23 Colour dilution alopecia
Colour dilution alopecia is a developmental disorder resulting in the weakening of hair shafts and progressive alopecia due to abnormal melanin pigment distribution within hair shafts, and within epidermal and follicular keratinocytes. It is a common condition in dogs with blue or fawn dilute colour hair coats. This case report describes a case of colour dilution alopecia in a collie with a blue hair coat.
The onset of symptoms in colour dilution alopecia is usually between 6 months and 3 years of age and may present initially as gradual onset dorsal alopecia or as dorsal folliculitis. Only hairs in the dilute coat colour area are involved. When dogs present with folliculitis, there is a gradual progression of alopecia following each episode of folliculitis. Unless there is pyoderma, this is a non-pruritic disease and there is no systemic involvement. The history in this case was as follows:
General physical examination was unremarkable.
Examination of the skin revealed generalized, partial to complete alopecia over the pinnae and trunk (Figs 23.1 and 23.2). White areas were unaffected.
There was loss of primary hair shafts over the pigmented areas, revealing a blue to brown, dilute coloured undercoat.
In this case, the history and clinical signs were strongly suggestive of colour dilution alopecia, although other causes of non-pruritic alopecia were potential rule-outs. The involvement of a staphylococcal folliculitis could be ruled out on the basis of no response to the previous antibacterial therapy. In tardive onset cases, causes of hair follicle arrest, such as hypothyroidism and hyperadrenocorticism, should also be considered as differentials.
Therefore, the differential diagnosis list was: