Introduction

There are a large number of bacterial diseases of the skin in dogs, with different histopathological and clinical aspects. Some are superficial and benign (the basement membrane is not destroyed by the infectious process) and some are deep and severe (the basement membrane is destroyed). Pseudopyodermas are not real pyodermas since infection plays only a secondary role and anti-infectious therapy is not effective. Staphylococcus intermedius is the most common infectious agent cultured in canine pyoderma. It can multiply easily in the dog's skin, due to the thinness of the stratum corneum and the lack of sebum plug in the hair follicles. Inflammation of the skin, particularly due to allergy, is the most common cause of pyoderma, more than real immunodeficiency.

Superficial pyoderma

1. Skin fold pyoderma (intertrigo)

These lesions are seen in anatomical defects where there is an important bacterial colonization: lip, facial, vulvar, caudal, obese and mammary folds. The dermatosis is localized with erythema, exudation, suppuration and bad odour.

2. Impetigo

In juvenile impetigo, subcorneal pustules are present on the ventral side of the body, with crusting. The disease is self-limited. In adult impetigo, large pustules ("bullae") are seen all over the body. In general, adult impetigo is severe and secondary to an underlying disease (hyperadrenocorticism, glucocorticoid therapy...) or multiple traumas (e.g., during hunting).

3. Folliculitis

 Juvenile folliculitis: numerous follicular pustules are present on the ventral side of the body. The condition often heals at puberty.

 Short-haired dog pyoderma: there are generalized follicular pustules, epidermal collarettes and crusts, with a "moth-eaten" hair. Pruritus disappears when the lesion heal.

 Secondary folliculitis: this common disease is characterized by follicular pustules, epidermal collarettes and crusts which are often generalized. Pruritus is still present after lesions healing in case of underlying pruritic dermatosis. The disease may also generate pruritus in a usually nonpruritic dermatosis (in such cases pruritus disappears when lesions heal).

 "Bacterial hypersensitivity" and/or superficial spreading pyoderma: bacterial hypersensitivity is an uncommon disease based on a clinical triad: erythematous follicular pustules, target lesions/seborrhoeic plaques, haemorrhagic bullae. There is sometimes a severe pruritus. The existence of a real bacterial allergy is presumed and debatable. In superficial spreading pyoderma, nummular areas of alopecia and erythema are centrifugally expanding, with epidermal collarettes and crusts. These lesions are often associated to intact but transient follicular pustules.

 Deep folliculitis: it is the so-called acral lick dermatitis, most often a deep follicular bacterial infection with retrograde hidrosadenitis secondary to a psychogenic and/or an allergic cause.

 Pyotraumatic folliculitis: some cases of folliculitis (e.g., in Labradors, Retrievers) appear as oozing suppurative plaque with pain. They are surrounded by satellite pustules of folliculitis or even furunculosis, which help to differentiate them from the "classical" pyotraumatic dermatitis.

Deep pyoderma

1 Furunculosis

 Acne: papulo-pustules and pustules are seen on the face, particularly the chin, in young dogs.

 Secondary furunculosis: localized or generalized pustules are associated or secondary to a folliculitis and the disease is triggered or aggravated by an excessive therapy (e.g., glucocorticosteroids).

 Nasal pyoderma: pustules and crusts are present on the bridge of the nose and eyelids. There may be an unpleasant scaring. This true bacterial nasal pyoderma of unknown cause should be differentiated from the sterile eosinophilic furunculosis possibly due to arthropod bites.

2. Cellulitis

A. Localized cellulitis

 Pressure points pyoderma: there are necrotizing lesions of the elbows, the rump, the stifles, the hocks and the lateral digits. They are due to permanent trauma in heavy dogs.

 Various localized cellulites: These are other localized necrotizing lesions (e.g., perianal). Their cause is often unknown; they are sometimes secondary to a furunculosis.

B. Generalized cellulitis

 Pyodemodicosis: There is an extensive necrotizing skin disease, which is secondary to a generalized demodicosis (an immunodeficiency status).

 Various generalized cellulites: necrotizing lesions are extensive and often secondary to other immunodeficiencies.

3. The interdigital pyoderma complex

There are very numerous causes of non infectious pododermatitis with erythema, oedema, oozing and alopecia. The same lesions are present in interdigital pyoderma along with furunculosis, ulcerations, fistulae and necrosis (cellulitis). Interdigital pyoderma is often secondary.

Pseudo-pyoderma

1. Pyotraumatic dermatitis

The typical lesions have an acute onset and are characterized by alopecia, erythema, oozing, suppuration, pruritus and/or pain. These lesions are common, and most often associated to pruritic skin disease. They are poorly understood (sometimes due to vasculitis?). There is a spontaneous healing in a few days, but a short treatment is useful.

2. Juvenile pyodermas

Juvenile pyoderma of new-born puppies: crusty lesions are present on the face, thorax and dorso-lumbar area. They might be due to trauma. No treatment is required since there is a spontaneous healing.

Juvenile cellulitis: The aetiology of this disease is unknown. The typical clinical aspect is a facial oedema and furunculosis, with fistulae, crusting and a suppurative otitis externa. Adenopathy and sterile abscesses (cellulitis) are present. The onset of this uncommon disease occurs before 4 months of age in one or several puppies of a litter. There is a spontaneous healing in a few weeks with scaring but treatment is required.

Diagnosis of canine pyoderma

Diagnosis of canine pyoderma is based on history, physical examination and complementary examinations: cytology, histopathology and bacteriology.

1. Cytology

In intertrigo (skin fold pyoderma), images of "bacterial colonization" are observed, i.e., healthy neutrophils, Cocci and Bacilli in an extracellular position and degenerated neutrophils in a state of phagocytosis. In impetigo and folliculitis, impaired (degenerated) neutrophils are only found. The pictures of Cocci phagocytosis are not particularly numerous. This is an image of "bacterial invasion", i.e., the penetration of pathogenic germs into the skin. The significance of the pictures of phagocytosis differs considerably depending on whether they are observed on the surface or in a cutaneous lesion. In effect, when they are observed inside the skin (epidermis, hair follicles, dermis) one might consider that the phagocytosed germs are pathogenic and that there is a real bacterial pustulosis. In contrast, phagocytosis observed on the surface indicates multiplication of germs which are not necessarily and probably rarely pathogenic. In deep pyoderma cytology is less likely to reveal the germs and pictures of phagocytosis, although they must be looked for. Frequently there are a granulomatous reaction, eosinophils and red blood cells. Bacterial colonization is observed in pyotraumatic dermatitis as in intertrigo but it is not significant and treatment with antibiotic does not result in remission. In juvenile cellulitis, the degenerated neutrophils are very numerous, with a granulomatous reaction. No germs are seen.

2. Histopathology

This will show typical lesions, but is relatively rarely performed for the diagnosis of canine pyoderma, except in case of difficult differential diagnosis.

3. Bacteriology

This can confirm the bacterial infection and allows sensitivity testing.

Treatment of canine pyoderma

Systemic (antibiotic) and topical therapy can be used in canine pyoderma.

1. Selection of antibiotics

The criteria for the choice of an antibiotic are as follows: appropriate kinetics and good cutaneous penetration, activity against Staphylococci, activity in pus and reactive tissues, bactericidal activity rather than bacteriostatic activity particularly in severe cases, easy administration (oral, q12h or q24h), absence of secondary effects, reasonable cost. The choice can be empirical, particularly in superficial pyoderma, after cytological examination of pus from an intact pustule which shows bacterial invasion. Bacteriology and sensitivity testing must be used in case of deep pyoderma, recurrent pyoderma, when cytology shows a complex flora with rods, and in case of empirical antibiotic therapy failure. They can be repeated during therapy.

2. Dosage and duration of treatment

Ideal doses must be used and duration of treatment must be long enough (a few weeks to several months depending of extension and depth of lesions, and always beyond clinical cure). Maintenance pulse treatment (e.g., 2 to 3 days a week) can be used in chronically relapsing pyoderma but it could theoretically select resistant strains as well as the use of subminimal doses. They are both used for economical reasons but the former is preferable.

3. Antibiotics useable in canine pyoderma

Antibiotics useful in canine pyoderma are included in the following table. They all have a good cutaneous diffusion (because of their liposolubility) and can be given orally, which is useful because of long therapeutic courses (ease of administration). They are all bactericidal except macrolides which are bacteriostatic.

Penicillin G (which is injectable) and A are sensitive to penicillinases. Aminoglycosides have a low cutaneous diffusion (they are hydrosoluble), are injectable and toxic. Chloramphenicol has a bad reputation in humans and the cat (haematologic toxicity). Tetracyclines have a very low activity against Staphylococci. These antibiotics are never or rarely used in canine pyoderma. Rifampicin is effective against Staphylococci but, as it is still used to treat human tuberculosis, it should be used when there is no other possibility (5 to 10 mg/kg SID). In addition, it should be then associated to a betalactamine to prevent the selection of resistant strains of Staphylococci. Mupirocine, a topically active bactericidal antibiotic, in a polyethylene glycol base is effective against Gram+ Cocci, is not systematically absorbed and is not chemically related to other antibiotics. It can be used in localized pyodermas (acne, pressure point pyoderma, interdigital pyoderma).

4. Associated treatments

Topical therapy is always beneficial in canine pyoderma, particularly in superficial staphylococcal disease. Clipping can be useful and is necessary in deep pyoderma such as cellulitis. The main useful topical products are chlorhexidine (lotion and/or shampoo), povidone-iodine (lotion and/or shampoo), benzoyl-peroxide (shampoo and eventually gel), ethyl-lactate (shampoo). They should be used frequently, e.g., once a day, at the beginning of therapy. Later, frequency of application may decrease. Each shampoo should be followed by the application of an appropriate humectant. Topical or systemic glucocorticoids should never be used in true canine pyoderma, even in case of pruritus, because they cause severe relapses ("rebound effect"). In contrast they can be used and are effective in pseudo-pyoderma (e.g., oral prednisolone: 1 mg/kg/day for pyotraumatic dermatitis and 2 mg/kg/day for juvenile cellulitis).

Conclusion

Canine pyoderma is a group of various skin diseases and an accurate diagnosis is mandatory. An appropriate antibacterial therapy is required in most cases of canine pyoderma, in association with topical therapy. Antibiotics must be selected carefully and used with appropriate dosage and duration of treatment.

References

References are available upon request.