Seasonal biology of malaria vectors in South Africa
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Date
2020
Authors
Burke, Ashley Morgan
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Abstract
Residual malaria transmission refers to all forms of transmission beyond the reach of
current control measures. Identifying the entomological drivers of residual transmission is a
central component of South Africa’s agenda to interrupt local transmission and to subsequently
prevent re-establishment of transmission. This study aimed to 1) evaluate and compare the
efficiency of different mosquito collection methods for routine surveillance, 2) determine the
seasonal Anopheles assemblage within two malaria-affected regions, and 3) investigate winter
dormancy in malaria vector mosquitoes. The merits of passive mosquito shelters (traditional
clay pots and modified plastic buckets), used both indoors and outdoors, were assessed in
parallel with active methods (human landing catches and CO2 -baited nets) in two malaria
affected villages. Passive vector surveillance was conducted weekly and active surveillance
conducted monthly. The species of all Anopheles collected was determined using
morphological keys and appropriate polymerase chain reaction (PCR) protocols. Every female
mosquito was screened for Plasmodium falciparum circumsporozoite protein (CSP). The
metabolic rate of wild anophelines, determined by CO2 production during closed system
respirometry, was measured. The effect of manipulated photoperiod on the metabolic rate of
colonized An. arabiensis mosquitoes was determined by laboratory-based experiments. The
outdoor clay pots accrued significantly more wild Anopheles than the plastic buckets and
showed productivity alike to the active methods thus showing useful for routine vector
surveillance. Changes in local population density of known vector and non-vector species was
consistent with seasonal rainfall patterns. Malaria parasite screening of all females collected
gave the first ever account of P. falciparum CSP-positive An. vaneedeni and An. parensis
females in the wild. Although now considered secondary vectors, their role in malaria
transmission in South Africa is likely minimal owing to their strong zoophilic tendency. There
were no significant seasonal disparities in metabolic rate data obtained from wild-caught
Anopheles nor amongst colonized An. arabiensis placed under simulated seasonal photoperiod
conditions. The occurrence of vector species alongside malaria incidence throughout the dry
season suggests that local vector populations do not enter a dormant phase in the adult stage.
Due to the decrease in populations during winter leading to reduced malaria transmission, the
study recommended control by winter larviciding of known breeding sites, which now forms
part of South Africa’s malaria vector control and elimination strategy for the period 2019 –
2023
Description
A thesis submitted in fulfilment of the requirements for the degree of
Doctor of Philosophy to the Faculty of Health Sciences, School of Pathology, University of the Witwatersrand, Johannesburg, 2020