An estimation of the number of malaria sporozoites ejected by a feeding mosquito

Restrained Anopheles stephensi mosquitoes infected with Plasmodium falciparum were made to produce time-dependent series of saliva droplets in mineral oil. The relative volume of each droplet and the number of sporozoites each contained were determined microscopically; gland sporozoites were estimated with an enzyme-linked immunosorbent assay. Median gland infection was 8170 and median number of sporozoites ejected was 15 (range, 0-978). Inoculum size was positively correlated to the number of sporozoites in the salivary glands. Most mosquitoes ejected sporozoites only at the beginning of salivation; this suggests that only those parasites in the common and secondary salivary ducts at the time of feeding can be ejected. The small size of inocula may explain some aspects of malaria transmission, including the often observed discrepancy between inoculation and incidence rates.