Mosquito direct skin feeding bioassay: 15 years of experience and a standardised approach in Mali

Abstract Background New malaria control tools are urgently needed. Transmission-blocking vaccines (TBVs) target sexual parasite stages in mosquitoes to prevent disease spread. TBV testing requires specialised mosquito transmission assays, such as the Direct Skin Feeding (DSF) bioassay. DSF is a xenodiagnostic tool that mimics parasite transmission to mosquitoes as it naturally occurs but has not previously been scaled up nor standardised for use in clinical trials. Methods DSF bioassays were performed on large cohorts of participants, including children five years and older, during observational and interventional studies conducted over a 15-year period at sites around Bamako, Mali. Human, mosquito and parasite parameters were monitored to assess DSF safety and acceptability, vector performance, and individual- and population-level transmission dynamics. Standardised procedures developed for DSF included mosquito colony maintenance, mosquito preparation for DSF, feeding procedures, quality control metrics, ethical approaches and volunteer follow-up. Results From 2011 through 2025, 37,984 DSF bioassays were performed on 2,796 rural study participants aged 5 years and older, at two Mali study sites. DSFs were well-accepted and safe, with a low refusal rate (0.8% of subjects in vaccine studies) and rare adverse events (AE) that met grading criteria (11 subjects; 0.032%). The few immediate and short-term skin reactions that met AE grading criteria were mild or moderate in severity, all resolving within 48 h. DSF infrastructure was progressively scaled up to a capacity of 120 assays per day requiring 36,000 female mosquitoes per week. Rates of DSF positivity were highest in studies where feeds were conducted on individuals with known Plasmodium falciparum parasitaemia (average 18.4%) vs studies where feeds were conducted on all participants irrespective of blood smear status (average 1.6%). Conclusions The DSF bioassay is a xenodiagnostic tool to detect transmissible malaria parasites, and a scalable and safe method for evaluating TBV efficacy. DSF offers several advantages including close mimicry of naturally occurring transmission, simplicity of performance and standardisation, and scalable throughput to support late-stage vaccine trials. While parasite transmission rates measured by DSF were low overall at study sites in Mali, sufficient transmission endpoints are generated to assess efficacy of interventions that interrupt transmission, supporting the DSF bioassay as a surrogate efficacy endpoint for TBV studies.