Assays to Support Development of Malaria Transmission Blocking Vaccines

National Institute Of Allergy And Infectious Diseases

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Abstract

In 2018, the focus of this project shifted from developing molecular assays to the set up, and conduct, of a community malaria epidemiology study in Mali that could be used to evaluate transmission dynamics. Samples collected in this study will be evaluated using standard laboratory techniques such as blood smear microscopy as well as via application of molecular assays previously developed to determine sub-patent parasite and gametocyte levels, parasite diversity and forensic genotyping of bloodmeal. The protocol titled Community Dynamics of Malaria Transmission and Mosquito Feeding in Bancoumana and Doneguebougou, Mali was approved by the NIAID IRB in September 2017 and enrollment began in February 2018. To date, 913 individuals have been enrolled in Bancoumana. Enrollment is into three different study cohorts: Direct skin feeding (DSF) cohort (n=683), Parasite Surveillance (PS) cohort (n=192) and Genotype cohort (n=38). The DSF and PS cohort undergo monthly study visits where they are evaluated by blood smear for any asexual or sexual parasites, have their hemoglobin level measured, provide a small blood sample for molecular analyses at a later date, provide consent for live and spray mosquito catches in their compound and, in the case of the DSF cohort, undergo a DSF. The Genotype cohort undergo only a single visit at enrollment to provide a small blood sample that may be used at a later date for forensic genotyping of spray caught mosquitoes to determine feeding fidelity. Unlike most vaccine studies, the community study enrolls from all age groups across the community providing valuable date on the contribution of individuals <18 years of age to parasite transmission dynamics. This information is especially valuable as we move closer to a community wide vaccine trial intervention which would necessitate vaccination of all age groups and not be restricted to adult only studies. To date the study has largely been performed in the dry season and is already providing useful information on parasite carriage rates at this time of year with parasite prevalence rates of 9-20% through the months of Feb-July which are considerably higher than those seen in adult only studies. The transmission season starts in late July/early August through December, so rates are expected to rise during this time. Up to the end of July 2018, a total of 3,188 visits have been made by the DSF cohort with 3,162 DSF assays conducted. 23 DSFs have been positive to date (0.7%) a rate of infection comparable to that seen during the transmission season in adults enrolled in TBV trials. These 23 positive DSFs have come from 20 unique individuals of which 19 are children (median age = 13) illustrating the importance of studying this group as they likely contribute significantly to the community infection reservoir. Moving forward, the study has been extended to include the village of Doneguebougou and enrollment is scheduled to begin there in August 2018. The community study overlaps with the ongoing Pfs230-EPA/AS01 vaccine trial that is currently being conducted in both Bancoumana and Doneguebougou. A fourth booster dose of this vaccine is scheduled to be administered in August 2018 followed by sixteen weeks of twice weekly DSF assays. In the 2017 transmission season (August-November 2017), a total of 4,861 DSF assays were conducted of which 40 (0.8%) yielded at least one malaria transmission event. Positive mosquitoes from these positive DSFs underwent molecular analyses demonstrating that 30 of these feeds contained Plasmodium falciparum parasites while the remaining contained either P. malariae or P. ovale (or both species). Eight of the 40 infections were mixed species with P. falciparum and P. ovale mixed species infections the most common. Further analysis of these midguts for parasite genetic diversity (using polymorphic DBL4 and CLAG2 sequences) and for selection of the vaccine target Pfs230 is underway. In addition to mosquito feeding assays, mosquito collections were performed in huts of Pfs230-EPA/AS01 volunteers in Bancoumana during the past transmission season. A total of 659 live collections were completed which yielded a total of 948 blood fed Anopheles mosquitoes; 616 (65%) survived to dissection 7 days later and of these, 11 infected mosquitoes were detected (1.8%). Similar collections are currently underway in the community study in Bancoumana with 3 infected mosquitoes collected to date during the dry season (0.8%).