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: dermatophytosis (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

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 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, mites, 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.

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