Optimizing Direct Membrane and Direct Skin Feeding Assays for Plasmodium falciparum Transmission-Blocking Vaccine Trials in Bancoumana, Mali

Abstract

Malaria transmission-blocking vaccines (TBV) have been evaluated in field trials in Mali since 2013. However, the assays currently used to measure serum antibody TB activity (TBA) after vaccination are highly variable, in part due to the lack of optimization and standardization for field assays in which mosquitoes feed on gametocytemic blood. Herein, we report a study conducted in Bancoumana village, Mali, where we identify and optimize the parameters that contribute to successful mosquito feeding outcomes in both direct skin feeds (DSFs) and direct membrane feeding assays (DMFA). These parameters include: 1) mosquito age, 2) duration of mosquito starvation prior to feeding, 3) membrane selection for DMFA, 4) anatomical location of DSF feeding (arm, calf, and ankle), and 5) time of day for DSF (dawn or dusk). We found that younger mosquitoes were significantly associated with higher feeding, survival, and infection rates. Longer starvation times were positively, but not significantly, associated with higher infection rates, but were negatively associated with feeding and survival. Membrane type and body location did not affect infection outcome significantly. Although dusk was found to be associated with higher infection rates, this may be confounded by the time from positive blood smear. Based on these findings, we make specific recommendations for optimal feeding parameters in the different assay types to maximize the chance of detecting parasite transmission in a standardized manner.

Malaria Transmission-Blocking Vaccines: Present Status and Future Perspectives

“…These DSF assays use a line of locally caught Anopheles coluzzii recently adapted for breeding in a contained insectary. In these studies [52], uninfected mosquitoes (generally 30-60 per assay) are fed on study volunteers on a regular basis during the malaria transmission season, with the expectation that an effective vaccine will reduce the number of infected mosquitoes compared to controls. We believe DSFs, in which insectary-raised clean mosquitoes are directly fed on infected individuals, may be more likely to be predictive of an intervention's impact on transmission than membrane feeds.…”

Optimization of the Feeding rate of Anopheles Farauti Colony Mosquitoes in Direct Membrane Feeding Assays

“…Coulibaly and colleagues compared the feeding rate for An. coluzzii mosquitoes between 3 days and 9 days post emergence and found that 3 day old mosquitoes had a signi cantly higher feeding rate compared to 6 and 9 day old mosquitoes [10].…”

“…Starving conditions is a key component that greatly impacts mosquito feeding rates and a balance needs to be established between starving the mosquitoes for too long, thereby increasing mosquito mortality or affecting their tness [10], and not starving for long enough so mosquitoes only partly feed or not at all.…”

“…Most studies describe dry starving for durations from 5-36 h [11][12][13][14] while other studies performed starving where the mosquitoes had access to water for 12 h [15,16]. A study conducted by Coulibaly and colleagues compared the feeding rates of mosquitoes dry starved 8 h, 14 h and 20 h and concluded that mosquitoes starved 8-14 h yielded signi cantly higher feeding rates than mosquitoes starved 20 h. [10] However, most studies did not directly report the impact of starving on the feeding rate.…”

“…Most studies reported using Baudruche membrane [8,18,19] while others used Para lm membrane [14,20]. Interestingly, a study done by Coulibaly and colleagues showed that there was no signi cant difference between the feeding rates, survival and infection rates from feeding experiments with either Baudruche or Para lm membranes, for Anopheles coluzzii mosquitoes [10].…”