Demodex musculi in the Skin of Transgenic Mice.

Although infestations by a number of Demodex mite species have been described in mice, the occurrence of Demodex musculi infestation was last reported by Hirst in 1917. This communication describes the occurrence of D. musculi infestation in two lines of transgenic mice and their F1-hybrid offspring. We first found the Demodex mite in mouse hair samples collected during efficacy screenings in an ongoing ectoparasite treatment trial for the fur mite Radfordia affinis. An investigation was undertaken to determine the extent of the Demodex infestation within the facility and the original source of the parasite. D. musculi was found in three of the four mouse genotypes present in the index room and in one of these genotypes in two other rooms. The mite was not found in sentinel mice, other strains, or stocks within the facility. The mites were more easily recovered from the immunodeficient B6,CBA-TgN(CD3E)26Cpt transgenic (Tg) and the hybrid double-Tg (B6,CBA-TgN(CD3E)26Cpt x B6,SENCARB-TgN(pk5prad1)7111Sprd)F1 mice than from the B6,SENCARB-TgN(pk5prad1)7111Sprd Tg mouse, which is believed to be immunocompetent despite its thymic abnormalities. Histopathologic examination showed D. musculi superficially in hair follicles but not in the preputial or clitoral gland or in serial sections of the head, eyelids, or ears, the locations favored by other mouse demodicids. Physical and microscopic examination revealed no dermatitis. The immune deficiency in the B6,CBA-TgN(CD3E)26Cpt mouse probably provided the permissive host conditions that contributed to the proliferation and subsequent detection of the Demodex. Preliminary transmission experiments conducted with other immunologic mutant mice and our sentinel strain demonstrated variation in mite transfer and in either detection or maintenance of infestation when na ve mice were housed with those carrying D. musculi. The original source of D. musculi was not conclusively identified, but this parasite appears to be of low pathogenicity in the examined genotypes.