Measuring the blockade of malaria transmission – An analysis of the Standard Membrane Feeding Assay

Abstract

The standard membrane feeding assay (SMFA) is currently considered to be the 'gold standard' for assessing the effectiveness of malaria transmission blocking interventions (TBIs) in vivo. The operation and analysis of SMFAs has varied between laboratories: field scientists often measure TBI efficacy as a reduction in the prevalence of infected mosquitoes whilst laboratory scientists are more likely to quote efficacy as a change in the number of oocysts within the mosquito. These metrics give outputs that differ widely, resulting in a need for greater understanding of how the SMFA informs TBI assessment. Using data from 536 different assays (conducted on Plasmodium falciparum and Plasmodium berghei, in either Anopheles gambiae or Anopheles stephensi) it is shown that the relationship between these metrics is complex, yet predictable. Results demonstrate that the distribution of oocysts between mosquitoes is highly aggregated, making efficacy estimates based on reductions in intensity highly uncertain. Analysis of 30 SMFAs carried out on the same TBI confirms that the observed reduction in prevalence depends upon the parasite exposure (as measured by oocyst intensity in the control group), with assays which have lower exposure appearing more effective. By contrast, if efficacy is estimated as a reduction in oocyst intensity, then this candidate demonstrated constant efficacy, irrespective of the exposure level. To report transmission-blockade efficacy accurately, the results of SMFAs should give both the prevalence and intensity of oocysts in both the control and intervention group. Candidates should be assessed against a range of parasite exposures to allow laboratory results to be extrapolated to different field situations. Currently, many studies assessing TBIs are underpowered and uncertainties in efficacy estimates rarely reported. Statistical techniques that account for oocyst over-dispersion can reduce the number of mosquitoes that need to be dissected and allow TBI candidates from different laboratories to be accurately compared.

Infectiousness of the Human Population to Anopheles arabiensis by Direct Skin Feeding in an Area Hypoendemic for Malaria in Senegal

“…16,22,23 We chose a minimum number of fed mosquitoes per batch to maximize the informativeness of individual feeding experiments. 24 Our findings confirm previous findings on a higher likelihood of mosquito infection from microscopically detectable gametocyte densities 16,24 and a relevant contribution of submicroscopic gametocyte carriers to onward malaria transmission. 18 Because of the methodology we used, direct skin feeding, we could not directly determine gametocyte density in the mosquito blood meal.…”

Anti-Pfs25 Human Plasma Reduces Transmission of Plasmodium falciparum Isolates That Have Diverse Genetic Backgrounds

“…2). This lack of significance may be due to the high oocyst density in the infection control typically observed in these assays (19) (Fig. 2).…”

“…As recently highlighted by Churcher and colleagues, variation in oocyst density associated with little or no change in prevalence occurs at high levels of infection, with high infection prevalence and high oocyst density. This is due to the relationship between prevalence and oocyst density, in which infection prevalence reaches a plateau close to 100% when oocyst density is high (19). Consequently, TBA is more dependent on control infection intensity than TRA, and a higher TBA may be observed when mosquitoes are fed on blood meals with lower infectious gametocyte densities.…”

Antibodies to a Single, Conserved Epitope in Anopheles APN1 Inhibit Universal Transmission of Plasmodium falciparum and Plasmodium vivax Malaria

“…Informed consent was provided directly by individuals Ն20 years of age or was provided by the legal guardian. Infective venous blood was collected and prepared as described previously (20)(21)(22). Transmission-blocking assays were performed using rabbit anti-AnAPN1 60 -195 IgG diluted in nonimmune human AB serum or AB serum alone as a control.…”

“…Generalized linear mixed-effects models were used to determine the efficacies of different anti-AnAPN1 60 -195 antibody dilutions across all feeding experiments (22). Data from Cameroon and Thailand were initially analyzed independently.…”