False detection of Plasmodium falciparum sporozoites in Anopheles marshallii group mosquitoes

A study was conducted to determine the role of members of the Anopheles funestus group in malaria transmission in the Mpumalanga Province, in the northeastern region of South Africa. Female anopheline mosquitoes were collected between January 1996 and November 1997 by means of human landing catches and tested for salivary gland Plasmodium falciparum infections by means of the enzyme-linked immunosorbent assay (ELISA) method with PF2A10 antibodies. Infection rates from April and May 1997 collections were 3.73% and 19.4%, respectively. None of the nonimmune collectors became infected with malaria. The ELISA-positive mosquitoes were tested with a polymerase chain reaction (PCR) malaria detection assay based on sequence variation present in the small subunit ribosomal RNA gene. Only 1.09% of ELISA-positive mosquitoes were PCR-positive for malaria. Initially, all mosquitoes were assumed to belong to the An. funestus group but subsequent molecular taxonomy showed this assumption to be false. The use of a single-strand conformation polymorphism (SSCP) assay revealed only 1 member of the An. funestus group, An. rivulorum. All other specimens produced banding patterns not seen before. Those samples were identified morphologically as An. demeilloni and An. marshallii s.l. These 2 species are not recognized malaria vectors and thus it is possible that the ELISA results are misleading.