Pyrethroid resistance in a major African malaria vector Anopheles arabiensis from Mamfene, northern KwaZulu-Natal, South Africa

Introduction Malaria vector control in southern Africa relies mainly on indoor residual house spraying (IRS) with insecticides. Synthetic pyrethroids and DDT (dichloro-diphenyl-trichloroethane) are used in IRS applications, while permethrin-treated bed-nets are used for personal protection in some households. DDT was introduced for malaria control in 1946 in South Africa and remained in use until 1996 when the choice of insecticide was changed to the pyrethroid deltamethrin. Following a major upsurge of malaria cases in 1999/2000 and the detection of pyrethroid resistance in the major malaria vector Anopheles funestus, DDT was re-introduced in 2001 and a marked reduction in malaria morbidity and mortality was recorded. Deltamethrin is currently used in modern-style houses with painted walls while DDT is used in traditional houses. Anopheles arabiensis is a major malaria vector in southern Africa, second only to Anopheles funestus. Historically, it has been considered to be susceptible to all insecticides. Standard bioassay susceptibility studies in three provinces of South Africa (KwaZulu-Natal (KZN), Limpopo and Mpumalanga) in 1996 showed full susceptibility to DDT, deltamethrin and fenitrothion in all provinces, while resistance to propoxur was reported in Limpopo province. Biochemical analyses on these samples provided little information on the mechanism causing the propoxur resistance. The first record of DDT resistance in An. arabiensis from South Africa was reported in 2003 in KZN. Bioassay data from this study also showed reduced susceptibility to deltamethrin. Biochemical analyses on these samples showed increased levels of non-specific esterases (EST) and glutathione S-transferases (GST), enzymes that are commonly involved in metabolic resistance. However, there was a lack of correlation between elevated enzyme levels and the bioassay data. This study presents the first report of permethrin resistance in An. arabiensis in South Africa.