Special Pathogens Unit, National Institute for Virology and Department of Virology, University of the Witwatersrand, Private Bag X4, Sandringham 2131, South Africa
Address for correspondence: Peter G. Jupp, Ph.D., D.Sc., Special Pathogens Unit, National Institute for Virology and Department of Virology, University of the Witwatersrand, Private Bag X4, Sandringham 2131, South Africa.Voice: +27-11-882-3164; fax: +27-11-882-3741.
juppy{at}mweb.co.za
This paper reviews studies done on West Nile virus (WNV) in
South Africa, mainly between 1962 and 1980 on the temperate
inland plateau (Highveld and Karoo). The virus is maintained
in an enzootic transmission cycle between feral birds and the
ornithophilic mosquito
Culex univittatus. About 30 avian species
have been shown to be involved without mortality. Humans, and
other mammals, although they may have antibodies, are considered
blind-alleys in the transmission cycle except perhaps some dogs.
Cx. univittatus also transfers infection to humans, almost invariably
causing only a mild illness. Its usually low anthropophilism
may explain why annual human infection on the Highveld is limited
to sporadic cases. Besides multiple isolations from field collections
of
Cx. univittatus, this mosquito is both highly susceptible
to the virus and an efficient transmitter.
Culex theileri is
a minor vector. In the summer of 1974 there was a large epidemic
in the dry Karoo after unusual rains: there were many human
cases, the infection rate in
Cx. univittatus was 39.0/1000,
and postepidemic immune rates in humans and birds were high.
In 1984 there was an epizootic in Gauteng Province in the Highveld
with an infection rate in
Cx. univittatus reaching 9.6/1000
and more human infections than usual. The much lower immune
rates in the KwaZulu-Natal coastal lowlands than on the plateau
and the single isolation from
Cx. neavei, which replaces
Cx. univittatus in the lowlands, are explained by the low susceptibility
of
Cx. neavei to the virus. Genetic relatedness of isolates
from different countries showed two lineages, with one lineage
comprising only African isolates, including 25 South African
strains, which had a sequence homology of 86.3-100%. This suggests
that the viral enzooticity does not depend on annual importation
of virus in migrant birds.
