Standard selection treatments with sulfadiazine limit Plasmodium yoelii host-to-vector transmission

Some early antimalarial drugs have been repurposed for experimental applications, thus extending their utility well beyond the point when resistance becomes prevalent in circulating parasite populations. One such drug is sulfadiazine, which is an analog of p-aminobenzoic acid (pABA), and acts as a competitive inhibitor of dihydropteroate synthase, which is an essential enzyme in the parasites folate synthesis pathway that is required for DNA synthesis. Sulfadiazine treatment of mice infected with P. yoelii and P. berghei is routinely used to enrich for gametocytes by killing asexual blood stage parasites, but it is not well known if the exposed gametocytes are perturbed or if there is a detrimental effect on transmission. To determine if there was a significant effect of sulfadiazine exposure upon host-to-vector transmission, we transmitted Plasmodium yoelii (17XNL strain) parasites to Anopheles stephensi mosquitoes and evaluated the prevalence of infection (percent of mosquitoes infected) and intensity of infection (number of oocysts per infected mosquito) under different sulfadiazine treatment conditions of the mouse or of the mosquitoes. We observed that parasites exposed to sulfadiazine either in the mouse host or in the mosquito vector had a reduction in both the number of mosquitoes that became infected and in the intensity of infection compared to untreated controls. We also observed that provision of freshly prepared pABA in the mosquito sugar water could only marginally overcome the defects caused by sulfadiazine treatment. In contrast, we determined that gametocytes exposed to sulfadiazine were able to be fertilized and develop into morphologically mature ookinetes in vitro, and thus that sulfadiazine exposure in the host may be reversible if the drug is washed out and the parasites are supplemented with pABA in the culture media. Overall, this indicates that sulfadiazine dampens host-to-vector transmission, and that this inhibition can only be partially overcome by exposure to fresh pABA in vivo and in vitro. Because gametocytes are of great interest for developing transmission blocking interventions, we recommend that less disruptive approaches for gametocyte enrichment be used in order to study minimally perturbed parasites.