Chapter 37: Feline Demodicosis

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Feline Demodicosis



Feline demodicosis is considered a relatively rare parasitism; however, in certain regions this condition is endemic and is not uncommon. Compared with canine demodicosis, little is known about feline demodicosis. Infected cats may manifest a wide variety of clinical signs, and demodicosis should be considered in the differential diagnosis of many feline skin disorders. Three species of mites have been recognized to cause feline skin disease. Concurrent infestation with different species has been reported with all three species of feline Demodex.



Demodex gatoi



Clinical Signs and Differential Diagnosis


Demodex gatoi is a short, broad mite that inhabits the stratum corneum (the most superficial layer) of the epidermis. The mite is similar in appearance to Demodex criceti, the superficially dwelling mite causing skin disease in hamsters. The primary clinical sign resulting from infection with this mite is pruritus, and the intensity of the pruritus is suggestive of a hypersensitivity response. The most common clinical presentation is self-inflicted alopecia caused by overgrooming. The alopecia often is symmetric and typically is extensive. Most commonly the ventral abdomen, flanks, and anterior forelimbs are affected. This is more likely because these areas are most easily reached during grooming than because these areas are more commonly inhabited by the mites. However, facial and acral lesions also may occur, as can otitis externa. Other clinical signs typical of the cutaneous reaction patterns commonly associated with feline allergic dermatoses may be present: miliary dermatitis and the eosinophilic dermatitis complex (eosinophilic plaques, indolent lip ulcers, and eosinophilic granulomas).


Occult cases of D. gatoi infestation appear to be common. This is similar to the pattern in canine scabies and further supports the presence of a hypersensitivity component. The scarcity of mites on many pruritic cats with D. gatoi infestation can lead easily to a missed diagnosis. It is suspected that affected cats are physically removing and ingesting the mites by licking and overgrooming, which makes the mites difficult or impossible to find through examination of routine skin scrapings. The difficulty in detecting these mites in skin scrapings and the ease with which they can be overlooked is another significant factor that can contribute to misdiagnosis.


It is important to note that some infested cats demonstrate no clinical signs whatsoever, which further supports the possibility that the pruritus induced in affected cats may be associated with a hypersensitivity response to the parasite. In a multiple-cat household there may be a combination of cats demonstrating clinical signs and cats that are asymptomatic. The feline house mates of affected cats always should be screened for parasites. The differential diagnoses for a cat demonstrating a pattern of self-induced alopecia include flea-allergic dermatitis, atopic dermatitis, cutaneous adverse food reaction, cheyletiellosis, Otodectes cynotis infestation, and psychogenic alopecia; if miliary or plaque lesions are present, dermatophytosis, pyoderma, and cutaneous neoplasia should be considered.




Pathogenesis


The pathophysiology of D. gatoi infestation is not well known. Some cases reported in the literature were in cats that had a history of glucocorticoid therapy, although it is unclear whether this truly is a risk factor for the development of the disease. It is more likely that the cats were treated with glucocorticoids for the pruritus induced by the mite. Other sources of systemic or localized immunosuppression have been reported in conjunction with D. gatoi infestation, including retroviral infection, diabetes mellitus, and cancer chemotherapy. However, infestation with D. gatoi has been diagnosed in cats that have no history of glucocorticoid therapy and no associated systemic disease. Infested cats often have a history of roaming or exposure to other cats. Introduction of an asymptomatic infected cat from a shelter to a household with other cats may lead to disease in previously unaffected cats.


It is unknown whether D. gatoi is part of the normal microfauna of feline skin, but the author suspects that it is not. There appears to be marked regional variability in the distribution of this parasite since the condition is commonly diagnosed in the Gulf Coast region of the United States and is reported less commonly in other areas.



Diagnosis


In cases involving D. gatoi, in which occult infestation is common, detection of a single mite in the composite of multiple scrapings is diagnostic. A scraping technique similar to that used for scabies is appropriate: broad, superficial scrapings of large surface areas. Areas that are not easily reached during grooming (e.g., dorsal neck, thorax) may be more likely to produce positive results since the mites cannot be physically removed from these areas by grooming. Negative findings on scrapings cannot be relied on to rule out this condition. This is particularly true in endemic areas. Alternative diagnostic measures include fecal flotation (because pruritic cats appear to groom away and ingest the mites) and therapeutic trial of a miticide. The mites also may be found on occasion by using cellophane tape to collect samples from the surface of the skin. It is strongly recommended that all cats in contact with the affected cat be screened for infestation since some asymptomatic cats may have positive results on skin scrapings.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Chapter 37: Feline Demodicosis
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