The first major distinction is between focal and multifocal alopecia (inflammatory type) and diffuse and/or bilateral alopecia (non-inflammatory type). Those represent a large group of skin diseases, united by thinning or lack of hair, absence of itching (which differentiates them from scratching alopecia due to allergic causes), and negative results of fungal tests and skin scrapings.[1; 2]
Patchy (or focal) alopecia can be caused by bacterial, fungal or parasitic infections affecting the hair follicle. Among the most common bacterial infections is pyoderma, which leads to the formation of pustules and scabs on the skin. The most common fungal infections are ringworm and dermatophytosis. Finally, the most common parasitic infection is red mange (or demodectic mange).
The breed predisposition to some forms of alopecia can help in the differential diagnosis. E.g. Pomeranian Fox and Chow Chow are predisposed to alopecia X, Doberman and Yorkshire terrier are predisposed to colour dilution alopecia.
The age of the patient, besides helping us to distinguish congenital from acquired alopecia, can direct us towards some diseases more frequent in adulthood or old age.
In general, a slow progression is typical of systemic diseases, endocrinopathies, metabolic or nutritional imbalances. A rapid onset e.g. after physiological phenomena (childbirth and breastfeeding), or after severe systemic pathologies (shock or surgery) suggests telogen outflow. Rapid onset alopecia (anagenic outflow) may occur as a consequence of the administration of cytotoxic agents such as methotrexate and cyclophosphamide.
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