Sporozoites are an invasive stage of the malaria parasite in both the mosquito vector and the vertebrate host. We developed an in vivo assay for mosquito salivary gland invasion by preparing Plasmodium gallinaceum sporozoites from infected Aedes aegypti mosquitoes under physiological conditions and inoculating them into uninfected female Ae. aegypti. Sporozoites from mature oocysts were isolated from mosquito abdomens 10 or 11 d after an infective blood meal. Salivary gland sporozoites were isolated 13 or 14 d after an infective blood meal. Purified oocyst sporozoites that were inoculated into uninfected female mosquitoes invaded their salivary glands. Using the same assay system, sporozoites derived from salivary glands did not reinvade the salivary glands after inoculation. Conversely, as few as 10 to 50 salivary gland sporozoites induced infection in chickens, while only 2 of 10 chickens inoculated with 5,000 oocyst sporozoites were infected. Both sporozoite populations were found to express a circumsporozoite protein on the sporozoite surface as determined by immunofluorescence assay and circumsporozoite precipitation test using a circumsporozoite protein-specific monoclonal antibody. We conclude that molecules other than this circumsporozoite protein may be responsible for the differential invasion of mosquito salivary glands or infection of the vertebrate host.

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