Role of Culex and Anopheles mosquito species as potential vectors of rift valley fever virus in Sudan outbreak, 2007

Rift Valley fever (RVF) is an acute febrile arthropod-borne viral disease of man and animals caused by a member of the Phlebovirus genus, one of the five genera in the family Bunyaviridae. RVF virus (RVFV) is transmitted between animals and human by mosquitoes, particularly those belonging to the Culex, Anopheles and Aedes genera [1, 2]. Transmission is mostly horizontal, but a vertical mode was described for some Aedes species [3, 4]. RVFV is carried in the eggs of Aedes mosquitoes which breed in isolated depressions called dambos found in the vast grassland areas. At flooding of the dambos during periods of extensive and widespread rainfall, the eggs of the Aedes mosquitoes hatch and the subsequent adults transmit the virus to domestic animals including sheep, goats, cattle, camels, and buffalos [5]. These depressions also serve as good habitats for Culex and Anopheles mosquito species. When Aedes mosquitoes infect domestic animals with RVFV, virus amplification occurs in these vertebrate hosts, leading to propagation into various Culex and Anopheles species that are capable of transmitting the virus to a wider area beyond the area of the original outbreaks by wind-borne dispersal [6]. Although RVFV is, in most cases, transmitted to humans by mosquitoes, it may also be transmitted through direct contact with secretions of infected animals and meat. The professional nature of some groups, such as doctors and veterinarians working in slaughterhouses, made them more vulnerable than others to infection (because the virus is not vital outside the body). Good cooking of meat may help to eliminate the virus from it [7].

The disease in animals is characterized by high rates of abortion and death of young ruminants [1]. Human disease symptoms are often limited to a flu-like syndrome with transient fever, rigor (shivering), headache, severe muscle and joint pains, photophobia and anorexia, sometimes with a petaechial rash, nausea, vomiting and epistaxis. The symptomatic course is 4 to 7 days leading to full recovery in 2 weeks. However in severe cases, case fatality rates are about 1% and usually associated with hemorrhagic manifestations of the disease. Meningoencephalitis also occurs in about 1% of infections, but does not usually result in death. Ocular sequellae occur in a moderate percentage of cases and may result in impaired vision or even blindness, which might be permanent [2]. Human cases are mainly caused by direct contact with infected animal body fluids after abortion or slaughtering viremic animals and by mosquito bites. When an epizootic occurs in animals, it is easily transmitted to humans leading to an epidemic [8].

Since livestock immunization against RVF has so far appeared difficult to implement efficiently in areas of endemicity, strengthened surveillance, early detection, management of cases seemed to be among the best options to prevent extension of RVF epidemic foci. Precise estimation of specific weight for each risk factor is a considerable guide to construct an effective outbreak control plan. Insufficient entomological studies and surveys were conducted and resulted in a vague evaluation of the vector role in RVFV transmission and dissemination in Sudan. For that reasons, this study was conducted to answer some raised questions about the role of vector(s) in RVFV outbreak, 2007 in Sudan.

This work was developed by an expert Egyptian team on request of the Sudanese Federal Ministry of Health and the author (A.S.) was one of this team.

The low diversity of the mosquito species collected in this survey may be due to natural abundance of mosquito species (high abundance of Culex and Anopheles species). Alternatively, it might be because the mosquito captures were undertaken at the end of the rainy season, when most breeding sites had dried up and/or due to the intensive use of pesticides to control blood sucking insect vectors during outbreak. This hypothesis was further strengthened by the small number of mosquitoes caught (978) and the low number of Aedes mosquitoes, a vector species of RVFV in West Africa [18]. Among all adult and immature mosquito stages collected, Cx. pipiens complex (51.1%) and An. gambiae arabiensis (40.9%) were the most represented species. Most of the collected mosquito species, especially Culex species [19], were previously incriminated in RVFV transmission and dissemination (Tables 1, 2 and 3). In addition, feeding preference studies showed that Cx. pipiens fed preferentially on pigeons but also on human at indoors. However, Anopheles species fed preferentially on human but also on other mammals. These considerations on Culex and Anopheles biology and ecology as well as its experimentally demonstrated vectorial competence to RVFV (Table 1) constituted clues pointing to their relative role in RVFV transmission cycles. Excluding gravid females (to avoid false positive results), the relative role of An. gambiae arabiensis in transmission of RVFV in White Nile and Khartoum states was 1.5% and 0.4%, respectively. Meanwhile, the relative role of Cx. pipiens complex in transmission of RVFV was 0.8% and 4.8% in White Nile and Khartoum states, respectively. In case of White Nile state, the relative roles of An. coustani, Cx. poicilipes and Ae. aegypti were 0.6, 0.2 and 2.1%, respectively. High relative role in case of Ae. aegypti may be attributed to the high number of positive detections in relation to the number of collected samples. Although the choice of a 1-month period, for monitoring mosquito natural abundance is quite a short time, it was assumed to be a critical period for RVFV outbreak. In addition, we were quite a bit late in the transmission season and tried to follow up the outbreak as soon as we could launch it.

RT-PCR assay is considered as specific, sensitive tool for RVF diagnosis in the early phase of the disease and its results do not differ significantly from those obtained by virus isolation [20]. Successful RT-PCR detection of RVFV in female mosquito species was considered as a precursor for viral circulation in these species (incriminated in dissemination or acquired the virus in its midgut only). Meanwhile, successful RT-PCR detection of RVFV in male and larval stages, although at very low rates, referred to transovarial (vertical) transmission of the virus within these mosquito species. It may also refer to possible venereal RVFV transmission when a male being infected vertically and then infecting the female during mating. RVFV was known to be carried in the eggs of Aedes mosquitoes which can survive for several years in the dried mud [21]. These mosquitoes breed in isolated depressions called dambos found in the vast grassland areas. On flooding, Culex mosquito species (bread in dambos) play important role in virus circulation [22, 23]. The survival of RVFV during interepizootics was believed to depend on transovarial transmission of the virus in floodwater Aedes mosquitoes [4].

Satellite monitoring (June-September, 2007) showed that most of the central Sudan could be unusually subjected to heavy rainfall, and generated RVF risk warnings for central and southern Sudan in July-September. In late October, RVF outbreaks were reported by World Health Organization (WHO) in humans in Sudan in White Nile, Sinnar, and Gazira states. By early November 2007, 329 human cases, including 96 deaths were reported. The cases being reported in Gazira State are in an area close to irrigation canals and are linked to naturally occurring cycles involving livestock and mosquitoes which are abundant in the irrigation zone. In parallel, many previously studied outbreaks were reported to occur during heavy rainfall seasons in various sub-Saharan countries including Kenya, Somalia, Tanzania, Senegal, Eastern Africa, Mauritania, Egypt and more recently in Saudi Arabia and Yemen. Earlier works suggested that epizootics and epidemics of RVFV occur periodically after heavy rains that flood natural depressions in the grasslands of sub-Saharan Africa [24, 25]. In addition to three outbreaks recorded in neighboring countries (Table 1), Sudan was subjected to the heaviest rainfall season reported in the last forty years. Given the wide geographic and ecological range of RVFV, it is necessary to monitor large areas for conditions that may trigger the emergence of mosquito vectors that could spread RVF. Furthermore, there has been increased scientific interest in the connection between global climatic anomalies and disease outbreaks [26]. RVFV is one example of a disease whose outbreaks have been shown to be closely coupled with climate anomalies [25].

Overall connection between human and entomological results with regard to recorded outbreaks in neighboring countries were definite warnings for the incoming RFV outbreak in Sudan, especially that conditions of emerging epidemics were almost certain. Furthermore, the potential of RVF as a disease emerging in new areas was first documented in Egypt in 1977 [2], and since then, epidemics have occurred in Mauritania (1987 to 1988 and 1998), Madagascar (1990 to 1991), Egypt (1993), and eastern Africa (in Kenya, Somalia, and Tanzania) [27, 28]. Recently, the outbreak on the Arabian Peninsula (in Yemen and Saudi Arabia) represented the first case of RVF outside Africa [29, 30].

Although virus isolation (VI) is considered as gold standard method, IgM-ELISA method avoids false positive results due to the presence of rheumatoid factor and antinuclear antibodies. On the other hand, anti-RVF IgM antibodies were estimated to persist at a detectable level for up to 6 months in chronic infections [31]. However, combination of ELISA and RT-PCR assays is very important for rapid and efficient identification of RVFV during outbreaks. The present data and those obtained during the epidemics of RVF in Mauritania [11], in Kenya [32], as well as in Saudi Arabia and Yemen [33] demonstrated the importance of combining diagnostic assays for accurate and comprehensive detection of RVFV infection. The present results indicated that males of 15-29 years old were more susceptible than females. In parallel, housewives and farmers were the most susceptible people to RVFV infection (Table 5). These results may be related to their more vulnerability to the vector as well as to socioeconomic/professional activities which allow a direct contact with infected animals. Agreeable results were presented by Woods et al. [32] who stated that children <15 years of age were significantly less likely to have had recent RVFV infection. A relation between virus infection and direct contact with animals was reported by Faye et al. [11]. Indeed, those at highest risk include butchers and others who come in contact with animals (e.g., slaughterhouse workers, tanners, and herdsmen), who represent a large part of the population living in these areas. Although WHO estimated that the human mortality rate due to RVFV is ≈1%-2% of infected patients, the number of recorded deaths during this outbreak was 193 among 451 infected patients when the laboratory data were considered exclusively. This means that the number of infected but not officially recorded is about 19303 cases.

Conclusively, the present work reported that Culex and Anopheles were collected in relatively high numbers during Sudan outbreak, 2007. Because the mere presence of a mosquito species during the outbreak does not mean that it is involved in transmission cycle of RVFV, RT-PCR was used and successfully detected RVFV in female, male and larval stages of Culex and Anopheles species. Furthermore, connection between human and entomological studies results in important human case-vulnerability relatedness findings. It is noteworthy that validated RVF forecast models may provide early warning (~3 months) for RVF epidemics in Africa. Model performance, integrated with epidemiologic and environmental surveillance systems, should be assessed systematically for RVF and other mosquito-borne diseases using historical epidemiologic and satellite monitoring data. Case management related interventions; health education and vector control efforts are extremely effective in preparedness for viral haemorrhagic fever and other seasonal outbreaks. Further confirmatory transmission studies are very important to determine the vectorial capacity of the incriminated species in this study.