The 2000 epidemic of Rift Valley fever in Saudi Arabia: mosquito vector studies

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, six isolations of RVF virus were made from 15 428 Culex (Culex) tritaeniorhynchus Giles and seven from 8091 Aedes (Aedimorphus) vexans arabiensis Patton [corrected]. 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 log10 FFU/mL of virus; transmission rates were 10% (post-infection day 14) and 26% (post-infection 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.