The relevance and applicability of oocyst prevalence as a read-out for mosquito feeding assays

Abstract

Mosquito feeding assays are important in evaluations of malaria transmission-reducing interventions. The proportion of mosquitoes with midgut oocysts is commonly used as an outcome measure, but in natural low intensity infections the effect of oocyst non-rupture on mosquito infectivity is unclear. By identifying ruptured as well as intact oocysts, we show that in low intensity P. falciparum infections i) 66.7–96.7% of infected mosquitoes experienced oocyst rupture between 11–21 days post-infection, ii) oocyst rupture led invariably to sporozoite release, iii) oocyst rupture led to salivary gland infections in 97.8% of mosquitoes, and iv) 1250 (IQR 313-2400) salivary gland sporozoites were found per ruptured oocyst. These data show that infectivity can be predicted with reasonable certainty from oocyst prevalence in low intensity infections. High throughput methods for detecting infection in whole mosquitoes showed that 18s PCR but not circumsporozoite ELISA gave a reliable approximation of mosquito infection rates on day 7 post-infection.

Targeting Human Transmission Biology for Malaria Elimination

“…Furthermore, all methods assume that the presence and level of oocysts in the mosquito midgut is indicative of mosquito infectivity. This relationship was recently demonstrated to be consistent for low-density infections despite a portion of the oocysts failing to release viable sporozoites [93,94]. Even in the cases in which oocysts burst and sporozoites successfully migrate to the salivary glands, not all bites from an infected mosquito are equally infectious.…”

The role of submicroscopic parasitemia in malaria transmission: what is the evidence?

“…Experimental studies whereby mosquitoes are fed blood-stage parasites, either by direct skin feeding or via a membrane feeding apparatus, and subsequently dissected to look for malaria parasites (oocysts in the mosquito midgut), represent a more direct way for assessing human-to-mosquito infectious potential [33,34,35,36]. …”

Transmission blocking malaria vaccines: Assays and candidates in clinical development

“…Two outcome measures of the SMFA are the proportion of infected mosquitoes (oocyst prevalence) and the infection burden (oocyst density) in mosquitoes. These outcomes are commonly determined by microscopical examination of mosquito midgut but can also be determined by the detection of parasite DNA by molecular methods [64,65] or parasite protein by immuno-assays [65,66]. Recently, fluorescence and luminescence-based SMFA approaches have been proposed that use P. falciparum strains expressing green fluorescent protein [67] and firefly luciferase protein [68].…”

“…In order to reduce malaria transmission in a vaccinated human population an SSM-VIMT will have to reduce oocyst prevalence -and not just mean oocyst densities -in mosquitoes feeding on the vaccinated population. This is because even mosquitoes with only one oocyst will produce salivary gland sporozoites [65] and may be (equally) infectious, therefore, as mosquitoes with many oocysts. Therefore, reducing oocyst densities may have little effect on overall malaria transmission unless it also results in significant reduction in oocyst prevalence in the mosquitoes Although oocyst prevalence and density are positively associated in experiments, substantial reductions in density are needed to obtain lower prevalence rates [70,71,77].…”