Melanization of plasmodium falciparum and C-25 sephadex beads by field-caught Anopheles gambiae (Diptera: Culicidae) from southern Tanzania.

The melanization responses of field-captured Anopheles gambiae s.l. toward oocysts of the malaria parasite Plasmodium falciparum or negatively charged (C-25) Sephadex beads were determined. Only two of 431 infected mosquitoes harboured melanized oocysts. However, 90% of field-captured mosquitoes melanized C-25 Sephadex beads. The effects of age, glucose concentration and blood meal on the melanization response of an An gambiae s.s. laboratory colony toward C-25 beads were also assayed. All newly emerged females (which did not blood-feed) melanized the beads. By 4 d postemergence, there was a marked reduction in melanization response, particularly among those mosquitoes that had not blood fed. A blood meal, however, taken by 4-d-old mosquitoes increased their immune response as did high glucose concentrations in the nonblood-fed group. These data indicate that C-25 Sephadex beads can estimate the general strength of An. gambiae's immune response. However, C-25 beads do not accurately model An. gambiae's susceptibility to P falciparum oocysts in natural populations. To the best of our knowledge, this is the first report of field refractoriness in An. gambiae s.l.