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==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
=====Endocrine-related hair cycle problems=====
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''
[[Category:To Do - Helen]]
[[Category:Integumentary System - Developmental Pathology]]
[[Category:Integumentary System - Developmental Pathology]]