Malaria risk and receptivity: Continuing development of insecticide resistance in the major malaria vector Anopheles arabiensis in northern KwaZulu-Natal, South Africa

Malaria incidence in South Africa is highest in the three endemic provinces: KwaZulu-Natal, Mpumalanga and Limpopo. The contribution to malaria transmission by several mosquito species, variation in their resting behaviours and low levels of insecticide resistance makes it necessary to periodically monitor Anopheles species assemblages and resistance phenotypes in vector populations. The aim of this study was therefore to assess Anopheles species assemblage in northern KwaZulu-Natal and to collect insecticide susceptibility data for An. arabiensis, the primary vector of malaria in that province. Anopheles specimens were collected from Mamfene, Jozini, northern KwaZulu-Natal from November 2019 to April 2021. Progeny of wild-collected An. arabiensis females were used for standard insecticide susceptibility tests and synergist bioassays. Anopheles arabiensis contributed 85.6% (n=11 062) of the total catches. Samples for subsequent insecticide susceptibility bioassays were selected from 212 An. arabiensis families. These showed low-level resistance to DDT, permethrin, deltamethrin, and bendiocarb, as well as full susceptibility to pirimiphos-methyl. Synergist bioassays using piperonyl butoxide and triphenyl phosphate suggest oxygenase-based pyrethroid and esterase-mediated sequestration of bendiocarb. These low levels of resistance are unlikely to be operationally significant at present. It is concluded that northern KwaZulu-Natal Province remains receptive to malaria transmission despite ongoing control and elimination interventions. This is due to the perennial presence of the major vector An. arabiensis and other secondary vector species. The continued detection of low-frequency insecticide resistance phenotypes in An. arabiensis is cause for concern and requires periodic monitoring for changes in resistance frequency and intensity.Significance:• Insecticide resistance in the major malaria vector Anopheles arabiensis in northern KwaZulu-Natal Province is cause for concern in terms of resistance management and ongoing vector control leading toward malaria elimination.• Despite ongoing control interventions, northern KwaZulu-Natal remains receptive to malaria owing to the perennial presence of several Anopheles vector species

The effect of pollution on the competitive dynamics of Anopheles arabiensis Patton, 1905 and Culex quinquefasciatus Say, 1823 (Diptera: Culicidae)

Culex quinquefasciatus Say, 1823 and Anopheles arabiensis Patton, 1905 (Diptera: Culicidae) are often found breeding in the same water sources and engage in interspecific competition. Although Cx. quinquefasciatus is known to proliferate in a range of polluted environments, the ability of An. arabiensis to proliferate in polluted water has only been reported relatively recently. The effects of pollution and insecticide resistance on this competitive interaction are unknown. This study examined the effect of pollution on the dynamics of the interspecific competition. Three laboratory strains were used in this study: an insecticide susceptible and an insecticide resistant An. arabiensis, and an insecticide resistant Cx. quinquefasciatus. Larval pollutant tolerances of each strain were assessed and compared by determining the lethal concentration at 50% mortality (LC50). The larvae from each strain were exposed to either inorganic fertiliser or copper nitrate, following which eclosion success was assessed. The results showed that the insecticide resistant strains had higher emergence rates when reared in polluted conditions without competition, with the Cx. quinquefasciatus strain showing the highest rate of eclosion. This species also had a higher tolerance for metal pollution than the An. arabiensis strains. The effects of pollutants on oviposition choice were examined. Pollution altered adult oviposition choice. The effect of larval metal exposure had variable effects on the activity of metabolic detoxification enzymes. An insecticide resistant phenotype had greater tolerance to pollutants and possibly developmental advantages based on a variable detoxification response to the pollutant. Pollution can therefore alter interspecific competition dynamics between the malaria vector An. arabiensis and Cx. quinquefasciatus.

Additional evidence on the efficacy of different Akirin vaccines assessed on Anopheles arabiensis (Diptera: Culicidae)

[Background]: Anopheles arabiensis is an opportunistic malaria vector that rests and feeds outdoors, circumventing current indoor vector control methods. Furthermore, this vector will readily feed on both animals and humans. Targeting this vector while feeding on animals can provide an additional intervention for the current vector control activities. Previous results have displayed the efficacy of using Subolesin/Akirin ortholog vaccines for the control of multiple ectoparasite infestations. This made Akirin a potential antigen for vaccine development against An. arabiensis.[Methods]: The efficacy of three antigens, namely recombinant Akirin from An. arabiensis, recombinant Akirin from Aedes albopictus, and recombinant Q38 (Akirin/Subolesin chimera) were evaluated as novel interventions for An. arabiensis vector control. Immunisation trials were conducted based on the concept that mosquitoes feeding on vaccinated balb/c mice would ingest antibodies specific to the target antigen. The antibodies would interact with the target antigen in the arthropod vector, subsequently disrupting its function.[Results]: All three antigens successfully reduced An. arabiensis survival and reproductive capacities, with a vaccine efficacy of 68–73%.[Conclusions]: These results were the first to show that hosts vaccinated with recombinant Akirin vaccines could develop a protective response against this outdoor malaria transmission vector, thus providing a step towards the development of a novel intervention for An. arabiensis vector control.This project was supported by funding from the National Research Foundation (NRF) Competitive Programme for Rated Researchers (CPRR), Incentive Funding (UID; 85538), and the DST/NRF Research Chair Initiative grant awarded to L.L.K. (UID: 64763), as well as the Medical Faculty Research Endowment Fund awarded to B.D.L, and funding Y.D.M. from the South African Medical Research Council—Self-Initiated Research Grant.Peer reviewe

Malaria risk and receptivity: Continuing development of insecticide resistance in the major malaria vector Anopheles arabiensis in northern KwaZulu-Natal, South Africa

Malaria incidence in South Africa is highest in the three endemic provinces: KwaZulu-Natal, Mpumalanga and Limpopo. The contribution to malaria transmission by several mosquito species, variation in their resting behaviours and low levels of insecticide resistance makes it necessary to periodically monitor Anopheles species assemblages and resistance phenotypes in vector populations. The aim of this study was therefore to assess Anopheles species assemblage in northern KwaZulu-Natal and to collect insecticide susceptibility data for An. arabiensis, the primary vector of malaria in that province. Anopheles specimens were collected from Mamfene, Jozini, northern KwaZulu-Natal from November 2019 to April 2021. Progeny of wild-collected An. arabiensis females were used for standard insecticide susceptibility tests and synergist bioassays. Anopheles arabiensis contributed 85.6% (n=11 062) of the total catches. Samples for subsequent insecticide susceptibility bioassays were selected from 212 An. arabiensis families. These showed low-level resistance to DDT, permethrin, deltamethrin, and bendiocarb, as well as full susceptibility to pirimiphos-methyl. Synergist bioassays using piperonyl butoxide and triphenyl phosphate suggest oxygenase-based pyrethroid and esterase-mediated sequestration of bendiocarb. These low levels of resistance are unlikely to be operationally significant at present. It is concluded that northern KwaZulu-Natal Province remains receptive to malaria transmission despite ongoing control and elimination interventions. This is due to the perennial presence of the major vector An. arabiensis and other secondary vector species. The continued detection of low-frequency insecticide resistance phenotypes in An. arabiensis is cause for concern and requires periodic monitoring for changes in resistance frequency and intensity. Significance: Insecticide resistance in the major malaria vector Anophelesarabiensis in northern KwaZulu-Natal Province is cause for concern in terms of resistance management and ongoing vector control leading toward malaria elimination. Despite ongoing control interventions, northern KwaZulu-Natal remains receptive to malaria owing to the perennial presence of several Anophelesvector species