Plasmodium yoelii sporozoite infectivity varies as a function of sporozoite loads in Anopheles stephensi mosquitoes

Mechanisms by which Plasmodium sporozoites survive and maintain their infectivity within the salivary glands of mosquitoes are unknown. In this study we establish a relationship between the number of sporozoites present in the salivary glands of individual mosquitoes (sporozoite load) and sporozoite infectiousness (or "quality") as measured by infections in BALB/c or ICR mice. When Plasmodium yoelii-infected Anopheles stephensi mosquitoes were each allowed to feed on a single mouse, we noted that sporozoites from mosquitoes with higher sporozoite loads were more infectious in 13 of 30 (43%) mice. In a second experiment, we inoculated mice with known numbers of sporozoites from individual mosquitoes. Eleven of 18 (61%) and 16 of 18 (89%) mice that received 25 and 100 sporozoites, respectively, became infected. For inoculations using 100 sporozoites, again we noted that sporozoites from mosquitoes with higher sporozoite loads were more infectious to mice. In a third and final experiment, the overall infectiousness of sporozoites from individual mosquitoes was evaluated first by allowing individual mosquitoes to feed on individual mice and then by intravenous inoculations of 100 sporozoites in a second mouse. There was a significant difference in host infections as a function of sporozoite loads in 14 of 19 (74%) mice. Analysis of the feeding times for infected versus noninfected mosquitoes did not show a significant difference between the 2 groups. The mean total feeding times for 50 infected and 45 noninfected An. stephensi mosquitoes were 306 (standard deviation [SD] = +/- 230) and 441 (SD = +/- 273) sec, respectively. Further, among infected An. stephensi mosquitoes there was no difference in probing times between cohorts that transmitted infectious sporozoites to mice and cohorts that failed to transmit infectious sporozoites. Our findings that sporozoite load influences sporozoite infectiousness or quality suggest that this may be an important factor in malaria parasite transmission.