Bunyaviridae family. The virus was first isolated in 1930 from a sheep during an epizootic at a farm by Lake Naivasha in the Rift Valley Province of Kenya

In December 2006, Rift Valley fever (RVF) was diagnosed in humans in Garissa Hospital, Kenya and an outbreak reported affecting 11 districts. Entomologic surveillance was performed in four districts to determine the epidemic/epizootic vectors of RVF virus (RVFV). Approximately 297,000 mosquitoes were collected, 164,626 identified to species, 72,058 sorted into 3,003 pools and tested for RVFV by reverse transcription-polymerase chain reaction. Seventyseven pools representing 10 species tested positive for RVFV, including Aedes mcintoshi/circumluteolus (26 pools), Aedes ochraceus (23 pools), Mansonia uniformis (15 pools); Culex poicilipes , Culex bitaeniorhynchus (3 pools each); Anopheles squamosus , Mansonia africana (2 pools each); Culex quinquefasciatus , Culex univittatus , Aedes pembaensis (1 pool each). Positive Ae. pembaensis , Cx. univittatus , and Cx. bitaeniorhynchus was a first time observation. Species composition, densities, and infection varied among districts supporting hypothesis that different mosquito species serve as epizootic/ epidemic vectors of RVFV in diverse ecologies, creating a complex epidemiologic pattern in East Africa. * Address correspondence to Rosemary Sang, Arbovirology/ Hemorrhagic Fevers Laboratory, Centre for Virus Research, Kenya Medical Research Institute (KEMRI), P.O. Box 54628, Nairobi, Kenya. E-mail: [email protected] 29 RVF IN KENYA, 2006–2007: ENTOMOLOGIC INVESTIGATIONS Vector-Borne Infectious Diseases (CDC-DVBID), Fort Collins, CO and the Navy Medical Research Unit No. 3 Cairo, Egypt. The team focused on evaluating the entomologic parameters that contributed to the epidemic; specifically, to determine the mosquito species composition, the abundance of known and potential mosquito vector species, and to conduct virus testing to identify the species most likely involved in virus transmission in the affected areas. Our goal was to learn more about the vectors involved in virus maintenance and transmission during outbreaks, information that could be used to forecast risk and facilitate improvement of prevention and response tools for use in preventing or controlling future outbreaks. MATERIALS AND METHODS Study sites. Studies were undertaken at four ecologically distinct communities in eastern, central, and western Kenya where RVFV activity was detected in humans or livestock ( Figure 1A ). The first cases of RVF were reported from the Garissa district, which is located in the North Eastern Province of Kenya bordered by the Tana River to the west and Somalia to the east ( Figure 1B ). The district is characterized as an arid area with Somali Acacia-Commiphora bushlands and thickets. Rainfall is sporadic, averaging approximately 200–500 mm per year, with occasional torrential storms causing extensive flooding. The average temperatures range from 20 to 38°C and the altitude varies from 70 to 400 m above sea level. The soil is generally sandy with scattered areas of dark clay that tend to retain water after the rains and serve as watering holes and grazing land for livestock and wild animals. The sparse population (~7 people/km 2 ) 26 of the district is found concentrated around the water sources and also around small market centers. The people are largely nomadic, moving between districts with their large herds in search of water and pasture land. The collection of arthropods was conducted between December 15, 2006 and March 3, 2007 at 20 sites within an estimated 100 km radius of the provincial capital of Garissa ( Figure 1B ). The city of Kilifi, located in Kilifi district, Coast Province, is 318 km south of Garissa ( Figure 1C ). The district contains a moderately dense population (~114 people/km 2 ), 26 and the vegetation is characterized by a mix of East African mangroves and northern Zanzibar-Inhambane coastal forests that are comprised of dense woods, swamps, dry scrub, and commercial plantations. 27, 28 Annual rainfall for the district ranges from 750 to 1,200 mm, while the average temperature maximum is 30°C. 29 The soils are fertile and subsistence farming of corn, coconut, goats, chicken, and cattle is widespread. The collection of vectors was conducted between 12 January and 1 February 2007 at 10 homesteads that were associated with confirmed or suspected RVF cases or at sites nearby ( Figure 1C ). Nine of the homesteads were within a 120 km transect of each other and no further than 6 km from the coast. The remaining homestead was inland, about 30 km west of the other homesteads. Kirinyaga is located in the highland region of the Central Province of Kenya, approximately 100 km northeast of Nairobi, on the southern slope of Mount Kenya ( Figure 1A ). At 1,113 to 1,623 m above sea level, the collection sites were typified by East African montane forests and northern Acacia-Commiphora bushlands and thickets. The mean daily temperatures range from 16 to 26°C, with an annual rainfall of approximately 950 mm. 30 This densely populated district is home to more than 500,000 people (> 300 people/km 2 ). 26 The primary occupation is agriculture, including subsistence farming of corn, beans, and potatoes; cash crop farming of tea and rice, and the raising of both exotic and indigenous livestock. Mosquitoes were collected from four locations around the district from 6 to 8 February 2007 ( Figure 1A ). Baringo District is located in the Rift Valley Province of Kenya, 250 km northwest of Nairobi. The low lying arid part of Baringo consists of northern Acacia-Commiphora bushlands and thickets and has experienced severe land degradation caused by uncontrolled grazing. Harsh physical and climatic conditions have led to a sparsely populated district (average of 31 people/km 2 ) 26 where the local inhabitants, classified as agro pastoralists, subsist mainly on limited crop production and livestock rearing. The collection of arthropods was conducted at three sites near Lake Baringo (elevation ~980 m) where the annual rainfall ranges from 300 to 700 mm, and the daily temperature varies between 16 and 42°C. 31 Trapping was conducted from 13 to 15 February 2007 around flooded marshland ~2.2 km west of Lake Baringo and along the Molo river, south of the lake ( Figure 1D ). Vector collection and identification. Arthropods were collected from areas where confirmed or suspected RVF cases were previously reported. Mosquitoes and sand flies were sampled using CO 2 -baited CDC light traps placed outdoors approximately 1 hour before sunset and collected 1–3 hours after sunrise the next day. Mosquitoes were typically anesthetized using triethylamine, 32 identified to species, and pooled (≤ 25 mosquitoes per pool) by species, sex, collection date, and site and frozen at −80°C for later testing. When large numbers of mosquitoes were trapped, they were killed by freezing, immediately stored in 15 mL centrifuge tubes, and transported in a liquid nitrogen shipper to the laboratory where they were identified on ice and pooled as indicated for testing. During mosquito identification, all specimens with blood in their abdomens (blood fed) were sorted out and preserved singly in vials for subsequent blood meal analysis in a separate study. In selected locations, ticks were collected from infested animals, placed in 15 mL centrifuge tubes, stored in a liquid nitrogen shipper, and transported to the laboratory for identification and testing. Mosquitoes and ticks were identified to species using various taxonomic keys. 33– 38 Representative pinned specimens of the important species were sent to Walter Reed Biosystematics unit in Silver Spring, MD and to the taxonomy unit of the Arthropod Borne and Infectious Diseases laboratory, CDC, Fort Collins, CO, for verification of the identification. Parity determination. Parity was determined for a limited number of mosquitoes because of concerns that some mosquito collections might be composed largely of newly emerged, unfed females that would likely be uninfected. A sub-sample of pools of probable vector species, and of species with a high relative abundance was evaluated to estimate parous rates. Females from selected pools were placed on a microscope slide and their ovaries dissected into a drop of distilled water. 39 After drying, the ovaries were graded as parous (evidence of previous blood feeding and egg production) or nulliparous (no evidence of egg production). Forceps and other instruments used during the dissections were dipped in 70% ethanol and flame sterilized after dissections of a pool to eliminate transfer