Page 1496 - Clinical Small Animal Internal Medicine

1434 Section 12 Skin and Ear Diseases

Table 163.1 Differential diagnoses for inflammation‐induced
VetBooks.ir Common Less common Uncommon
alopecia

Pyoderma Idiopathic vasculitis Paraneoplastic
Demodicosis Vaccination‐induced alopecia
Dermatophytosis vasculitis Alopecia areata
Sebaceous adenitis Drug reaction Pseudopelade
Systemic lupus
erythematosus
Dermatomyositis

Figure 163.2 Adult male castrated toy poodle with focal alopecia
secondary to rabies vaccination‐induced vasculitis. Dog was
referred for nonresponsive dermatophytosis because of the
circular lesion.

is started. For some breeds such as the malamute, this
may be a few years. The telogen hairs are well anchored
and do not epilate easily. Thus, the presence of telogen
hairs is not necessarily pathologic. Dogs that require fre-
quent hair clipping, such as poodles, have a hair cycle
more similar to people in that they have a prolonged ana-
gen phase with fewer hairs in telogen at any point in
time. These dogs are more likely to develop alopecia
Figure 163.1 A 3‐year‐old male castrated English bulldog with from chemotherapeutic agents which inhibit actively
patchy alopecia of the trunk associated with sebaceous adenitis. dividing cells. Shedding, or exogen, is separate from the
hair cycle and is independent of whether a new hair cycle
is starting.
Noninflammatory Alopecia
Alopecia may also occur as a consequence of abnormal Alopecia Associated with Abnormal Hair Growth
hair cycling or hair development. These are grouped Abnormalities in hair growth occur during the anagen
under noninflammatory causes of alopecia and include phase of the hair cycle. There are only a few conditions
endocrine diseases as well as congenital/hereditary and that fall into this category and they include follicular
idiopathic causes of alopecia. To understand these con- dysplasias, congenital hypotrichosis, color dilution alo-
ditions, it is important to understand the hair cycle. pecia, and black hair follicular dysplasia. Follicular
Briefly, the anagen phase of the hair cycle is when the dysplasias, where abnormal follicles fail to produce
hair is actively growing. In most dogs and likely all cats, healthy anagen hairs, are most often associated with
this is a relatively short phase lasting at most a few alopecic breeds (e.g., Chinese crested dog, Mexican hair-
months until the hair reaches its maximum length. The less dog, and the Sphynx cat) in which the alopecia is a
length of the anagen phase, and thus the length of hair, is breed characteristic. Congenital hypotrichosis is poorly
genetically determined and may be of different durations described in small animals. The hypotrichosis/alope-
for different regions of the body. From anagen, the hair cia occurs either at birth or within the first six months of
goes through a short catagen phase in which it reverts life and may be associated with other ectodermal
into a nongrowing or telogen hair. The hair can remain in defects. Color dilution alopecia and black hair follicular
telogen for a prolonged period of time before a new cycle dysplasia are color‐restricted alopecias that begin