Wide cross-reactivity to the Anopheles gambiae and Anopheles funestus SG6 salivary proteins supports exploitation of gSG6 as a marker of human exposure to major Afro-tropical malaria vectors.

The availability of serological markers of human exposure to malaria vectors may represent a useful additional tool to monitor malaria transmission and evaluate vector control measures, especially in conditions of low malaria transmission and/or reduced vector density. We previously reported that the An. gambiae salivary protein gSG6 elicits in exposed individuals an IgG response that can be used as indicator of exposure to bites of An. gambiae. However, a marker of exposure covering not only An. gambiae, but also other major malaria vectors, would be desirable. Although the SG6 protein is highly conserved among members of the An. gambiae species complex (≥99%), the An. funestus homologue (fSG6) shows higher divergence (81%). As a consequence, it cannot be assumed a priori that human antibodies raised against gSG6 cross-react with fSG6, and viceversa. We expressed and purified the An. funestus SG6 protein and compared the humoral response to the gSG6 and fSG6 antigens in a population from a malaria hyperendemic area from Burkina Faso. Both vectors were present in the study area, although An. gambiae was largely prevalent (>80%). According to previous observations both anti-SG6 IgG levels and prevalence varied according to the transmission/rainy season with a typical decrease during the low transmission/dry season. No significant difference in the response to the two antigens was found, suggesting that wide cross-reactivity occurs and that the An. gambiae gSG6 protein may be exploited as a reliable marker of exposure to the three main malaria vectors in tropical Africa: An. gambiae, An. arabiensis and An. funestus. Information on SG6 family members from Asian malaria vectors is presently limited to An. stephensi, whose homologue share 81% identity with gSG6. This divergence is similar to the one existing between gSG6 and fSG6, suggesting the attractive possibility that the An. gambiae gSG6 protein may also be useful to monitor exposure to Asian malaria vectors.