Introduction

Alopecia is the loss or absence of hair on the body where hair is normally present. Hair growth in the dog follows the hair cycle, namely anagen (growing phase), catagen (intermediate phase) and telogen (resting phase). Hair is shed in the telogen phase as a new hair begins at the anagen phase.

Alopecia

The first step in determining the underlying cause(s) of alopecia is to make sure that the alopecia is not congenital i.e., from birth. This is characteristic for certain breeds such as Mexican Hairless Dog, Chinese Crested Dog, and Peruvian hairless dog. It can also be abnormal in dogs affected by ectodermal dysplasia (has concurrent teeth abnormalities).

The next step is to differentiate if alopecia is due to self-trauma or spontaneous. Self-traumatic alopecia is usually due to pruritic causes e.g., allergies and will not be covered in this presentation. If you are unsure, a trichogram can assist in this differentiation i.e., broken hair tips suggestive of self-traumatic alopecia vs. smooth tapered hair tips for non-pruritic alopecia.

Non-pruritic alopecia can be broadly divided into inflammatory vs. non-inflammatory and symmetric vs. non-symmetric alopecia. Inflammatory causes usually result in non-symmetric (focal to multifocal) alopecia. Conversely, non-inflammatory causes usually result in symmetric or diffuse alopecia.

Non-pruritic non-symmetrical inflammatory alopecia

Diagnostic Work-up

Signalment and History

Age

• Demodicosis and dermatophytosis typically affect young dogs
• Epitheliotrophic lymphoma most commonly in old dogs

Breed

• Sebaceous adenitis more common in Standard Poodles, Akita Inus, Vizslas, and Hovawarts
• Dermatomyositis usually affects Collies, Shetland Sheepdogs, and crosses

History

• Concurrent muscle atrophy/weakness suggestive for dermatomyositis

Physical and Dermatological Examinations

• Comedones due to follicular plugging by demodex mites or sebaceous adenitis
• Follicular casts for sebaceous adenitis and demodex mites
• Excessive scaling suggestive for dermatophytes or sebaceous adenitis
• Papules, pustules, and crusting for bacterial folliculitis

Diagnostic Tests

• Wood’s lamp to check for dermatophytes i.e., apple-green fluorescence, and to select hair for KOH and fungal culture
• Skin scrapes: both deep and superficial for demodex mites
• Cytology for bacterial pyoderma and demodex mites
• Trichogram: Follicular casts for sebaceous adenitis, demodex mites around hair shafts, fungal hyphae/arthrospores along and within hair shafts
• Tissue biopsies for histopathology is required diagnose sebaceous adenitis, ischemic dermatopathy, alopecia areata, and epitheliotropic lymphoma

Non-pruritic symmetrical non-inflammatory alopecia

Various hormones affect the hair cycle by stimulating anagen or prolonging telogen. While hormonal disorders can result in non-pruritic symmetrical non-inflammatory alopecia, there are also many other causes.

Diagnostic Work-up

Signalment and History

Age

• Middle aged to older dogs for hormonal disorders
• Young dogs for follicular dysplasia and pattern baldness

Breed

• Hyperadrenocorticism: predisposed breeds include Miniature Poodles, Dachshunds, and Boston Terriers
• Hypothyroidism: predisposed breeds include Boxers, Old English Sheepdogs, and Golden Retrievers
• Plush coated breeds such as Pomeranians, Alaskan Malamutes, Samoyeds, and Siberian Huskies suggestive of hair cycle arrest
• Recurrent cyclic flank alopecia: predisposed breeds include Airedale Terriers, Boxers, and Schnauzers
• Colour dilution alopecia affect dilute (blue or fawn) coated breeds such as Staffordshire Bull Terriers and Dobermans
• Pattern baldness: predisposed breeds include Dachshunds and Whippets

History

• Polyuria, polydipsia and polyphagia, and history of exogenous corticosteroids for e.g., allergies suggestive of hyperadrenocorticism
• Weight gain and lethargy suggestive for hypothyroidism
• History of owner using topical estrogen hormonal replacement therapy or use of estrogen for urinary incontinence, could indicate estradiol induced alopecia
• Feminisation behaviour in intact dogs e.g., nursing or attraction of male dogs to other males could indicate sex hormone dermatosis
• History of seasonal alopecia and regrowth could indicate recurrent cyclic flank alopecia
• Prior stressful events e.g., disease, surgery, or pregnancy could suggest anagen (alopecia within days) or telogen (usually 1–3 months prior to alopecia) effluvium

Physical and Dermatological Examinations

• Thinning of skin, comedones, hepatomegaly, pendulous abdomen, and calcinosis cutis suggestive of hyperadrenocorticism
• Overweight, bradycardia, and facial myxedema suggestive of hypothyroidism
• Gynecomastia, pendulous prepuce, vulvar enlargement, and/or preputial dermatosis suggestive of hyperestrogenism
• Well demarcated alopecia and hyperpigmentation affecting lateral flanks/thorax suggestive of recurrent cyclic flank alopecia
• Alopecia affecting only black coloured hair is suggestive for black hair follicular dysplasia
• Palpate testes for asymmetry suggestive of testicular tumour

Diagnostic Tests

• Based on clinical suspicion to allow ranking of ddx and diagnostic steps
• Trichogram
• Irregular pigment granules along hair shafts suggestive of colour dilution alopecia
• All hair bulbs in telogen phase for endocrine alopecia
• Complete blood count, biochemistry, urinalysis and culture may provide clues but are not specific e.g.
•  Stress leukogram with elevated liver enzymes (especially ALP), hyposthenuria or isosthenuria for hyperadrenocorticism
• Non-regenerative anemia and elevated fasting cholesterol for hypothyroidism
• Anemia, thrombocytopenia, leucocytosis, or leucopenia due to myelosuppression by estrogen

Endocrine Testing

• Hypothyroidism e.g., fT4 (ED), TT4, TSH
• Hyperadrenocorticism: Screening (e.g., LDDST, ACTH simulation test) and differentiating tests (e.g., LDDST, eACTH, abdominal ultrasound evaluating adrenal glands)
• Baseline and post ACTH stimulation tests for sex hormones (estrogen levels may be normal due to overlap between healthy vs. affected dogs)
• Typically, if endocrine tests are negative/normal, the next step is to perform skin biopsies for histopathology and/or castration or ovariohysterectomy with histopathology of testes/ovaries

References

1.  Wiener DJ, Rufenacht S, Koch HJ, Mauldin EA, Mayer U, Welle MM. Estradiol-induced alopecia in five dogs after contact with a transdermal gel used for the treatment of post-menopausal symptoms in women. Vet Dermatol. 2015;26:393–396.

2.  Berger DJ, Lewis TP, Schick AE, Miller RI, Loeffler D. Canine alopecia secondary to human topical hormone replacement therapy in six dogs. J Am Anim Hosp Assoc. 2015:51:136–142.