Anopheles salivary gland architecture shapes plasmodium sporozoite availability for transmission

Plasmodium sporozoites (SPZs) must traverse the mosquito salivary glands (SGs) to reach a new vertebrate host and continue the malaria disease cycle. Although SGs can harbor thousands of sporozoites, only 10 to 100 are deposited into a host during probing. To determine how the SGs might function as a bottleneck in SPZ transmission, we have characterized Anopheles stephensi SGs infected with the rodent malaria parasite Plasmodium berghei using immunofluorescence confocal microscopy. Our analyses corroborate findings from previous electron microscopy studies and provide new insights into the invasion process. We identified sites of SPZ accumulation within SGs across a range of infection intensities. Although SPZs were most often seen in the distal lateral SG lobes, they were also observed in the medial and proximal lateral lobes. Most parasites were associated with either the basement membrane or secretory cavities. SPZs accumulated at physical barriers, including fused salivary ducts and extensions of the chitinous salivary duct wall into the distal lumen. SPZs were observed only rarely within salivary ducts. SPZs appeared to contact each other in many different quantities, not just in the previously described large bundles. Within parasite bundles, all of the SPZs were oriented in the same direction. We found that moderate levels of infection did not necessarily correlate with major SG disruptions or abundant SG cell death. Altogether, our findings suggest that SG architecture largely acts as a barrier to SPZ transmission.