Two species of parasitic mites cause skin disease in cats. Demodex cati is thought to be a normal inhabitant of feline skin. It is a follicular mite, similar to but narrower than the canine mite. The other species of Demodex (D gatoi) is shorter, with a broad abdomen, and is found only in the stratum corneum. The mite may also inhabit the external ear canals and cause a ceruminous otitis externa, which is variably pruritic.
The mite is part of the normal microflora of the feline skin, but the reproductive process may liberate aspects which liberate antigens leading to the inflammatory process seen with the disease. It is often associated with underlying systemic disease such as FeLV, FIV, neoplasia such as lymphoma, diabetes mellitus or concurrent corticosteroid therapy.
In D. cati, transmission appears to be vertically from queen to kittens, probably during the sucking period before weaning. D. gatoi is unique amongst all Demodex species to require only casual contact to spread, and thus is horizontally transmitted.
In localized demodicosis, there are one or several areas of focal alopecia on the head and neck. The habitat of D. cati is the hair follicle and sebaceous glands and ducts. Like its canine counterpart, D. cati provokes follicular inflammation, and in doing so may result in any combination of alopecia, follicular plugging (comedones), scale, crusts and even erosions and ulceration. Large numbers of mites tend to be found at the site of an exploding mite population. Otitis may be present as the sole clinical manifestation of the disease or in conjunction with more widespread lesions.
In generalized disease, pruritus, alopecia, crusting, and secondary pyoderma of the whole body are seen. The generalized form has also been associated with other systemic diseases, especially diabetes mellitus. In some cases, ceruminous otitis externa is the only clinical sign. Pruritus is variable; both species can cause similar disease, but cats infested with D. gatoi are more frequently pruritic.
Diagnosis is made by deep skin scrapings, although mite numbers are often small. Medical evaluation is indicated in cats with generalized disease. Dermatophyte cultures are essential, because dermatophytosis and demodicosis can be concomitant conditions.
Differential diagnoses would include ringworm, pemphigus foliaceous, bacterial folliculitis, Notoedres spp, Otodectes spp and some of the more unusual causes of erosive and ulcerative dermatitis such as squamous cell carcinoma and epitheliotrophic T-cell lymphoma.
Prognosis of generalized demodicosis is unpredictable because of its potential relationship with systemic disease. Some cases spontaneously resolve.
Weekly lime-sulfur dips (2%) are safe and usually effective. The cat is usually left to soak in the solution for a minimum of 5 minutes. The solution should not be washed off and allowed to air dry. An Elizabethan collar is recommended to prevent ingestion of the wash. Lime sulfur may cause staining of the coat and is malodorous while wet.
Weekly ivermectin injections have been reported as ineffective, but daily or every second day oral dosing with ivermectin at 300 μg/kg PO is effective in treating both species of mites. For D. cati, treatment should be continued until negative scrapings are obtained. For D. gatoi it is recommended to continue for 2 weeks after negative scrapings are obtained. Because of the potentiality for toxicity with ivermectin (including Heinz-body anaemia resulting from the propylene glycol vehicle), lime sulfur is preferred over this treatment.
Amitraz (0.025-0.05%) has been used, but is not approved for use in cats and can cause anorexia, depression, and diarrhea.
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