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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 Gland - Anatomy & Physiology|pineal]], [[Pituitary Gland - Anatomy & Physiology|pituitary]], [[Thyroid Gland - Anatomy & Physiology|thyroidal]], [[Adrenal Glands - Anatomy & Physiology|adrenal]] and [[Reproductive Hormones Overview - Anatomy & Physiology|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 [[Leishmania|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''': [[dermatophytosis]] (''Microsporum canis, Trichophyton mentagrophytes'')
'''Sterile folliculitis''' includes cases of: [[Sebaceous Adenitis|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|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
==Clinical Signs==
The signs can be '''obvious or subtle''', depending on the disease. Congenital hair loss is often symmetrical and not accompanied by any inflammatory changes.
In acquired alopecia, the clinical signs are influenced by the cause. The alopecia can be '''localised, multifocal, symmetric or generalised'''.
Inflammatory changes such as: '''lichenification, hyperpigmentation, erythema, scaling and pruritus''' are common. Some causes of alopecia predispose animals to developing '''secondary skin diseases''' such as bacterial pyoderma or [[Seborrhea|seborrhoea]].
In '''endocrine disorders''', the pattern of loss is symmetrical, often in wear areas first, and pruritus is not generally present.
==Diagnosis==
A thorough history and physical examination are very important.
'''Key points in the history''': breed (any congenital predispositions), duration and progression of lesions, presence of pruritus, evidence of contagion, other non-dermatological problems.
On '''physical examination''': the type and distribution of lesions should be noted.
'''Hairs''' should be examined to determine if they are being shed from the follicle or broken off, which suggests pruritus.
Diagnostic tests:
:'''Skin scrapings''': for ectoparasites (especially ''Demodex'')
:'''Hair brushings''': for [[Fleas|fleas]], [[:Category:Mites|mites]], [[:Category:Lice|lice]]
:'''Tape strips''' for bacterial or yeast infections
:'''Fungal culture''' for evidence of [[ringworm]]
:'''Skin biopsy''': if the other tests are normal. The hair follicles and structures can be examined and any inflammatory patterns can be noted.
:'''Haematology and biochemistry''' are helpful in the diagnosis of endocrinopathies, and specific '''endocrine function tests''' should be performed if such a disease is suspected.
==Treatment==
This will depend on the underlying cause of the alopecia.
{{Learning
|flashcards = [[Small Animal Dermatology Q&A 12]]
}}
==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''
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[[Category:Dermatological Diseases - Dog]]
[[Category:Dermatological Diseases - Cat]]
[[Category:Integumentary System - Pathology]]
[[Category:Expert Review - Small Animal]]
[[Category:Integumentary System - Developmental Pathology]]
[[Category:Integumentary System - Developmental Pathology]]