ETYMOLOGY:Demos = tallow and dex = woodworm; this mite has not been assigned a specific name.
SYNONYMS: This mite has been recognized as separate from Demodexcati, but no specific name has been given.
HISTORY: Conroy et al. (1982) sent specimens of a Demodex recovered from a cat to Drs. Nutting and Desch for identification. They were informed that the speciments appeared distinct from Demodex cati and were similar in appearance to Demodexcriceti which resides in the stratum corneum of the hamster. Chesney (1989) in his review of feline demodicosis summarized seven reports of feline infestations with this mite. Chesney also felt that the reports by Gabbert and Feldman (1976) and White and Ihrke (1983) probably dealt with this unnamed species based on the photographs of the mite that accompanied these case presentations. Chesney went on to discuss the report of Keep (1981) and stated that that these mites were also likely to be the unnamed species based on the description of the mites provided by Keep. Since the review of Chesney, additional cases of demodicosis due to this unnamed species have been reported, 3 cases by Guaguère (1993) and 3 cases by Morris (1996).
GEOGRAPHICAL DISTRIBUTION: This unnamed species of mite has been reported from the United States (8 cases), from Europe (5 cases), and from Australia (one case). Chesney (1989) reports that the unnamed species occurred in the USA in a domestic longhaired cat (Trimmier, 1966); a Siamese cat (Gabbert and Feldman, 1979), a Siamese cat (Conroy et al, 1982), a domestic shorthaired cat (Muller, 1983), a domestic shorthaired cat (McDougal and Novak, 1986), and in two domestic shorthaird cats (Medleau et al., 1988). Morris (1996) describes infection in three Siamese cats in the United States. Chesney (1989) describes two cases from Europe: one from France in a domestic shorthaired cat (Carlotti et al., 1986) and one from England in a domestic longhaired cat (Chesney, 1988). Guaguère (1993) has described three additional cases in domestic shorthaired cats in France. One Australian case is that of Wilkinson (1983) in a domestic shorthaired cat. Thus, of the 14 cases described, 5 have been described in Siamese cats, and all cases in Siamese cats have occurred in North America.
LOCATION IN HOST: The unnamed Demodex that occcurs is cats is found in the stratum corneum rather than in hair follicles. Two other species of Demodex that have been described from this location in their host include Demodexcriceti of the hamster (Nutting and Rauch, 1958) and a species of Demodex which parasitizes Onychomysleucogaster, the grasshopper mouse (Nutting et al., 1973).
IDENTIFICATION: Chesney (1988) described two forms of the parasite, a longer form that measured about 143 to 148 m in total lenght (gnathosoma 17 to 19 m; podosoma 56 to 58 m; and opisthosoma being 66 to 73 m) with a maximum width of 37 to 41 m. The shorter form was about 110 m long and 28 m wide (gnathosoma 16.5 m, podosoma 47 m, opisthosoma 47 m). Foley (1995) gives measurements of 80 to 90 m by 30 to 35 m. In the undamed Demodex of the cat, the opisthosoma makes up around 40% of the total body length, and in some of the illustrations it appears that significantly less than 40% of the total body length is made up of the opisthosoma. In Demodexcati, the opisthosoma makes up about two-thirds of the total body length. There is no anus on any of the life cycle stages in the genus Demodex.
LIFE CYCLE: The location of the mite in the stratum corneum and the morphological resemblance to Demodexcriceti has suggested that the biology is similar to that described for Demodexcriceti. Studies with Demodexcriceti have indicated that this mite is transferred from mother to young during suckling. Very little else is known concerning this mite, and the different life-cycle stages have not been described in detail. It is assumed that the life cycle is similar to that described for Demodexbrevis. Thus, a six-legged larva hatches from the egg, develops to a six-legged protonymph, which matures to a nymph, which then matures to an adult male or female (Nutting, 1983). Work with Demodexcaprae of the goat has revealed that only males will develop from fertilized females and that the males are haploid and the females diploid. It is believed that this form of development may be considered advantageous to transfer because a single female, fertilized or not, could ultimately establish a complete colony of mites on a new host. The span of time per stage or for the entire life cycle is not assurred for any species of Demodex (Nutting, 1983). Demodexauratus of the golden hamster, Mesocricetusauratus, can build up to large numbers of all life stages in 35 days; Demodexfollicularum has been estimated to complete its cycle in 14 ½ days; and Demodexcaprae has been estimated to undergo a complete cycle in several to 15 weeks (Nutting, 1983). Morris (1996) suggests that a cat which began to cohabitat with two infested cats contracted its infestation from these already infested cats.
PATHOGENESIS AND CLINICAL SIGNS: In the 13 cases of demodicosis due to the unnamed species of Demodex, Chesney (1989) considered 10 of the cases to represent generalized demodicosis. The 3 cases described by Guaguère (1993) were all of the localized type; while in the 3 cases described by Morris (1996), numerous mites were obtained from superficial skin scrapings of multiple sites on all animals. This mite typically induces pruritis, excessive grooming, alopecia, scaling, hyperpigmetation, erythema, and excoriation associated with self-abuse with signs suggestive of flea allergy dermatitis, atopy, immune-mediated or eosinophilic skin disease, food allergy, contact dermatitis, notoedric mange, or diabetic neurodermatitis (Foley, 1995).
Many of the cases of infestation with the unnamed species of Demodex have been diagnosed after treatment with corticosteroids for the initial presentation of skin lesions, e.g., Chesney (1988); Conroy et al. (1982); Medleau et al. (1988); Morris (1996); Wilkinson (1983). Morris (1996) felt that exogenous corticosteroid therapy as an immunosuppressive factor must remain speculative. Signs in one of his three cases appeared prior to the corticosteroid injections and the signs resolved after lime-sulfur treatment. Two other cases were shown to actually have developed in cats that had an underlying food allergy. Human cases of demodicosis have developed following the long-term administration of corticosteroid therapy (Hakugawa, 1978; Sato et al., 1965); and rosacea-like demodicosis has been reported in HIV-positive children (Barrio et al., 1996). It is assumed that the mites are already present in these cases and that the immunotherapy causes the mites to increase in numbers.
TREATMENT: Foley (1995) recommends that cats infested with the unnamed species of feline Demodex be treated with parenteral ivermectin (300 g ivermectin per kg bodyweight) along with 2.5% l;ime-sulfur immersion. He believes the mite is vulnerable to treatment with the lime-sulfur regimen alone in many cases because of this mites localization in the stratum corneum. Foley states that the prognosis for cats infested with this mite is good.
EPIZOOTIOLOGY: Very little is known about the epizootiology of these mites. The report by Morris (1996) would suggest that infestations can be transferred between cats, but this has not been demonstrated in other situations.
HAZARDS TO OTHER ANIMALS: It is believed that this mite is specific for the fieline host.
HAZARDS TO HUMANS: It is believed that this mite does not infest humans.
CONTROL AND PREVENTION: Control is by the isolation and treatment of infested cats. It is expected that cats are infested while nursing although there is no direct evidence that this occurs.
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