Alopecia and Hypotrichosis
Introduction
Hair growth follows an intrinsic rhythm and is regulated by multiple growth factors, cytokines and local mediators. It is modified by systemic factors and pineal, pituitary, thyroidal, adrenal and reproductive hormones.
There is also a seasonal regulation in colour, thickness and length.
Alopecia refers to the absence of hair where it is normally present, or the loss of hair.
Hypotrichosis is a congenital alopecia, such as that seen in certain breeds with a hairless trait: Mexican Hairless and Chinese Crested dog and Sphinx cat.
Alopecia can be congenital, such as those breeds and cases of ectodermal dysplasia and hair shaft defects.
Its main form is acquired, and the main mechanisms for alopecia are presented below.
Traumatic/pruritic alopecia
Any pruritic dermatosis will cause alopecia due to the behaviour of the animal who will scratch, rub itself on objects, overgroom or bite off hair.
This is a common cause of bilaterally symmetrical alopecia in cats.
Non-traumatic
Secondary inflammatory
Any marked epidermal or dermal inflammatory disease which does not selectively target the hair follicles can still damage adjacent hair follicles and lead to alopecia. Examples include leishmaniasis or dermal neoplasia.
Scarring can also lead to alopecia through follicular destruction or displacement.
Folliculitis
Inflammation targeting the hair follicle will lead to destruction or damage of the hair shaft.
Follicular infections include:
Parasites: Demodex, Straelensia, Pelodera
Viruses
Bacteria: Staphylococcal pyoderma, Dermatophilus congolensis
Fungi: dermatophitosis (Microsporum canis, Trichophyton mentagrophytes)
Sterile folliculitis includes cases of: sebaceous adenitis, alopecia areata, idiopathic lymphocytic mural folliculitis, linear alopecia
Non-inflammatory
These diseases tend to slow of inhibit follicular growth.
On histopathology, there is telogen and catagen growth with very few hairs in anagen.
There is possible atrophy of the follicular epithelium and adnexae and varying degrees of surface and follicular hyperkeratosis.
Coat colour-linked
Colour-dilution alopecia, black hair dysplasia, follicular lipidosis
Hypothyroidism, hyperadrenocorticism, testicular cell tumours
Miscellaneous hair cycle problems
Post-clipping alopecia, traction alopecia, pattern alopecia, cyclic flank alopecia, alopecia X, mane and tail dystrophy
Ischaemic/atrophic
Post-rabies vaccine, dermatomyositis, topical glucocorticoid-induced, paraneoplastic syndrome
Neoplastic
Epitheliotropic lymphoma
Nutritional/metabolic/debilitation/toxicosis
Iodism, plant toxicoses, selenium, anhidrosis, starvation, chronic hepatic/renal disease
References
Gross, T. (2005) Skin diseases of the dog and cat: a clinical and histopathological diagnosis Wiley-Blackwell
Mecklenburg, L. (2009) Hair loss disorders in domestic animals John Wiley and Sons
Kahn, C. (2005) Merck Veterinary Manual Merck and Co