Abstract: 　AIM: To better understand the differences in haemozoin formation and pathogenicity between chloroquine sensitive(N) and chloroquine resistant(RC) strains of Plasmodium berghei. METHODS: IRC mice were grouped as follows: group XbaⅠ (Normal control, NC), group Ⅱ (control treated with chloroquine alone, CC), group Ⅲ (mice infected with N strain), group Ⅳ (mice infected with RC strain,RC) and group Ⅴ (mice infected with RC strain and treated with chloroquine, RCC). Morphologic features of the parasites, parasitaemia, histological and ultrastructural changes of livers among the groups were compared. RESULTS: Severe damages of the hepatic cells of the N group including increased lysosomes and swollen and fused mitochondria were detected. On the contrary, the prominent features in liver section of the RC group were inflammatory cell (notably mononuclear) infiltration and Kupffer cell activation. Numerous trophozoites and schizonts were sequestrated in hepatic sinusoids and less degeneration of parenchymal cells was found except for some swollen and vacuolated mitochondria. Internal food vacuoles containing haemozoin were found in the parasites of the N group, whereas external food vacuoles without haemozoin granules were arranged in foamy appearance within the parasitized cell of the RC group. CONCLUSION: P.berghei RC strain may modify the mode of ingestion and degradation of hemoglobin in the parasites, resulting in impeding haemozoin formation. The difference in virulence between the N and RC strain of P.berghei is probably attributed to the significant differences in the induction of immune response of the host.

Key words: Plasmodium berghei, chloroquineresistance, haemozoin, pathogenicity, external food vacuole, inflammatory response