General Clinical Approach to Alopecia in Dogs

Mechanisms of Hair Loss

There are a number of possible mechanisms of hair loss, some being reversible and some irreversible. These are illustrated in Figure 1 and include:
• Self trauma – the hair loss is caused by scratching, licking or chewing.
• Folliculitis – the hair loss is caused by follicular inflammation, follicle wall damage and hair shaft breakage.
• Follicular invasion – the hair loss is caused by follicular damage.
• Hair follicle atrophy – this typically involves hair follicle cycling abnormalities due to telogenisation as seen with endocrine diseases and metabolic conditions.
• Scarring – the hair follicles are replaced by fibrous tissue.

[Figure 1]

Conditions associate with these various pathogenetic mechanisms are summarised below:

[Table 1]

Diagnostic Approach to Hair Loss

The first thing to do when presented with alopecia is to determine the predominant pattern. Although many factors and disease processes can influence the hair follicle directly and can be involved in the pathogenesis of non-pruritic alopecia, the animal nearly always presents with one of three morphological patterns – focal, multifocal or symmetrical. These three patterns are helpful clinically because they allow differentials to be more easily generated.
To prioritise these differential diagnoses, careful assessment of the skin and coat can provide further clues. The hairs around the areas of alopecia should be carefully assessed to see if they are easily epilated (endocrine disease) or if they are broken off at the skin surface (dermatophytosis). Comedones may be seen with demodicosis, hypothyroidism and hyperadrenocorticism but are uncommon with the other conditions. Follicular casts are a common feature of demodicosis and sebaceous adenitis. The skin should also be carefully examined for the presence of other lesions such as papules or epidermal collarettes which can indicate the presence of staphylococcal pyoderma. Non-pruritic, diffuse erythema in addition to alopecia can be a hallmark of epitheliotropic lymphoma. If secondary hyperpigmentation is present, a diffuse blackening of the skin may suggest endocrine disease whereas a slate-grey pigmentation is more suggestive of demodicosis. The skin thickness should also be assessed because it can appear atrophic in hyperadrenocorticism and alopecia X. The colour of the coat should also be noted as this is important in colour dilution alopecia and black hair follicular dysplasia. In some cases, the history and clinical signs are so suggestive of a particular condition that it may be possible to make a diagnosis without any further tests (medication site application, injection site reaction, pituitary dwarfism, cyclic flank alopecia, black hair follicular dysplasia, congenital alopecia).
Unfortunately, even the most experienced clinician is not able to make a definitive diagnosis of all cases of alopecia just by looking at them. Even though a detailed dermatological examination will allow some of them to be diagnosed, in many cases we are still left with a list of differential diagnoses. Figures 1, 2 and 3 provide a basic diagnostic approach for the three major patterns of alopecia that should allow a definitive diagnosis to be made in all cases. [...]