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Parasitic Skin Diseases

Elizabeth E. Toops, DVM, MS, DACVD (Dermatology)

Virginia Veterinary Specialists, Charlottesville, VA, USA

Demodicosis the development of adult‐onset demodicosis. Demodex
injai has only been reported in adult dogs.
Canine Demodicosis The reason for development of demodicosis in some
dogs versus others is not precisely known. We do know
Etiology/Pathophysiology that certain breeds develop disease more commonly than
Canine demodicosis is currently known to be caused by others, suggesting the influence of genetics. Breeds more
three species of mites: Demodex canis, D. injai, and D. cor- commonly diagnosed with demodicosis caused by
nei. Demodex canis and D. injai are thought to be two sepa- D. canis include the shih tzu, West Highland white terrier,
rate species. However, D. cornei, a short‐bodied mite, is English bulldog, and Boston terrier. In the author’s expe-
believed to be a morphologic variant of D. canis. Demodex rience, pit bull terriers and Yorkshire terriers also seem
canis and D. injai live within the hair follicles or sebaceous overrepresented. Demodicosis caused by D. injai has
glands, and D. cornei lives on the stratum corneum. been noted at greater frequency in terriers and shih tzus.
Recent studies using real‐time polymerase chain reac- Other factors suggested to play a role include parasit-
tion (PCR) have validated the widely held belief that all ism, inadequate nutrition, and stress. However, many
dogs, healthy and diseased, harbor D. canis mites in their affected patients do not have these problems. Various ex
skin. Puppies are believed to acquire D. canis from their vivo studies have investigated the immune response of
mothers at birth. However, the mode of transmission of dogs with juvenile‐onset generalized demodicosis caused
D. injai and D. cornei is currently not known. by D. canis, but a defect has yet to be identified.
Demodex canis mites have a life cycle involving four Nevertheless, the current belief is that these dogs inherit
stages. They begin as fusiform eggs. These eggs develop an immune dysfunction of variable severity linked to a
into six‐legged larvae, then eight‐legged nymphs, subset of T cells capable of recognizing specific mite
followed by the adults. The life cycle of D. cornei and antigens and keeping the mites in small numbers. This
D. injai has not been investigated but it is likely to mirror theory, however, needs to be validated. Overexpression
the life cycle of D. canis. Adult D. injai mites are much of the Toll‐like receptor 2 gene may result in the clinical
longer than the D. canis species (Figure 165.1). Adult findings associated with demodicosis.
D. cornei mites have a short tail (Figure 165.2).
Demodicosis caused by D. canis can occur in juvenile
(≤18 months) or adult dogs (>4 years). Older dogs that History and Clinical Signs
develop demodicosis may have another disease process There are two types of infestations of Demodex mites:
leading to an immune dysfunction that allows the mites localized and generalized. The localized variant consists
to multiply and cause disease. Some examples include of one or more lesions usually on the face and forelegs.
hyperadrenocorticism, hypothyroidism, diabetes melli- These cases typically self‐resolve and affect puppies. An
tus, or malignant neoplasia. Immunosuppressive therapy infestation is considered generalized when large body
can also result in a proliferation of mites. This includes areas, several localized lesions, an entire body region, or
topical and oral corticosteroids and oclacitinib maleate two or more paws are affected. This form primarily
(Apoquel® for dogs; Zoetis). Allergic dermatitis is often a begins at the puppy stage and may resolve, but typically
concurrent diagnosis. Often, no cause is identified for progresses without treatment as the puppy ages.

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