative of Staphylococcal hypersensitivity.

Figure 2. Acute moist dermatitis in a crossbred male dog.

Figure 3. Superficial spreading pyoderma on the ventrum of a 6 month old female boerbull.

Figure 4. Deep pyoderma involving the carpus and paw of a 5 year old female boerbull.

Figure 5. Muzzle furunculosis in a 5 month old female Rhodesian ridgeback.

Figure 6. Feline acne in a 6 year old female domestic short hair.

Figure 7. Pyotraumatic dermatitis in a one year old male golden retriever.

Figure 8. GSD pyoderma of the hindquarters in a 6 year old female German shepherd cross. The area has been shaved under general anaesthesia to expose hyperpigmentation, furunculosis and draining fistulas.

Figure 9. Chronic recurrent deep furunculosis and cellulitis in a 10 year old male bull terrier.

Classification of bacterial skin disease
Pyoderma can be classified as localised or generalised, primary or secondary, and also according to the depth of the affected tissue. Surface pyodermas include acute moist dermatitis ('hotspot'), and intertrigo (fold dermatitis). Superficial pyoderma involves the epidermis and often the hair follicles. Included here are impetigo and superficial folliculitis. It is important to treat these cases adequately to prevent recurrence and progression to deep pyoderma. Deep pyoderma may be an extension of a surface or superficial pyoderma, or may occur after a primary insult. Deep pyodermas include muzzle folliculitis, pyotraumatic folliculitis, bacterial pododermatitis, German shepherd dog pyoderma, and subcutaneous abscessation.

Surface pyoderma
This involves colonisation of the epidermis only. Clinical signs include erythema, papules, pustules, and alopecia. Self-excoriation may result in larger alopecic areas. The hallmark finding, especially early in the disease process, is intact pustules (Figure 1). These may enlarge in the epidermis and rupture, resulting in a circular alopecia with scale at the periphery - 'epidermal collarette'. Gently removing the roof of an intact pustule gives an uncontaminated sample. An impression smear can be made from the pustule contents. A stain such as Kyro-Quick stain (Kyron) enables cell cytology to be performed. To achieve a pure growth of the causative organism, samples for culture are taken from intact pustules.

Acute moist dermatitis
Acute moist dermatitis is commonly encountered in practice. There is usually a single erythematous lesion, starting in the haired areas, which may rapidly enlarge. Erythema, folliculitis and crusting may be evident under the hair coat beyond the edge of the alopecic area (Figure 2). The ability to spread rapidly like a veld fire has lead to the term 'hotspot'. The rump, dorsum, tail base, and flanks are the most common sites involved. Fleas are usually incriminated. Erythema of the skin indicates enlarged dermal blood vessels which probably further attract fleas to an easy blood meal. For this reason, corticosteroids at anti-inflammatory levels are often sufficient on their own. In early hotspots topical glucocorticoids may be sufficient. Where systemic glucocorticoids are required, a covering antibiotic effective against skin pathogens should be considered. Self-excoriation and the resultant hair loss may make it difficult to find evidence of flea involvement, but strict flea control is necessary. Deeper pyodermas involving usually the peri-auricular and facial areas (known as 'pyotraumatic dermatitis') require more intensive investigation and therapy.

Skin fold pyoderma (intertrigo)
Any of the body folds (e.g. lip, facial, tail, and vulva fold) can be involved, but also the interdigital spaces of the paws. Irritant substances and lack of ventilation combine with sweating, self-excoriation, and eventually swelling of the folds. Where these folds rub together, as in the paws, intense inflammation results. Colonisation by bacteria and the yeast organism, Malassezia pachydermatis causes further inflammmation. Where swollen folds rub together, as in the paws, a cycle of inflammation, pruritus, swelling, and infection is perpetuated.

Mucocutaneous pyoderma
This has recently been recognised as a distinct entity which involves the oral mucocutaneous junction. There can also be concurrent mucocutaneous involvement of the anus. Superficial pustules and crusts involve the full extent of the lips as opposed to lip fold pyoderma which is less extensive, involving the dimple (fold) in the lip only. Ulceration leading to deeper infection may occur. Histopathologically, the dermis contains a dense, predominantly plasmacytic, interface dermatitis. Pigmentary incontinence may also be present.

Superficial pyoderma
Superficial pyoderma is a deeper invasion of bacteria with involvement of all layers of the epidermis. The hair follicle is invaded and the hair shaft may fracture resulting in alopecia. In both cat and dog pyodermas, Staphylococcus is the most frequently isolated bacterium. Cytology and culture may fail to reveal a causative organism. This is indicative of non-Staphylococcal, or aseptic pyodermas which can mimic a bacterial pyoderma. Pemphigus, juvenile cellulitis, sterile nodular panniculitis, subcorneal pustular dermatosis, eosinophilic folliculitis and furunculosis, sterile nodular pododermatitis, linear immunoglobulin A pustular dermatosis and sterile eosinophilic pustulosis have all been described as aseptic pyodermas or 'pyoderma impersonators' occuring in dogs.

Impetigo
The term 'impetigo' is used to denote a superficial pyoderma affecting dogs which have not yet reached puberty. Puppies from 6 weeks to 7 months old are affected. The clinical finding in impetigo is the presence of pustules on the ventrum which are not centred on the hair follicle. Verminosis, systemic disease, and nutrition may all play a role. However, often no inciting cause can be found. The problem may self-cure, however, antibacterial shampoos and antibiotics are sometimes necessary. Impetigo may occur in older pets that are immuno-incompetent. In these patients, an immunosuppressive condition should be searched for.

Superficial folliculitis
In folliculitis, the infection is limited to the hair follicle. The hallmark finding, a pustule with a hair in the centre, may only be found early in the disease process. Superficial folliculitis occurs in young and older pets, and is generally secondary to other conditions. Allergic skin disease, demodicosis, hypothyroidism and lack of adequate hygiene should be investigated. Control or eradication of the underlying causes can be combined with systemic antibiotics, antibacterial shampoos and/or antiseborrhoeic shampoos.

Superficial spreading pyoderma
Expanding papular and macular areas indicate a spreading S. intermedius pyoderma (Figure 3). Differentials include other common dermatoses such as dermatophytosis, demodicosis, and scabies. In the early stages pustules and epidermal collarettes are seen, sometimes with hyperpigmentation of the centre. These may coalesce to form an alopecic area which may be pruritic. Intense erythema indicates a hypersensitivity to Staphylococci present within the pustule (Figure 1). The pruritus associated with this hypersensitivity is so intense that the condition is only seen in a pet that has had adequate restraint (e.g. with an Elizabethan collar). Self-excoriation often results in a penetration of the infection into the dermis. The circular lesions of superficial pyoderma (Figure 3) have a close resemblance to ringworm lesions and hence this pyoderma is commonly misdiagnosed as a dermatophytosis. Features which assist in distinguishing between these two are listed in Table 1.

Deep pyoderma
Deep pyoderma occurs when the infection extends through the epidermis or hair follicle and involves pyogenic inflammation of the dermis or subcutis (Figure 4). The hair follicle ruptures and the infection spreads into surrounding dermal structures (furunculosis), or becomes disseminated through the deeper dermal tissues into the subcutis (cellulitis). Since demodicosis may be an underlying cause in all deep pyodermas, repeated skin scrapings are necessary. Although deep pyoderma is the rarest form of pyoderma, it is also the most severe form, requiring intensive systemic therapy.

Muzzle folliculitis and furunculosis
In dogs, muzzle folliculitis and furunculosis is more prevalent in puppies approaching maturity (Figure 5). However, in cats, this condition known as 'feline acne', may occur at any age. In dogs, mild cases self-cure, but furunculosis and cellulitis require both topical and systemic therapy. Since this condition is found in short coated dogs, it is usually not necessary to shave the area. Benzoyl peroxide in a shampoo or gel is effective. Malassezia dermatitis should be treated with topical products containing an antifungal agent such as miconasole (Daktarin, Janssen-Cilag), systemic antibiotics effective against S. intermedius are required and short courses of corticosteroids (anti-inflammatory doses) may be necessary. In dogs, this condition will usually resolve after puberty and adequate therapy but may, as in cats with feline acne, be a lifelong problem.

Feline acne
Feline acne (Figure 6) is considered a defective primary keratinization in areas rich in sebaceous glands. The presence of comedones and follicular casts in the skin of the chin of cats confirms the condition. Invading organisms include Pasteurella, Streptococcus, Malassezia, Demodex and dermatophytes. Feline acne can be distinguished from eosinophilic granuloma by the fact that comedones are not present in the latter disease. Cleansing and flushing with benzoyl peroxide and chlorhexidine are beneficial. Topical treatments include the antibiotic, mupirocin, and the antifungals, cotrimazole and miconazole. An ointment containing benzoyl peroxide combined with miconazole (Acnidazil, Jannsen-Cilag) is useful. The systemic antibiotic clindamycin (Antirobe, Pharmacia & Upjohn) can be administered for a four to six week course. Synthetic retinoids have been recommended for stubborn cases. However, as in man, a cautious approach to this last group of drugs is advised.

Pyotraumatic folliculitis and furunculosis
As the term denotes, this involves trauma (abrasion, self-excoriation) and a purulent discharge. It is often secondary to otitis externa, foreign body, atopy, and dietary allergy. Initially, there may be an acute moist dermatitis which extends deeper, especially in the facial and subauricular areas (Figure 7). Golden retrievers, bull mastiffs, and rottweilers are at risk. Self-excoriation, wound soiling and contamination, inadequate therapy, and demodicosis can all result in pyotraumatic folliculitis. E coli, Proteus, and S. intermedius are often present. Shaving must extend beyond the border of involved skin. After careful scraping for mites, both topical and systemic therapy is administered. Ear canals and surrounding areas must be thoroughly evaluated. Sedation, bandaging up the paws and other forms of restraint are necessary to minimise self-excoriation.

Pressure point pyoderma
Localised infection of the elbows and hind limbs may be precipitated by lying on hard surfaces. A blanket on hard surfaces does not provide sufficient protection for the pressure points in large and giant breeds. A foam rubber mattress (covered in an impervious material) provides an insulating bedding.

Pododermatitis
Deep bacterial infections in the paws may be an extension of intertriginous pyoderma of the interdigital spaces. Malassezia dermatitis may be involved, either alone, or as a mixed infection. Other inciting factors include trauma, foreign bodies, atopy, contact allergy/irritant dermatitis, neoplasia and migrating nematodes. Deep draining fistulas and painful pododermatitis may require sedation or even general anaesthesia to allow for deep scrapings to rule out demodicosis. Bacterial paronychia is common in cats as a nail bed infection. Chronic nail bed infections may be secondary to underlying immune modulated disease and the immunosuppressive viruses should be screened for. Furunculosis of the paws indicates deep pyoderma with/without demodicosis. However, dermatophytoses, particularly those caused by Trichophyton species, should always be considered - especially in Jack Russell terriers, hunting dogs, digging and rooting dogs and where one paw only is involved.

Nasal pyoderma
This is encountered in rooting/digging dogs and outdoor/hunting dogs. Factors to be investigated include trauma, geophilic fungi, insect/arthropod hypersensitivity, auto-immune and allergic skin disease.

German shepherd dog (GSD) pyoderma
GSD pyoderma is the term given to frequent episodes of deep folliculitis and furunculosis in the German shepherd dog and its crosses. Hallmarks of this disease are middle aged and older GSDs and their crosses of either sex with a furunculosis, discharge and pain (Figure 8). It may be an extension of a surface pyoderma and begin as a mild infection, often unnoticed in the thick coat. Later, serous and bloody discharges cause matting of the coat, which becomes 'glued' to the lesions. Scarring can result in permanent deep draining fistulas. GSD pyoderma has been described as a syndrome of disproportionate severity and with frequent relapses. The familial nature and severity should be made clear to owners. Recent studies have shown that affected dogs have unusual lymphocyte characteristics indicating an immunodeficiency. Other inciting factors must be searched for. Ectoparasites, especially fleas, but also scabies and demodicosis are the most common predisposing causes in the author's experience.

Abscessation
Abscesses are common in cats, and are usually from fight wounds. Pasteurella multocida is the most common bacterium isolated. Subcutaneous abscesses must be lanced, drained and flushed. P. multocida is usually well controlled with penicillins. However, deep abscesses e.g. tail root abscesses, and those involving anaerobic bacteria require extended courses of clindamycin.

Rare bacterial infections
Atypical mycobacteria which are present in the soil may invade the subcutaneous skin. Feline leprosy is caused by rat bites. Nocardia is a filamentous bacteria which may affect cats and dogs.


Topical therapy

Creams, ointments and gels
Localised pyodermas such as fold dermatitis, feline acne, otitis externa and impetigo may respond well to topical creams, ointments and gels. Kanamycin, neomycin, bacitracin, polymixin B, nitrofurazone and mupirocin are examples of topical antibiotics. Mupirocin (Bactroban, SmithKline Beecham) is especially useful in certain stubborn surface pyodermas. Cat fight abscesses should be lanced, drained and flushed with a 2 % hygrogen peroxide and/or a 0.5% chlorhexidine solution. Dilute povidone iodine solution may also be used.

Baths, soaks and shampoos
Clipping and shaving the coat and cleansing with antibacterial shampoos will be beneficial. Antibacterial shampoos are particularly beneficial in surface and superficial infections. Chlorhexidine is both anti-bacterial and anti-fungal and is available in a shampoo (Pyoderm, Virbac). In deep pyodermas, pyodemodicosis, and stubborn cases of furunculosis, the follicular flushing effect of benzoyl peroxide may assist. In severe furunculosis, such as GSD pyoderma, it is essential to shave the effected areas (even if this is the whole body!). The lesions can be painful and this may have to be done under deep sedation or even general anaesthesia. Shaving exposes areas of infection previously hidden under a dense coat (Figure 8). It may be necessary to warn the patients owners as fragile skin may peal away leaving unsightly deep draining fistulas. Exposure of the deeper lesions is necessary, however, to allow for adequate cleansing and access for topical therapy. Washing helps to remove crusts, thereby improving ventilation and drainage and it can also have a soothing effect. Chlorhexidine is particularly effective and often less irritant. Avoid corticosteroids because of the possibility of underlying immuno-incompetence and/or demodicosis. The systemic antibiotics that have been advised are the fluoroquinolones and cephalexin.

Systemic therapy
S. intermedius, is the most commonly isolated bacterium in pyodermas. Occasionally there will be mixed infections, and rarely, other bacteria will predominate. Antibiotic selection may be empirical (based on the clinicians preference and experience), or based on culture and sensitivity results. Therapy, however, should always include a beta-lactamase resistant antibiotic with known activity against Staphylococci. Antibiotics fulfilling these criteria are listed in Table 2. More specific antibiotics can be based on sensitivity results especially in recurrent infections, deep pyodermas, non-responsive pyodermas and immuno-incompetent patients. Cultures from draining sinuses may yield non-pathogenic contaminants. Cultures taken from intact pustules will give more accurate sensitivity results. Antibiotic sensitivity results generally give a good guide, but several strains of Staphylococcus may be present at any one time giving different results. The strain cultured may not be representative of the pathogen present. Furthermore, the correlation between in vitro and in vivo performance of antibiotics is not absolute. Sensitivity results must be interpreted in conjunction with clinical symptoms, and other factors such as drug costs, tolerance and availability. Furthermore, therapeutic failure may be due to insufficient penetration into affected tissue. Potentiated sulphonamides and ampicillin give mixed results and amoxycillin and tetracyclines have generally given poor results. Table 2 lists antibiotics which are useful in bacterial skin disease in small animals. Fluoroquinolones (enrofloxacin, marbofloxacin and orbifloxacin) are capable of good intracellular and intercellular penetration and also a high activity within phagocytes. Antibiotics can be divided into two groups, according to their pharmacodynamics; those that work in a concentration-dependant fashion (e.g. fluoroquinolones) and those which have a time dependant effect (e.g. cephalosporins). The significance of this is that for fluoroquinolones, efficacy is a function of peak plasma concentration rather than half-life whereas for cephalosporins, the duration of plasma and tissue concentration at high enough levels is more important. Fluoroquinolones are most effective if given once daily; and also, cephalosporins must be administered twice daily.

Duration of therapy
This is at least as important as the choice of antibiotic to be used. The duration of therapy must be based on factors such as patient age and weight, depth of infection, concurrent therapy, type of infection (localised or generalised; superficial or deep) and immunosuppressive factors. Surface and superficial pyodermas need 10 days of therapy, whereas deep pyodermas require 6 weeks or more. For pyodemodicosis, GSD pyoderma, and in immunocompromised patients treatment must be continued for a minimum of three weeks after what appears to be clinical cure. Glucocorticoids suppress the inflammation, reducing the blood supply at the site of infection and also the hosts immune response. The skin will appear normal, but will still be infected. Pet owners must be made aware that response to therapy may take weeks and premature drug withdrawal will only result in relapses, drug resistance, and extra costs. Some pet owners are reluctant to administer an adequate course of antibiotics. Re-assuring the pet owner of the safety of antibiotics, especially in relation to the risks posed by not treating, is central to successful control of bacterial skin disease.

Chronic recurrent pyoderma
This is a common and frustrating problem (Figure 9). These cases require a review of the history, a thorough clinical examination and a repeat of the laboratory tests. Withdrawal of all therapy at this stage may be beneficial. There may be inappropriate concurrent therapy, or long-term antibiotic therapy may have resulted in antibiotic resistance. Searching for underlying causes (e.g. poor nutrition, demodicosis, atopy, flea, food and/or other allergy), while repeating culture and sensitivity regularly is necessary. Very old or very young animals may be immuno-incompetent, as are those with neoplastic disease or receiving immunosuppressive drug therapy. It has been proposed that an absolute neutrophilia as well as a lymphocyte count of at least 1000 cells/microlitre should be seen in dogs with a bacterial pyoderma. If these two criteria are not met, immuno-incompetence is suspected and underlying immunosuppressive disease processes should be searched for.

Systemic antibiotics
Antibiotic resistance of S. intermedius, the bacterium involved in small animal dermatology, is slow when compared to the rapid development of resistance which occurs in man with S aureus. However, high levels of resistance to penicillin G, ampicillin and amoxycillin, and to the tetracyclines are common (25 - 70% in a recent worldwide study). Resistance to trimethoprim and sulphonamide combinations is about 5%. Resistance to synthetic penicillins such as oxacillin, cloxacillin, and methicillin is uncommon. Marbofloxacin, enrofloxacin, cephalexin and amoxy-clav also have minimal resistance build-up. Where Gram-negative infections are encountered, a drug effective against Staphylococcus is usually sufficient except where Pseudomonas is involved.

Deep granulomatous pyodermas may respond to anti-mycobacterial therapy. Rifampicin is used for tuberculosis infections in man. Resistance to this drug builds up rapidly and it does have some hepatotoxicity. For these reasons short (two week) courses are used. Covering antibiotics (e.g. cephalexin) are administered concurrently to prevent the development of resistance.

Pulse therapy
Long term, low dose daily administration of antibiotics is not advised due to the development of antibiotic resistance. Pulse therapy, however, using the recommended dosage for one week a month (or week on, week off) has allowed many pets to live a relatively normal life. Cephalexin is advised in pulse therapy along with regular re-examinations and strict ectoparasite control.

Immunomodulation
Several drugs, such as levamisole and cimetidine have been used in an attempt to stimulate the immune system. These drugs are not licensed for this use and there is little support in the literature. The effect of autogenous vaccination is also not yet clear. Bacterial products derived from Staphylococcus aureus phage lysate and Propionibacterium acne are available commercially in some countries. These are administered as adjunctive therapy in an attempt to stimulate the immune system. Thorough treatment of acute and superficial cases of bacterial skin infections with appropriate products remains the most effective method of preventing development of deep pyodermas.

Successful therapy of pyoderma involves the identification and elimination of underlying inciting causes combined with appropriate antibacterial treatment. Systemic and topical antibacterial therapy may be necessary as well as immunostimulation. In those cases where the underlying causes cannot be identified and eliminated, prolonged and repeated therapy may be necessary.

Bibliography

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2. Briggs O.M. 2001 Skin disease of the extremities. Part II Vetmed 15: 5 - 8.
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