REPORTS Demodex musculi in the Skin of Transgenic Mice LORI R. HILL, DVM,1 PAM S. KILLE, LAT,1 DALE A. WEISS, LATG,1 THOMAS M. CRAIG, DVM, PHD,2 AND LEZLEE G. COGHLAN, DVM1 Abstract ͉ 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 demon- strated variation in mite transfer and in either detection or maintenance of infestation when naïve 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 patho- genicity in the examined genotypes. Demodex mites, a diverse group of permanent semiendopara- copulatory transfer (26–29). Copulatory transfer was also postu- sites of mammals, are host- and microhabitat-specific. Demodex lated for D. flagellurus, a parasite residing in the preputial and have been isolated from humans, dogs, cats, cattle, swine, horses, clitoral glands of feral Mus musculus (22). In contrast, several sheep, goats, and numerous other animals (1). Among labora- investigators have shown that Demodex mites found in tory animals, Demodex has been described in the gerbil (2, 3), extracutaneous areas are usually nonviable, probably resulting golden hamster (4–9), rat (10–12), and rabbit (13). In mice, at from lymphatic drainage of the affected region of skin (30, 31). least four (possibly five) species of Demodex have been reported, How Demodex organisms physically move between hosts and with no contemporary accounts in laboratory mice (10, 14–18). whether this movement involves adult or immature stages have The immunodeficient SCID-beige mouse, when engrafted with not yet been documented. Although most authors regard the dog skin and infected with D. canis, has been used to study ca- presence of small numbers of Demodex mites as harmless com- nine demodectic disease. The lack of mite translocation to mouse mensals (1, 20, 30, 32, 33), descriptions of cutaneous disease are tissues in this model further affirmed the remarkable host speci- common, especially in the dog, where generalized demodicosis ficity of Demodex (19). can be fatal (1, 24, 30, 33–36). Historically, disease states have The various species of demodectic mites reside not only in been thought to be associated with excessive numbers of prolif- the general epidermis but also in the excretory ducts of seba- erating mites, especially the immature forms represented by egg, ceous glands associated with hair follicles and within the larva, and nymph (30); however, the actual correlation between secretory and excretory portions of specialized glands, such as mite numbers and clinically apparent disease remains unclear the ceruminous glands of the ear canal and the meibomian (23). Decreased host immunity is thought to allow for progres- glands of the eyelids (11, 14–16, 20, 21). In mice, specimens sion to disease states (36, 37). have been recovered from the stratified epithelia of the oral This communication describes our discovery of D. musculi in cavity, tongue, and esophagus and from the ears, skin, eyelids, the skin of B6,CBA-TgN(CD3E)26Cpt (CD3E), a homozygous and preputial and clitoral glands (10, 14–18, 22). The speci- transgenic (Tg) mouse lacking mature T lymphocytes and natu- mens recovered appear to represent unique, anatomically ral killer cells (38); B6,SENCARB-TgN(pk5prad1)7111Sprd specific Demodex species. (Prad1), a Tg mouse overexpressing human cyclin D1 and mani- Despite more than 150 years of collective investigation, very festing severe thymic hyperplasia (39); and the double-Tg F1 little is known about demodectic mite transmission and the fac- offspring of these two lines. Demodex was not recovered from the tors regulating subclinical carriage as opposed to clinically room sentinels—SSIN/UTSP//Vsp (SSIN; 40)—or from other apparent demodicosis (19, 20, 23). In the dog, cow, and golden genotypes within the facility. In addition, we present the pre- hamster, mites were transmitted to neonates by direct contact liminary results of transmission experiments with the sentinel with the dam during the first few days of life (7, 24, 25). The strain and several immunologically impaired mouse strains. finding of demodectic mites in the internal tissues and genitouri- nary systems of dogs and goats have suggested intrauterine and Case Report In May 1998, mice with a history of Radfordia affinis infesta- The University of Texas M. D. Anderson Cancer Center, Science Park, Depart- tion were being monitored for the presence of mites during the ment of Veterinary Sciences, Route 2, Box 151-B1, Bastrop, Texas, 786021, course of an experimental acariasis treatment. This treatment Department of Veterinary Pathobiology, The Texas Veterinary Medical Center, involved a novel use of an avermectin compound and was taking Texas A&M University, College Station, Texas, 77843-44672 place simultaneously in several rooms in the facility (41). An Volume 38, No. 6 / November 1999 CONTEMPORARY TOPICS © 1999 by the American Association for Laboratory Animal Science 13 FIG. 1. Photomicrograph of Demodex musculi located superficially in the hair follicle of a CD3E mouse. Note the lack of inflammatory response FIG.2. Demodex musculi adult female from the skin of a CD3E mouse. in the adjacent tissues. Hematoxylin and eosin; 500X magnification. Wet mount; 400X magnification. external parasite examination was performed by plucking hairs Wayne® Autoclavable Rodent Blox® #8656 (Harlan Teklad, Madi- from the dorsum of the neck, placing them in mineral oil on a son, WI) and autoclaved, reverse osmosis water were provided slide, and observing the slide under a dissecting microscope. ad libitum. Room temperature and humidity were 20 to 22ЊC Radfordia was present in samples collected from all four geno- and 55%, respectively, with a 12-h light/dark cycle and 15 air types of mice in the room. The samples collected from CD3E changes per hour. Incoming animals, including sentinels, un- mice contained several unidentified, cigar-shaped mites typical derwent a 3-week quarantine and were determined to be free of of Demodex. With repeated examinations, a few Demodex were even- major pathogens prior to release to investigation. Representa- tually recovered from two of the other genotypes, Prad1 and the tives from each shipment underwent a tape test of the anus, fecal double-Tg F1 hybrid offspring of CD3E and Prad1. examination (gross examination of cecal contents, fecal concen- Cellophane tape tests of the muzzles and rear limbs of pups tration, and wet mount for protozoa), pelt examination, gross that were 1 or 2 weeks of age and from the infected cages were necropsy, and serum collection for at least a six-pathogen en- negative for demodectic mites. Demodex was not recovered from zyme-linked immunosorbent assay panel (Sendai virus, the room sentinels, SSIN mice, despite their receiving soiled Pneumonia virus of mice, Mouse hepatitis virus, Mycoplasma bedding twice weekly from the cages of the three other geno- pulmonis, Theiler’s murine encephalomyelitis, and Minute virus types. When tested further in June 1998, these sentinels were of mice). negative for all assayed pathogens except Radfordia; Demodex was Shipments of the CD3E mice (The Jackson Laboratory, Bar not detected by examination of the pelt or by histopathologic Harbor, ME) had been received periodically beginning in June examination of the skin. 1995. After release from quarantine, CD3E mice were segregated Because the species of Demodex was not yet determined and into two rooms. One room remained a closed breeding colony, the involved mouse genotypes had immune and/or thymic ab- and Demodex has not been detected by gross or microscopic ex- normalities, we considered that the mice could be serving as amination in these mice or in subsequent shipments from the accidental hosts or that the mites could have been aberrantly same vendor. The CD3E mice in the other room were used for located in these transgenic lines. Therefore we examined the outcrossing and subsequent experimentation. Radfordia was be- anatomic locations in which Demodex mites had been previously lieved to have been introduced by the Prad1 mice, whose reported in the mouse (10, 14–18, 22). The examination was Radfordia infestation went undetected at the time of their trans- performed on two animals that were positive for Demodex and fer into the index room in January 1996. The quarantine period included the preputial or clitoral gland and serial sections of and screening of the Prad1 founders (B6,SJL-TgN the decalcified head including the oral cavity, tongue, esopha- (pK5prad1)7111) were as described.