NONINFECTIOUS SPOROZOITES IN THE SALIVARY-GLANDS OF A MINIMALLY SUSCEPTIBLE ANOPHELINE MOSQUITO

In studies to evaluate vector-malaria parasite relationships, we have found that Anopheles albimanus is minimally susceptible to the rodent malaria parasite Plasmodium yoelii. Normally, less than 10% of A. albi manus develop oocyst infections compared to 80-100% for Anopheles step hensi and Anopheles freeborni mosquitoes. Although sporozoites produce d in A. albimanus invade the salivary glands, they are not infectious to BALB/c or ICR mice. In 11 experiments with sporozoites from A. albi manus, intravenous inoculations of up to 24,000 sporozoites in individ ual mice failed to produce host infections. In contrast, inoculation o f 300 sporozoites obtained from the salivary glands of A. stephensi an d A. freeborni always infected mice. The noninfectious sporozoites fro m A. albimanus were morphologically similar to the infectious sporozoi tes from A, stephensi and yielded 4+ circumsporozoite precipitin react ions when incubated with a monoclonal antibody against the circumsporo zoite protein of P. yoelii. The presence of noninfectious sporozoites in the salivary glands of A. albimanus suggests that this minimally su sceptible vector either possesses a toxic factor that abolishes sporoz oite infectiousness or lacks a critical substance needed by the sporoz oite to become infectious. Sporozoite infectiousness was neither atten uated by incubation of infectious sporozoites with A. albimanus saliva ry glands nor restored when noninfectious sporozoites were incubated w ith A. stephensi salivary glands. These studies provide a starting poi nt for defining the biological basis of sporozoite infectivity.