Infection with a babesia-like organism (WA1) in selected strains of laboratory mice

A newly identified intraerythrocytic babesia-liko organism, causes infection in humans in the Western U.S. C57BL/10), BALB/c and C3H.RKK mice were inoculated intra- peritoneally with WA1 parasitized rod blood cells (PRBC). Parasitemia was estimated by couating PRBC on Giemsa-stained mouse blood smears collected at 3 day intervals after initial inoculation. C57BL/10 mice exhibited a 20% peak para parasitemia 14 days after inoculation and subsequen- tly recovered naturally from their primary infection. BALB/c mice exhibited a 30% peak parasitemia and 40% of the animals die 14-16 days after inoculation. C3H.RKK exhibited a 40% peak parasitemia. Over 95% of these animals die 14-16 days after inoculation. Red blood cell counts and hematocrit values indicated a severe anemia Also to determine whether major histocompatibility complex (MHC) genes affected the susceptibility to this parasite, we compared congenic mice differing only at the MHC. C57BL/10(H-2_b, B10.BR(H-2^k.C3H.RKK(H-2^k and C3H.SW(H-2^b were used in this study. B10.BR(H-2^k mice developed a similar peak parasitemia and subsequent outcome as C57BL/10(H-2^b. However C3H.SW(H-2^b developed a 20% peak parasitemia and all animals recovered naturally in contrast to C3H.RKK(H-2^k. These initial results indicate that susceptibility to WA1 is not only strain specific but also dependent of MHC haplotypes.