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Approach to Alopecia
Linda A. Frank, DVM, MS, DACVD

Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee Institute of Agriculture, Knoxville, TN, USA

Etiology/Pathophysiology alopecic sites but not from other locations on the body.
While pruritus can be associated with these diseases and
Alopecia or hypotrichosis are common reasons for may even be directed at the lesions, it is not the cause of
presentation to a veterinarian. Alopecia implies complete the alopecic spots.
hair loss while hypotrichosis implies thinning of the hair Vasculitis from immune‐complex deposition second-
coat. The presentation of alopecia/hypotrichosis may be ary to infectious or immune‐mediated diseases may
localized or generalized and focal, patchy or diffuse, cause multifocal alopecia due to local ischemia. Clinically,
depending on disease severity, duration, and the cause of the lesions may resemble dermatophytosis in that they
the hair loss. Alopecia may be traumatically induced, with are circular and alopecic; however, they appear nonin-
the patient scratching or licking the hair off, or due to hair flammatory, lacking erythema and crusting with thin
falling out or failing to regrow following normal shedding. rather than thickened skin, unless cutaneous necrosis
and ulceration occur (Figure 163.2).
A common example of focal ischemic alopecia is rabies
Trauma‐Induced Alopecia vaccination‐induced vasculitis. Vasculitis may also be
Trauma‐induced alopecia is most often secondary to seen with drug reactions, tick‐borne diseases, and sys-
pruritus. Common causes include allergies, ectopara- temic lupus erythematosus. Dermatomyositis is an inher-
sites, bacterial skin infections, and Malassezia dermati- ited (autosomal dominant with variable penetrance)
tis. Less common causes in older animals include disorder that primarily affects collies and Shetland sheep-
neoplastic diseases such as epitheliotropic lymphoma or dogs. Vasculitis and ischemic dermatitis are thought to be
mast cell tumor and paraneoplastic syndromes. part of the pathogenesis of the disease.
Autoimmune/immune‐mediated diseases such as alope-
cia areata or pseudopelade and neoplastic conditions such
Inflammation‐Induced Alopecia
as epitheliotropic lymphoma could result in areas of
Alopecia not associated with trauma may be due to an inflammatory alopecia. Alopecia areata is an immune‐
inflammatory condition, such as pyoderma, follicular mediated attack on the hair bulb while pseudopelade is an
demodicosis, dermatophytosis, or sebaceous adenitis immune‐mediated attack on the follicle wall, resulting in
(Table 163.1). focal to multifocal alopecia. Clinically, the areas of alopecia
Sebaceous adenitis is an immune‐mediated destruc- may not show evidence of erythema, suggesting a nonin-
tion of the sebaceous glands and is associated with scal- flammatory mechanism; however, the pattern of alopecia
ing and alopecia. Breed predilections include the necessitates investigation into inflammatory differentials.
standard poodle, Samoyed, akita, and vizsla. In short‐ Paraneoplastic alopecia occurs in association with
coated dogs, these conditions present as a patchy or internal neoplasia. This is a presumed inflammatory alo-
“moth‐eaten” pattern of alopecia (Figure 163.1). pecia although the pathomechanism of the alopecia is
The lesions might not overtly appear inflammatory; unknown. Paraneoplastic alopecia has been described in
however, the multifocal pattern of the alopecia should cats in association with pancreatic and biliary carci-
make you suspicious of an inflammatory cause. Hairs may noma. The alopecia is acute and progressive with easy
epilate easily from the periphery of these inflammatory epilation of hair and smooth thin skin.

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical