Canine pyoderma – 5 ways to make a diagnosis

During this Covid pandemic, we’ve all been getting used to remote consultations using telephone and video calls. Whilst most of us find that it’s a reasonably good compromise, there are definitely some situations where the whole examination experience can’t be reproduced, and cases of canine pyoderma are one of these. After all, so much of our clinical judgement relies on using all our senses, including touch and smell.

Posted: 25 June 2020

Canine pyoderma – 5 ways to make a diagnosis


Pyoderma – it’s skin deep

Pyoderma can be caused by any infectious, inflammatory or neoplastic processes, but is generally a term used to describe a bacterial infection of the skin. Broadly speaking, there are two forms of pyoderma, and we refer to them according to the layers of skin that are affected. Skin infections affecting the epidermis and hair follicles are referred to as superficial pyoderma, whereas those that involve the dermis, deep dermis or cause furunculosis, are referred to as deep pyoderma.

A. Superficial pyoderma

A superficial bacterial pyoderma is usually triggered by an overgrowth or colonisation of the normal bacterial flora in the skin. In dogs, the pathogen typically found is Staphylococcus pseudintermedius. There is actually no such thing as a primary canine bacterial pyoderma, with a number of abnormal host trigger factors playing an important role. Such triggers include;

  • Hypersensitivities – think of atopy, flea allergic dermatitis and adverse food reactions
  • Endocrinopathies – examples include hyperadrenocorticism and hypothyroidism
  • Follicular dysplasia - Chinese crested dogs are particularly prone
  • Cornification abnormalities - sebaceous adenitis and ichthyosis
  • Parasitic infections – e.g. demodicosis

The sites that are usually affected are the areas of skin that are warm and moist. These include skin folds (lip, face, neck, tail base and vulva), axillary regions and the interdigital areas. Pressure points, such as the elbows and hocks, are also at risk due to irritation and rupture of hair follicles from repeated pressure.

The most common underlying triggers include;

  • Fleas
  • Flea allergy dermatitis
  • Atopic dermatitis
  • Food allergy
  • Hypothyroidism
  • Hyperadrenocorticism
  • Poor grooming

Clinical sign of superficial pyoderma

In the absence of being able to smell the animal, take particular care to look for the evidence. The areas frequently developing pyoderma include the bridge of the muzzle, chin, elbows, hocks and the spaces between the toes. Signs of superficial pyoderma include;

  • Excessive scaling – scale is an accumulation of loose fragments of the horny layer (stratum corneum) of the skin. Pyoderma tends to form a waxy, greasy scale.
  • Pruritis – this can be variable, with some animals showing no signs of pruritis and others being intensely itchy.
  • Epidermal collarette – this is a circular rim of scale from the remnants of a papule, pustule or vesicle and are often seen in superficial bacterial pyoderma.
  • Hyperaemia – look for areas of redness.
  • Welts - these are bumps in the skin that are caused by a fluid build-up directly below the surface.
  • Alopecia - superficial pyoderma commonly appears as multifocal areas of alopecia and are often within the epidermal collarette.
  • Papules – look closely at the follicles. Shorthaired breeds can develop multiple superficial papules that look similar to urticaria. The follicles and hairs epilate and progress to form focal areas of alopecia. Pustules are less common in short hair breeds.
  • Pustules – whilst they can form, they are infrequently found.

B. Deep pyoderma

With deep pyoderma, the infection breaks through hair follicles to involve the deep layers of the skin, producing more pronounced signs. In these cases, you’ll be finding;

  • Odour – the smell can be quite unpleasant
  • Furunculosis – this is especially common in the interdigital tissues
  • Cellulitis – the inflammation will result in erythema
  • Pain – at this stage of pyoderma the condition is often painful, requiring some form of analgesia
  • Ulceration and exudate – look for pus and a bloody discharge
  • Swelling – tissues become inflamed and oedematous
  • Scabs, crusts and blisters

The areas most likely to be affected by deep pyoderma are the bridge of the muzzle, chin, elbows, hocks, interdigital areas, and lateral stifles.

Pyoderma – 5 ways to make a diagnosis

Most cases of canine pyoderma are diagnosed from the evidence of clinical signs. However, we also need to rule out the common differentials, such as demodicosis, Malassezia dermatitis, dermatophytosis and pemphigus foliaceus. To achieve this, we;

1. Look for the underlying cause

As has already been mentioned, most cases of superficial pyoderma are secondary, so it’s most important to look for an underlying cause. At this time of year this can include a variety of triggers such as;

  • Fleas – is the owner using appropriate and effective flea control?
  • Allergies – there may be a pattern to a recurring pyoderma which coincided with a seasonal allergy.
  • Hypothyroidism – certain breeds of dog may be more likely to develop the condition, such as golden retrievers, Doberman pinschers, Irish setters, dachshunds, boxers, and cocker spaniels.
  • Cushing disease – this condition will frequently result in a nonpruritic pyoderma.
  • Poor grooming – some dogs just don’t like being brushed, whilst others are neglected. Bacteria may cause an infection secondary to local trauma from scratching.

2. Direct impression smears

Microscopy of impression smears are used to identify cocci, rods, inflammatory cell infiltrates and keratinocytes. These smears are usually taken from;

  • Intact pustules
  • Areas underlying crusts
  • Epidermal collarettes
  • Moist erythematous areas

3. Skin scrapes

Multiple deep skin scrapings are needed to exclude parasitic infections such as Demodex canis.

4. Bacterial culture

These are definitely recommended to ensure that resistant strains of bacteria, such as MRSA, are identified and managed appropriately. Samples are taken from intact pustules or induced rupture of deep lesions. Be aware that results from crusted lesions, papules, epidermal collarettes and fistulous tracts can often be contaminated, so the interpretation of the results may be questioned. Ideally, the best samples are from a closed lesion.

5. Dermatophyte cultures

These are used both in house and via an external laboratory to exclude dermatophytosis.