When a veterinary surgeon is faced with a street dog with a skin condition and does not have access to all the diagnostic aids that we perhaps take for granted in a regular primary care veterinary practice, it is possible to diagnose and treat that animal safely without compromising animal welfare. A trained dermatologist, whether in the human or veterinary field, can diagnose many of their clinical cases by taking a good clinical history, looking at the distribution pattern of the skin lesions and assessing the lesions themselves. While, of course, best practice would always suggest that an initial tentative diagnosis is confirmed by the performance of basic investigations such as skin scrapes, hair plucks and impression smears it is possible (but of course less than ideal) to institute a therapeutic regime without these tests if they can not be performed in the case of a ‘Street Vet’.
Distribution of skin lesions
There are exceptions to every rule, but many of the most common skin conditions have very characteristic distributions of skin lesions. As the dermatological disease process becomes more chronic the less typical the distribution pattern will become, but in the early stages of many problems the pattern of lesions together with more general signs such as the presence or absence of pruritus, can help narrow down the list of potential differential diagnoses.
1) Pattern distribution as an aid to differential diagnoses
a) Pruritic skin disease
Ectoparasites
There is some overlap between the presenting signs of some of the most common ectoparasitic problems but most have typical distribution patterns. Flea allergy tends to present with caudo-dorsal signs. Classically the flea allergic dog produces lesions within the ‘flea triangle’, which is an area of skin that extends as a broad-based triangle that has its base over the tail and forms its apex in the midline of the dorsum. Sarcoptes scabiei mites burrow through the skin and tend, therefore, to affect the glabrous areas such as the ear pinnae, especially the ear tips, as well as the extensor aspects of the elbow and hocks. Although dogs with S. scabiei infection will shake their head and exhibit a strong scratch reflex when the end of their ear pinna is scratched, they never exhibit signs of otitis externa. Trombicula autumnalis mites, picked up in the autumn time from grassland, affect contact areas of the skin so will be often found in the interdigital web. The foot licking that accompanies the mite infestation can mimic allergy, but close inspection of the affected area will reveal the paprika-coloured dots typical of the parasitic larval form of the mite. As well as producing pedal lesions, active outdoor dogs, especially those that forage will also pick up mites on their ventrum and in the small fold on the ear pinna, often referred to as Henre's pocket. Cheyletiella spp. mites produce generalised scaling mostly confined to the dorsum; there is usually sparing of the lower legs, face and ears.
Allergy
The distribution of lesions in cases of both cutaneous adverse food reactions (CAFR or food allergy) and atopic dermatitis (AD) can be identical. The only way to differentiate between the two is to food trial the dog with a novel protein or a hydrolysed diet. However, the pattern of lesions seen with both conditions is very different to those seen in ectoparasitic disease. Dogs have involvement of their feet especially their front feet, flexural surfaces especially the hocks and elbows, ventral skin in the axilla and groin, periocular skin and usually have otitis externa. In addition Favrot's criteria can be applied to help diagnose atopic dermatitis (Favrot et al, 2010). These are a list of criteria that can help establish a diagnosis of canine atopic dermatitis once ectoparasites, infections and other allergies have been ruled out. These are detailed in Table 1.
Table 1. Favrot's diagnostic criteria for atopic dermatitis
| Onset of signs under 3 years of age |
| Dog mostly living indoors |
| Glucocorticoid responsive pruritus |
| Pruritus before skin lesions |
| Affected front feet and concave surface of ear pinnae |
| None affected ear margins |
| None affected dorsolumbar area |
Malassezia dermatitis
Infection with yeast can be exceptionally itchy and can occur concurrently with other diseases such as ectoparasites and allergy. However Malassezia spp. infection is usually seen as an intertriginous problem. Lesions tend therefore to be confined to the interdigital spaces, neck and lip folds, chin folds, axilla and scrotal or skin folds in the groin. In allergic skin disease the lesions are usually confined to the dorsal aspect of the foot, whereas when Malassezia infection occurs lesions are found in the interdigital spaces.
b) Alopecic disease
The pattern of alopecia can help determine if the hair loss is inflammatory or non-inflammatory.
Inflammatory alopecia encompasses diseases that cause direct and indirect damage to the hair within the follicle. This group of diseases includes many of the most common causes of alopecia including direct damage through infections such as Staphylococcus spp., dermatophytosis and the follicular parasites Demodex canis. Less commonly, autoimmune, immune mediated and neoplastic disease cause direct damage and inflammatory alopecia. Indirect damage to the follicle encompasses the many causes of vascular damage. Inflammatory alopecia is usually asymmetrical, multifocal and the dog's coat has a moth-eaten type of appearance. There are commonly signs of inflammation manifested as erythema, excoriation and ulceration. Primary lesions may be present in the form of pustules and papules.
The non-inflammatory alopecia group contains the hair cycle arrest alopecias, which includes all of the endocrine diseases as well as the more unusual diseases such as cyclical flank alopecia; and follicular dystrophies, which comprises many of the colour dilute alopecic diseases. Although there are exceptions to the rule, non-inflammatory alopecia is usually bilaterally symmetrical the stereotypical ‘endocrine pattern’. There may be other changes associated with endocrine disease such as changes in the quality and colour of the hair coat and skin.
2) Lesion recognition as an aid to differential diagnosis
Most skin diseases present with some form of primary lesion and as such being able to find and identify a primary lesion can help to formulate a differential diagnosis. Primary lesions are directly associated with the disease process whereas secondary lesions are created through trauma, time and a degree of insult to the skin. Often primary lesions develop into secondary lesions. If afforded the luxury of biopsy collection, primary lesions hold far more information than the usually more chronic secondary lesions. Examples of primary and secondary lesions are detailed in Tables 2 and 3.
Table 2. Primary lesions
| Primary lesions | Description |
|---|---|
| Macules | Flat area of discolouration up to 1 cm in size |
| Patch | Flat area of discolouration >1 cm in size |
| Papules | Small solid elevation of the skin up to 1 cm in size |
| Plaques | Flat solid elevated lesion >1 cm in size |
| Nodule | Solid elevation of the skin >1 cm in size |
| Tumour | A large growth many cm in size |
| Pustule | Small circumscribed skin elevation containing purulent material usually inflammatory cells +/- infection |
| Vesicle | Small circumscribed elevation of the skin up to 1 cm in size filled with serum |
| Bulla | Circumscribed elevation of the skin >1 cm in diameter filled with serum |
| Wheal | Irregular elevated oedematous skin area that often changes in size and shape |
| Cyst | An enclosed cavity with a membranous lining that contains liquid or semi-solid matter |
Table 3. Secondary lesions
| Primary lesions | Description |
|---|---|
| Comedone | Blockage of follicle by sebaceous/epidermal debris leading to a ‘black head’ (comedone categorised by some authors as primary lesions) |
| Scale | Accumulation of superficial epidermal cells |
| Crust | Accumulation of epidermal cells with dried exudate |
| Erythema | Reddening of the skin due to increased blood flow |
| Erosion | Loss of superficial part of the epidermis down to but not including the basement membrane |
| Ulcer | Deeper loss to the epidermis including the basement membrane to expose the dermis |
| Fistula | Draining tract originating from the dermis or panniculus |
| Excoriation | Area of self inflicted trauma to the skin |
| Scar | Accumulation of abnormal fibrous tissue usually associated with full thickness skin damage |
| Fissure | Splitting of thickened usually lichenified or heavily crusted area of skin |
| Lichenification | Thickening of skin with accentuation of normal skin markings |
| Hyperpigmentation | Increase in cutaneous pigmentation |
| Hypopigmentation | Decrease in cutaneous pigmentation |
Table 4. Lesion type and common disease processes associated with them
| Lesion type | Most common disease process |
|---|---|
| Pustule | Infection (Staphylococcus spp.), immune mediated (pemphigus foliaceus) |
| Papule (usually with pruritus) | Ectoparasites (fleas, sarcoptic mange, pediculosis, Cheyletiella spp.) |
| Ulcer/erosion | Autoimmune and immune mediated diseases (lupus, vasculitis) mucocutaneous pyoderma |
| Sinus formation | Bite wound, foreign body sinus, deep pyoderma, anal furunculosis, panniculitis |
| Nodular | Neoplastic disease (mast cell, basal cell carcinoma); infectious granulomas (Cryptococcus spp., mycetoma, Staphylococcus spp., mycobacteria); sterile pyogranulomatous disease |
| Crust/scale | Sebaceous adenitis, leishmaniasis, zinc responsive dermatosis; secondary scale due to inflammation with parasites, allergy, infection |
| Comedone | Demodicosis, dermatophytosis, endocrine disease (hypothyroidism, hyperadrenocorticism) |
In view of the fact that the disease process in many skin problems can produce a range of different lesions and those lesions over time can develop into more chronic secondary lesions there is huge overlap between the presenting signs of different diseases. The guide below should therefore be used as a starting point only.
3) Empirical therapy as an aid to differential diagnosis
Careful use of empirical therapy can be invaluable to help establish a retrospective diagnosis of disease, or just to make an animal more comfortable when definitive diagnosis is not possible or is unnecessary.
Empirical therapy with ectoparasiticides is essential in any pruritic dog, and the author's drugs of choice would be one of new oral or topical isoxazalines. The high level of safety, broad spectrum of activity and rapid speed of kill makes these excellent choices to rule out ectoparasites and maintain routine flea control in any street dog.
In the author's opinion, empirical therapy with potent anti-inflammatory, anti-pruritic and immunosuppressive drugs (azathioprine, ciclosporin, oclacitinib), which would include high dose prednisolone, should never be undertaken until a definitive diagnosis has been made. Empirical therapy for skin disease with systemic antibiotics can also be rarely justified, in view of the need to practice appropriate antimicrobial stewardship and the availability of a wide range of effective, safe topical medication.
Topical products such as chlorhexidine based shampoos, mousses and wipes, often combined with climbazole, provide excellent activity against both bacterial and yeast pathogens. One study comparing the persistence of several antibacterial shampoos on dog hairs found that 2% and 3% chlorhexidine shampoo inhibited bacterial growth in vitro far more effectively than lower concentrations of chlorhexidine, benzoyl peroxide and ethyl lactate (Kloos et al, 2013).
While shampoo therapy provides the additional benefits of cleansing and soothing the skin, it is obviously not easily undertaken in dogs on the streets, therefore mousses, wipes and sprays form useful and more practical alternatives. A recent study considered the residual antibacterial activity of canine hair treated with antibacterial mousse products (Ramos et al, 2019). A 3% chlorhexidine 0.5% climbazole-based mousse was found to perform the best of those assessed and showed antibacterial effects for more than 10 days after application.
Summary
There is no doubt that it is possible to diagnose and successful treat many of the dogs on the street without the need for extensive diagnostic tests. By assessing the type and distribution of the clinical signs, an initial list of pertinent differential diagnoses can be formulated. By then considering appropriate empirical therapy, many differentials can be eliminated and successful treatment regimes can be put in place that can make patients comfortable.
KEY POINTS
- Pattern recognition of skin disease is useful to help formulate a list of differential diagnoses.
- Identifying primary lesions can help decide on the underlying disease process.
- Carefully chosen empirical therapy can be useful in many cases.