Mosquito vectors: a Southern African context

Previous author: P G JUPP

Current authors: A J CORNEL, L E O BRAACK and A P G ALMEIDA

Introduction

For the purposes of this chapter we consider South Africa, Lesotho, Eswatini (Swaziland), Botswana, Namibia, Zimbabwe, and southern regions of Angola and Mocambique as southern African because they share areas of similar subtropical and temperate humid, dry and plains, plateau and mountainous ecological zones (http://www.aag.org/galleries/publications-files/africa_ecosystems_booklet.pdf -Accessed April 2019) and diversity of mosquito and other arthropod vector species on animal and human diseases.

The “cosmopolitanizing” of Ae. aegypti and associated global outbreaks of dengue (DENV), Chikungunya (CHIKV), Zika (ZIKV) and yellow fever (YFV) virus  infections has stoked a “fire” in arboviral interest and reviews have recently been published that discuss the resurgence of interest and risks of infection by mosquito-borne arboviruses in Africa, including southern Africa.^{13, 14, 46, 148} These reviews provide summaries of the multi-continent re-emerging viruses such as West Nile (WNV), ZIKV, YFV, CHIKV and dengue viruses in the last two decades. Recent enhancements in surveillance in southern Africa have now revealed  considerable annual arboviral activity of the  better  known arboviruses such as Rift Valley fever  (RVFV), Wesselsbron (WESV) and WNV  but also the previously lesser known viruses such as Sindbis (SINV), Shuni (SHUNV), Middelburg (MIDV), and Usutu (USUV) viruses that infect both livestock and wild animals.¹⁴⁵ A list of these mosquito-borne arboviruses in southern Africa is given in Table 1. Unfortunately a good understanding of the natural cycles of many of these viruses, especially concerning the role of mosquito vectors and reservoir host vertebrates is lacking and is occasionally quite speculative.

Mosquito-borne arboviruses characteristically multiply in both the vertebrate host and the mosquito vector. Successful transmission is dependent upon the virus producing viraemia in the vertebrate, while the mosquito that feeds on the vertebrate must develop a salivary gland infection followed by excretion of virus in the saliva. The arboviruses belong to several families and genera and their taxonomic categories and host relations are discussed elsewhere. Apart from virus isolation from wild-caught mosquitoes, the lines of evidence incriminating a mosquito species as a vector are its susceptibility to infection with, and its ability to transmit, the virus concerned (vector competence) plus its relative density and ecological characteristics. Vector competence is assessed quantitatively in laboratory tests while data on density and ecology are obtained by field observations. Relevant aspects of bioecology are relative density, seasonal dynamics, feeding behaviour, including time of biting activity, host preferences, feeding frequencies, larval habitat associations and biology of the immature stages. Together, the aspects of adult bioecology that affect and condition transmission are usually expressed as vectorial capacity, a concept which was developed by Garrett-Jones²⁹ after the mathematical modelling for malaria by MacDonald⁸⁵

Climatic factors influence arboviruses both directly and indirectly. Temperature conditions directly affect the multiplication of the virus in the mosquito vector, low temperatures slowing down viral replication and lowering vector competence.^{56, 128} However, more importantly, climatic factors operate through their influence on the geographical distribution and prevalence of the viral hosts, both mosquito and vertebrate. In the case of mosquitoes, high rainfall and temperatures can lead to higher population densities of species belonging to all genera, particularly Aedes. Climate therefore has a considerable effect on the epidemiology of arbovirus infections.

Apart from some work in the temperate, moist highlands and subtropical, moist lowlands of Zimbabwe, the areas of southern Africa lying outside South Africa have largely only been studied for the presence of mosquito-borne viruses by means of serology, with little virus isolation from wild-caught mosquitoes being attempted. There is considerable arbovirus activity in the tropical region of Mocambique and the Zambezi valley and in the subtropical, moist northern belt that extends westwards from Zimbabwe and embraces the Okavango swamps in northern Botswana, and the Zambezi Region (formerly Caprivi strip) and Etosha pan in north-eastern Namibia.^{11, 35, 76, 80}

Arbovirus activity in southern Africa is more or less confined to the subtropical and tropical summer rainfall areas. The inland plateau has virtually no rain from May to November, while in the winter, from June to August, night temperatures are often near freezing and periodically fall below this. In eastern (northern South Africa and Mocambique) and western coastal areas (Angola) winters are milder with little to no frost .

In South Africa, there has been a change in the complexity of rangeland animal rearing, especially since the 1990s, and many private farmers rear wildlife for ecotourism, conservation, trophy hunting and game meat production. The game ranging industry is now worth more than $700 million annually and game animals are considered a valuable form of “livestock” that succumb to mosquito-borne arboviral infections. Many farmers have mixed livestock and game animal...