Vectors: Mosquitoes

Vectors: Mosquitoes

P G JUPP

Southern Africa

Introduction

Among the pathogens that cause disease in livestock in southern Africa, only a few viruses are transmitted by mosquitoes (Table 6.1). These and other arthropod-borne viruses (arboviruses) characteristically multiply in both the vertebrate host and the mosquito vector. Successful transmission is dependent upon the virus producing a viraemia in the vertebrate, while the mosquito which 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 (see Classification, epidemiology and control of arthropodborne viruses).

Of the mosquito-borne viruses that occur in southern Africa, Rift Valley fever virus (RVFV) is periodically responsible for serious disease in sheep and cattle in the subcontinent, while Wesselsbron virus (WSLV) causes a milder disease, affecting sheep in particular. West Nile virus (WNV) and Banzi virus (BANV), however, have only been experimentally incriminated as causes of congenital abnormalities in lambs in South Africa.12 In France, WNV has been documented as the cause of encephalomyelitis in horses.32, 33

Other mosquito-borne viruses of the families Togaviridae and Flaviviridae that do not occur in Africa, but which cause important diseases mainly in horses and humans outside Africa include the alphaviruses of Eastern, Western and Venezuelan encephalomyelitides and the flaviviruses of Murray Valley, St. Louis and Japanese B encephalitides (see the chapters on the specific diseases). It is possible that Akabane virus, which is a bunyavirus of the family Bunyaviridae, is also present in southern Africa, although this still remains to be demonstrated. This virus has been isolated from Culicoides spp. and from mosquitoes in Japan, suggesting it is also mosquito-borne.98 Akabane virus has been incriminated as the cause of abortion and congenital abnormalities in livestock64 (see Diseases caused by Akabane and related Simbu-group viruses)

Rift Valley fever, WSL and WN viruses are also important in causing human disease, while two other mosquito-borne viruses, namely chikungunya (CHIK) and Sindbis (SIN) viruses, only cause disease in humans. Their epidemiology and association with human disease have been summarized recently by McIntosh.78

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 ecology are feeding behaviour, including host preferences, larval habitat associations and biology of the egg stage.

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.38, 101 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, without virus isolation from wild-caught mosquitoes being attempted. There is considerable arbovirus activity in the tropical region of Mozambique 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 Caprivi Strip and Etosha Pan in north-eastern Namibia.13, 58, 62

Table 6.1 Mosquito-borne viruses causing disease in livestock and/or humans in southern Africa

  VIRUS MOSQUITO VECTOR WILD VERTEBRATE HOST HOSTS SHOWING DISEASE
        HUMANS LIVESTOCK
Togaviridae          
Alphavirus Chikungunya Ae. (Dic.) furcifer Primates + -
  Sindbis Cx. (Cux.) univittatus Birds + -
Flaviviridae          
Flavivirus Banzi Cx. (Eum.) rubinotus Rodents + +a
  Wesselsbron Ae. (Neo.) circumluteolus Rodents? + +
    Ae. (Neo.) mcintoshi      
    Ae. (Och.) caballus/juppi      
  West Nile Cx. (Cux.) univittatus Birds + +a
Bunyaviridae          
Phlebovirus Rift Valley fever Cx. (Cux.) theileri Antelope? + +
    Cx. (Cux.) zombaensis      
    Ae. (Neo.) circumluteolus      
    Ae. (Neo.) mcintoshi      
    Ae. (Och.) caballus/juppi      

a Disease produced after inoculation with virus in laboratory

Figure 6.1  Culex theileri

Figure 6.2  Aedes (Neo.) mcintoshi

Figure 6.3  Aedes (Och.) juppi

Arbovirus activity in South Africa is...

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