Sporozoite loads of naturally infected Anopheles in Kilifi District, Kenya

The number of salivary gland malaria sporozoites (sporozoite load) was determined by hemacytometer counts for 2,055 field-collected Anopheles mosquitoes from Kilifi District, Kenya. Of 48 gland-positive Anopheles gambiae s.l., sporozoite loads ranged from 125 to 79,875, with a geometric mean of 1,743 sporozoites per infected mosquito. About half of the infected mosquitoes had sporozoite loads < 1,000. Following hemacytometer examination of salivary gland samples, the same samples were subsequently tested for Plasmodium falciparum circumsporozoite (CS) protein by enzyme-linked immunosorbent assay (ELISA). The confirmation by ELISA of CS protein in 89.6% (43/48) of the salivary gland-positive samples compared to only 1.4% (28/2,007) of the dissection-negative mosquitoes indicated that dissection methods with hemacytometer counts of sporozoites were adequate for detecting even low numbers of sporozoites in field-collected mosquitoes. Detection of 17 or fewer sporozoites in blood meals of 7 freshly bloodfed An. gambiae s.l. provides a further indication that the actual number of sporozoites transmitted during bloodfeeding may be quite low.