Abstract. In mid-September 2000, Rift Valley fever (RVF) virus was diagnosed as the cause of infection in humans and livestock in Jizan Region, Saudi Arabia. This is the first time that this arbovirus has been found outside Africa and Madagascar. Collections of mosquitoes (Diptera: Culicidae) were therefore undertaken (from 25 September to 10 October) at eight sites during the epidemic to obtain mosquitoes for attempted RVF virus isolation. Among 23 699 mosquito females tested, isolations of RVF virus were made from six of 15 428 Culex (Culex) tritaeniorhynchus Giles and from seven of 8091 Aedes vexans arabiensis Patton. Minimum mosquito infection rates per 1000 at sites with infected mosquitoes were 0.3±13.8 Cx. tritaeniorhynchus and 1.94±9.03 Ae. v. arabiensis. Viral activity moved northwards as collecting was in progress and collectors`caught up' with the virus at the two most northerly sites on the last two trapping evenings. Other species occurred in small numbers and were identified but not tested. Both Cx. tritaeniorhynchus and Ae. v. arabiensis were susceptible to RVF virus and transmitted between hamsters, and an additional quantitative test with Cx. tritaeniorhynchus showed that 71±73% of mosquitoes became infected after ingesting 6.9±7.9 log 10 FFU/mL of virus; transmission rates were 10% (post-infection day 14) and 26% (postinfection day 20). It was concluded that both species were vectors on grounds of abundance, distribution, preference for humans and sheep, the virus isolations and vector competence tests.

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.

The effects of the extrinsic incubation temperature on the vector competence of Culex univittatus Theobald for West Nile (WN) virus were studied. A mean titer of 7.0 log10 CPD50/ml of mosquito suspension was reached in orally infected mosquitoes after 11, 15, and 16 d of incubation at 26 and 30 degrees C and at fluctuating temperatures in an outside cage (mean temperature, 23.5 degrees C), respectively. In contrast, 22 and 58 d were required to reach the same titers at 18 and 14 degrees C, respectively. Transmission rates of 100% were reached after 58 d (14 degrees C), 22 d (18 degrees C), and 15 and 16 d (30 degrees C and outside). Except at 30 degrees C, transmission rates fluctuated; e.g., at 18 degrees C from day 19, the transmission rate was 80-100%, whereas at 14 degrees C on day 36, the transmission rate was 60% and thereafter 20-100%. The maximum transmission rate occurred concurrently with maximum titers of virus secreted into capillary tubes during in vitro transmission attempts. Mosquito longevity increased as incubation temperature decreased and was maximum at 114 d at 14 degrees C. Mosquitoes that were transferred from 14 to 26 degrees C after 49 d subsequently oviposited, engorged on a pigeon, and transmitted virus, which indicated the possibility for overwintering of WN virus in adult Cx. univittatus. Vector competence at outside cycling temperatures was intermediate between that at 26 and 30 degrees C, indicating that incubation at 26 degrees C would give a fair reflection of the vector competence of Cx. univittatus during the summer near Johannesburg. Two human epidemics of WN virus are reevaluated in the light of these results; it is concluded that, in addition to abnormal rainfall, higher than normal temperatures were important factors for their occurrence.

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