Medical and Veterinary Entomology (2002) 16, 245±252 The 2000 epidemic of Rift Valley fever in Saudi Arabia: mosquito vector studies P. G. JUPP* ,A.KEMP* , A. GROBBELAAR* ,P.LEMAN* ,F.J.BURT* , A. M. ALAHMEDy,D.ALMUJALLIz,M.ALKHAMEESz and R. SWANEPOEL* *Special Pathogens Unit, National Institute for Communicable Diseases and Department of Virology, University of the Witwatersrand, South Africa, yDepartment of Plant Protection, College of Agriculture, Riyadh, Saudi Arabia and zMinistry of Agriculture and Water, Riyadh, Saudi Arabia 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 mosqui- toes for attempted RVF virus isolation. Among 23 699 mosquito females tested, isolations of RVF virus were made from six of 15 428 Culex (Culex) tritaenio- rhynchus 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 north- wards 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. Key words. Aedes vexans arabiensis, Culex tritaeniorhynchus, arbovirus vectors, mosquitoes, light-suction traps, Rift Valley fever virus, Jizan, Saudi Arabia. Introduction Yemen, and to have continued until the end of November. Although the epidemic was centred in Jizan Region and In mid-September 2000, it was confirmed that Rift Valley northern Yemen, it subsequently extended northwards fever (RVF) infection had occurred in both humans and into the Aseer and Al Quenfadah health regions. This is livestock in the Kingdom of Saudi Arabia (CDC, 2000a, b). the first time that RVF virus has been reported outside The epidemic appears to have started simultaneously in the Africa and Madagascar. Jizan Region in the south-west, as well as in neighbouring On 24 September, a team of four scientists from the Special Pathogens Unit of the National Institute for Vir- ology in Johannesburg, South Africa, arrived in Jizan at the Correspondence: Alan Kemp, National Institute for Communic- invitation of the Saudi Arabian Ministry of Agriculture and able Diseases, Private Bag X4, Sandringham 2131, South Africa. Water to help investigate the epidemic. The South African E-mail: [email protected] team members stayed in Saudi Arabia for up to 4 weeks. # 2002 The Royal Entomological Society 245 246 P. G. Jupp etal. One aspect of the study was a serological survey in livestock inundated by rain but some fields are irrigated with under- throughout Jizan Region and the concomitant vaccination ground water in the absence of rain. of these sheep and goats and a smaller number of cattle and camels. That work will be the subject of a separate paper, while the present paper deals with the entomological aspect Mosquito collecting of the study. In order to determine the vectors of the virus, mosquitoes were collected in the vicinity of human and/or Four methods were used for collection of adults: net traps livestock infections as soon as possible after such infections baited with solid carbon dioxide (CO2) (Jupp & McIntosh, had occurred. The identities of the prevalent mosquito 1967), battery-operated CO2-baited suction traps (ABC species were established, these were tested for the presence trap: Clarke Environmental Mosquito Management Inc., of RVF virus, their host preferences were determined and Roselle, Illinois, U.S.A.) run with or without a light source some observations were made on their breeding places (6 volt Phillips 150 ma bulb), and landing-biting catches on (main larval sites). Additionally, limited laboratory vector human bait. The suction traps were suspended inside and out- competence tests were undertaken. side on the walls of a sheep sleeping hut with a control trap sited 60 m away from the hut. The human-baited catches were also conducted in the manner described by Jupp & McIntosh (1967) by one or two catchers (immunized team Materials and methods members). All the adult collections were undertaken from 18.00 to 20.30 hours, sunset was at about 18.45 hours. Study area Owing to logistical problems and other difficulties, mosquito trapping was limited to 11 evenings. During the daytime Saudi Arabia has a `hotdesert'climate(Fig. 1). The Jizan a few larval collections were made at some of the sites. region is in the south-west, with its western boundary the Larvae were reared in the field to obtain both male and Red Sea, the Yemen Republic to the south and east and the female adults as well as their associated larval exuviae. Aseer region to the north. Jizan Region has been described Adult males were also collected resting in vegetation. briefly by (Ageel & Amin, 1997). There are three distinct zones: `mountain' (the Sarawat range) (2000±2500 m a.s.l., >300 mm rain/year); `hill' (400±600 m a.s.l., <300 mm/year) Mosquito identifications and `coastal plain' (<400 m a.s.l., little if any rain each year). Collection sites were selected after surveillance of the Culicine mosquitoes were identified according to the Jizan Region by helicopter in two phases. First, a south± keys and/or descriptions of Edwards (1941), Mattingly & north survey, close to the Yemen border and also in the Knight(1956), Hopkins (1952), Sirivanakarn (1976) and coastal plane was undertaken. This revealed that the only Jupp (1996). The designation of the Aedes (Aedimorphus) suitable habitat for mosquito breeding existed in the wadis, mosquito arabiensis as a subspecies of vexans followed which run from the Sarawat Mountain range down to the White (1975). Anophelines were identified from the keys Red Sea. Wadi Jizan was then chosen for a second survey of Gillies & De Meillon (1968). Mounts of larval exuviae because it had been associated with the RVF outbreak and and male genitalia were made for confirmation of species also because it was typical of the wadis of the region. Five of identification. our eight collection sites represented various stages of the Wadi Jizan and its tributaries, all within an 8 km radius of Jizan dam (90±300 m a.s.l.): near-Falas westof thedam Virus isolation (coastal plain), a site each on the south and east banks of the dam and two sites in the foothills of the Sarawat Moun- Most mosquito pools contained 200 mosquitoes, which tains, near the Yemen border. One other site in the coastal were processed to obtain a supernatant fluid as described by plain was situated further north near Baysh (50 m a.s.l.). Jupp et al. (2000). Each supernatant was tested by reverse The remaining two sites were situated in the hill zone: transcription-polymerase chain reaction (RT-PCR) to Al Khawbah to the south near the Yemen border identify the presence of RVF viral RNA. If the result of this (400 m a.s.l.) and Harub in the foothills of the mountains test was positive or possibly positive, the supernatant was to the north (500 m a.s.l.). Rainfall at all sites was considerably inoculated into suckling mice on the same day. If the mice higher than usual during 2000, for example, 262 mm fell died in the expected death time for RFV virus, mouse brain from June to September at Al Aridah, midway between was passaged into Vero cells, where the presence of RVF Jizan Dam and the Sarawat Mountains (Fig. 1), so that viral antigen was confirmed by the indirect immunofluor- sites were much wetter than usual, particularly those in the escent antibody test (IFAT) using the procedure described catchment of the Jizan Dam (the Sadd Wadi Jizan) and by Shope & Sather (1979). For those collection sites with Wadi Baysh (with Wadi Harub as one of its tributaries, viral activity, mosquito infection rates were calculated: the via Wadi Wusa and Wadi Shahdan), providing good `minimum infection' rate is the number of infected mosqui- mosquito larval habitats. Such habitats are described in toes per 1000 calculated arithmetically, whereas the `statis- Table 1 for each site. Agricultural fields may become tical infection rate' is the number per 1000 calculated by the # 2002 The Royal Entomological Society, Medical and Veterinary Entomology, 16, 245±252 Rift Valley fever in Saudi Arabia 247 Fig. 1. Map of Jizan Region of Saudi Arabia, showing mosquito collection sites: 1, Al Khawbah; 2, near Falas; 3, Jizan Dam east bank; 4, Jizan Dam south bank; 5, Al Refoud; 6, near Al Humayrah; 7, near Baysh and 8, Harub. # 2002 The Royal Entomological Society, Medical and Veterinary Entomology, 16, 245±252 248 P. G. Jupp etal. Table 1. Site descriptions and numbers of mosquitoes (per trap hour) collected in net and light (suction) traps baited with CO2, ateight different sites in Jizan Region, with proportions of the two principal species. Mosquitoes/trap hour (total collected) Species ratio Cx.