SUSCEPTIBILITY OF DIFFERENT STRAINS OF MICE TO HEPATIC INFECTION WITH PLASMODIUM-BERGHEI

Despite the low susceptibility of BALB/c mice to hepatic infection by Plasmodium berghei, this animal model is routinely used to investigate the basic biology of the malaria parasite and to test vaccines and the immune response against exoerythrocytic (EE) stages derived from sporozoites. A murine model in which a large number of EE parasites are established would he useful for furthering such investigations. Therefore, we assayed six mouse strains for susceptibility to erythrocytic and hepatic infections. The administration of 50 sporozoites by intravenous inoculation was sufficient to establish erythrocytic infections in five of five C57BL/6 mice compared with 10,000 sporozoites required to infect 100% of BALB/c mice. To assay for hepatic infections, mice received an intravenous inoculum of 10(6) sporozoites, and liver sections for light microscopy and histology were obtained at 29 and 44 h postinoculation. EE parasites were visualized by immunofluorescence, using an antibody to a P. falciparum heat shock protein. The mean number of EE parasites per 100 cm(2) for C57BL/6 and A/J strains was significantly higher than that for BALB/c (2,190 +/- 260, 88 +/- 38, and 6 +/- 2, respectively). The proportion of inoculated sporozoites transforming into liver schizonts was 8.2% in C57BL/6 and <1% in C3H/HeJ, DBA/1, and Swiss CD-1/ICR mice. Nonspecific inflammatory infiltrates around EE parasites were less prevalent in liver sections from C57BL/6 mice than in those from BALB/c mice, which contributed to the decrease in developing EE stages in BALB/c mice. These data indicate that the C57BL/6-P. berghei system is preferable for investigating the biology and immunology of liver stage parasites.