Infectivity of Plasmodium berghei sporozoites measured with a DNA probe.

A 2.3 kb, 32P-labeled repetitive DNA probe of Plasmodium berghei was used to measure the amount of parasite DNA in the liver of Norway Brown rats and mice infected with sporozoites. Standard hybridization curves were obtained by probing different amounts (100 pg to 1 microgram) of P. berghei DNA immobilized on nitrocellulose filters. Host DNA did not interfere with hybridization specificity and sensitivity. A 100-fold increase in hepatic parasite DNA was detected between 25 h post-infection and the peak of parasite proliferation, detected at 44 h. The amount of parasite DNA increased with the number of injected sporozoites. At 5 h post-infection, a large proportion of parasite DNA was found in the spleen. However, this diminished with time and was negligible in amount at 25 h. A significant number of viable sporozoites were probably cleared in the spleen, since considerably more parasite DNA was found in the livers of splenectomized rats than in sham-operated counterparts. Although older rats develop much lower parasitemias upon inoculation of sporozoites, no significant differences were observed in the amount of parasite DNA in rats, 43 and 152 days old, injected with equal numbers of sporozoites. The higher resistance to malaria displayed by older rats is probably controlled by post-hepatic events. The infectivity of sporozoites for A/J mice was calculated to be about 1/20th that of Norway Brown rats.