Abstract
The major genetic group of Toxoplasma gondii, known as type I, generally displays high lethality in laboratory Mus musculus (mouse) strains, with few exceptions. However, because rodents are the primary reservoir hosts for T. gondii, if this characteristic manifests in the wild, type I strains would be extinct. Therefore, we hypothesized that populations of wild rodents capable of harboring type I T. gondii asymptomatically exist globally and are not limited to a few localized areas, as previously thought. The strength of mouse resistance to T. gondii is known to depend on the affinity of the mouse-expressed immunity-related GTPases B2 (IRGB2) protein for the T. gondii-expressed rphoptry protein 5B (ROP5B) protein. Therefore, the Irgb2 gene sequences of 12 individuals mice captured at two animal farms in Gifu Prefecture, and on an island in Okinawa Prefecture, Japan were determined, and subjected to a molecular phylogenetic analysis together with those of various mouse strains worldwide. The Irgb2 gene of M. musculus individuals captured on one farm and one island showed diverse sequences. The sequences from two individual mice captured in an animal farm formed a single clade with a wild mouse derived CAST/EiJ strain, known for its exceptional resistance to type I T. gondii lethality. These results suggest that M. musuculus individuals resistant to the Type I T. gondii strain may be present in Japan, in addition to the previously known populations in South Asia, Thailand and India.