In many countries of East Asia, some people are fond of eating raw or undercooked meat and viscera of domestic and wild animals. These eating habits are important in the transmission of taeniasis (Fan et al., 1992b). Although meat and viscera are commonly eaten and Cysticercus cellulosae is frequently found, but T. saginata rather than T. solium is the dominant species (Cho et al., 1967; Huang et al., 1966; Kosin et al., 1972; Arambulo et al., 1976). In 1992, Eom and Rim proposed this causative agent as a new species of T. asiatica. After considering the closeness of this parasite with the classical T. saginata, we recently proposed this causative agent of taeniasis as a subspecies of T. saginata and named T. s. asiatica. The classical T. saginata was renamed as T. s. saginata (Fan et al., 1995).

Cysticerci of T. s. asiatica have been mostly recovered from the liver of pig, cattle, goat, monkey and wild boar (Fan et al., 1987; Chan et al., 1987). However, pig has been determined to be the favorable intermediate host of this new subspecies (Fan et al., 1990). In addition, some cysticerci in the liver of these intermediate hosts can develop to be degenerated or calcified within one month (Fan et al., 1995). Since this human taeniid cestode has a very high host-specificity, experimental infections can only be repeated in cattle or pigs and it is not easy to obtain information on the biological factors of these tapeworms. Therefore, the establishment of a model with small size animal should be beneficial to the understanding of their developmental biology, immunodiagnosis, pathology, vaccine development, and evaluation of taenicides. Recently, oncospheres of T. s. asiatica and T. solium have been reported to develop into cysticerci in severe combined immunodeficiency (SCID) mice. The mature cysticerci in the subcutaneous tissues of the SCID mice remained viable five months after infection (Ito et al., 1997a, 1997b). Moreover, we have succeeded in establishing a rodent model for study of taeniasis using immunosuppressed or normal mice (Wang et al., 1999a) and infecting hamster with rodent-derived cysticerci to sexual mature worms of T. solium (Wang, 1999b). However, no detailed information on the morphological characteristics of T. s. asiatica cysticerci from SCID mice is available. In the present study, we determined the long period of time for viable cysticerci of T. s. asiatica in SCID mice and also studied the morphological aspects of this metacestode in the rodent model.

T. solium, T. s. asiatica, and T. s. saginata may employ the pig as an intermediate host. After mature eggs are ingested by the intermediate host, oncospheres hatch from its membrane in the intestinal tract and penetrate the intestinal wall into blood vessels. Through the blood stream, oncospheres of T. solium are carried to various muscles (Beaver et al., 1984) and those of T. s. asiatica and T. s. saginata to the liver (Fan et al., 1992b, 1996). They develop into cysticerci at these sites. Therefore, collection of cysticerci for experimental infection studies require pigs. Fortunately, Ito et al. (1997a) recently succeeded in establishing a SCID mouse model for the development of cysticerci of T. solium and T. s. asiatica. This model has an advantage that the cysticerci of these two parasites remained viable five months after experimental infection. Moreover, we have demonstrated that cysticerci of T. solium and T. s. asiatica not only can develop in immunodeficient mice but also in immmunosuppressed mice and even in normal mice (Wang et al., 1999a). We also found that C57 mice is the most suitable laboratory intermediate host for T. solium among the immunosuppressed mice, since it has a high cysticercus recovery rate of 2.4%. In addition, normal C57 mice can also harbor cysticerci in their subcutaneous tissue, although the cysticercus recovery was relatively low (0.2%). In the establishment and maintenance of T. solium or T. s. asiatica cysticerci in immunosuppressed or normal mice, less efforts and costs are required (Wang et al., 1999a).

In the present study, we found that cysticerci remained viable on day 244 after infection. These cysticerci had a large diameter of 4 (1-6) mm. Moreover, we obtained a high cysticercus recovery rate of 4.6% in a SCID mouse sacrificed on day 62. These findings indicate that the rodent model can be employed not only in the study of the developmental biology, immunodiagnosis, host-parasite relationship, and vaccine development, and of human taeniid cestodes but also can be used as a tool for long-term maintenance of the viable biological materials. In addition, the cysticerci in SCID mice in this study did not become calcified/degeneration after a long period of 244 days. This interval was much longer than that reported by Ito et al. (1997a, 1997b). T. s. asiatica cysticerci recovered from SCID mice in the present study were also much larger than those from pigs (Fan et al., 1995). The increase in size of the cysticerci was found to be proportional to the days of infection. Although Ito et al. (1997a, 1997b) reported that no rostallar hooks were observed on the scolex of cysticerci of T. s. asiatica from SCID mice, we found that there were two rows of rudimentary hooks on the scolex of the cysticerci (Fig. 21). The large inner hooks were 16 (range: 6-25) in mean number and 9 (range: 3-18) µm in length. These findings were similar to the cysticerci obtained from the pig's liver (Fan et al., 1995). It has been suggested that the size of the cysticercus in the intermediate mammalian host might be controlled by some immune response which can not kill the established larvae (Mitchell et al., 1977; Lucas et al., 1980; Ito, 1985; Ishiwata et al., 1992; Dixon and Jenkins, 1995). However, further studies are required to confirm these suggestions. Moreover, in our recent experimental study, we found that the normal and immunosuppressed mice (C3H, C57, and ICR strains) have been demonstrated to be useful as a tool for maintenance of the viable cysticerci of T. solium and T. s. asiatica living longer than one year (Wang et al., unpublished data).