Re-emergence of Rift Valley fever virus in Barkedji (Senegal, West Africa) in 2002-2003: identification of new vectors and epidemiological implications.

The Rift Valley fever virus (RVFV) is a threat that must not be neglected, as the consequences of RVFV are dramatic, both for human and animal health. This virus is a zoonotic virus that already has demonstrated a real capacity for re-emerging after long periods of silence, as observed in Barkedji (Senegal, West Africa) in 2002. In this article we present the 2nd emergence in Barkedji after the 1st manifestation in 1993, and for the 1st time the circulation of RVFV during 2 consecutive years among mosquito populations in Senegal. As part of the entomological surveillance program undertaken since 1990 to detect circulation of the RVFV in Barkedji, 108,336 mosquitoes belonging to 34 species and 5 genera were collected in 2002-2003. Aedes vexans and Culex poicilipes, previously known to be vectors of RVFV in Senegal, comprised 88.7% of the total collection. In 2002, Ae. vexans was the most abundant mosquito, followed by Cx. poicilipes; the opposite situation was observed in 2003. In 2002, 29 and 10 RVFV isolates were obtained from Cx. poicilipes (minimum infection rate [MIR] = 0.13%) and Ae. vexans (MIR = 0.02%) pools, respectively and the MIR for the 2 species were significantly different (chi2 = 34.65; df = 1, P < 0.001). In 2003, 7 RVFV strains were isolated from Cx. poicilipes (3, MIR = 0.03), Mansonia africana (2, MIR = 0.08), Ae. fowleri (1), and Ma. uniformis (1, MIR = 0.05). The 3 latter species were found to be associated with RVFV for the 1st time in Senegal. A significant decrease in MIR was observed from 2002 to 2003 (chi2 6.28; df = 1, P = 0.01) for Cx. poicilipes, the only species involved in the transmission during the 2 sampling years.

[Integrated approach to yellow fever surveillance: pilot study in Senegal in 2003-2004]. / Approche intégrée de la surveillance de la fièvre jaune: étude pilote au Sénégal en 2003-2004.

The aim was to undertake a pilot study of integrated surveillance of yellow fever (YF) in Senegal, based on i) a human surveillance involving healthcare centers in the 11 administrative regions of the country ii) an entomological surveillance including domestic and sylvatic environment and iii) screening mosquitoes for YF virus using RT-PCR method. The integrated approach of human and entomological surveillance was conducted for 2 years (2003-2004). Surveillance in human population was based on screening samples of YF suspected cases (i.e. patients with acute (< or = 15 days) febrile illness with jaundice) for YF specific IgM antibodies. The entomological surveillance was carried out by collecting mosquitoes using human landing catch method and attempt to detect YF virus on them by RT-PCR. Forty five percent of the healthcare centres notified at least one suspected YF case during 2003-2004 periods. Among the 342 sera collected over 2 years, 2 revealed anti-YF IgM antibodies leading to investigations which allowed identification of the source and place of infection and implementation of a reactive focused YF immunization campaign. In addition, YFV was detected by RT-PCR from 49 out of 1762 mosquitoes tested and distributed as follows: in the sylvatic environment, 29 from Aedes furcifer and 1 from Aedes aegypti while in the domestic area, 15 Aedes aegypti and 4 Aedes furcifer. RT-PCR was found more sensitive and rapid than viral isolation for YF virus detection in mosquitoes. The pilot study in Senegal for YF surveillance integrating human and entomological parameters in domestic and sylvatic areas showed that this approach is very efficient in detecting yellow fever virus circulation due to the complementarity of the two systems. Therefore, in the light of the encouraging results presented herein, similar studies in different context and areas are needed to further validate and allow the extension of its application to other endemic regions of Africa.

Mosquito vectors of the 1998-1999 outbreak of Rift Valley Fever and other arboviruses (Bagaza, Sanar, Wesselsbron and West Nile) in Mauritania and Senegal.

Following an outbreak of Rift Valley fever (RVF) in south-eastern Mauritania during 1998, entomological investigations were conducted for 2 years in the affected parts of Senegal and Mauritania, spanning the Sénégal River basin. A total of 92 787 mosquitoes (Diptera: Culicidae), belonging to 10 genera and 41 species, were captured in light traps. In Senegal, Culex poicilipes (41%) and Mansonia uniformis (39%) were the most abundant species caught, whereas Aedes vexans (77%) and Cx. poicilipes (15%) predominated in Mauritania. RVF virus was isolated from 63 pools of Cx. poicilipes: 36 from Senegal in 1998 and 27 from Mauritania in 1999. These results are the first field evidence of Cx. poicilipes naturally infected with RVFV, and the first isolations of this virus from mosquitoes in Mauritania - the main West African epidemic and epizootic area. Additional arbovirus isolates comprised 25 strains of Bagaza (BAG) from Aedes fowleri, Culex neavei and Cx. poicilipes; 67 Sanar (ArD 66707) from Cx. poicilipes; 51 Wesselsbron (WSL) from Ae. vexans and 30 strains of West Nile (WN) from Ma. uniformis, showing differential specific virus-vector associations in the circulation activity of these five arboviruses.

Use of reverse transcriptase PCR in early diagnosis of Rift Valley fever.

Reverse transcriptase PCR (RT-PCR) for diagnosis of Rift Valley fever (RVF) was evaluated by using 293 human and animal sera sampled during an RVF outbreak in Mauritania in 1998. Results of the RT-PCR diagnostic method were compared with those of virus isolation (VI) and detection of immunoglobulin M (IgM) antibodies. Our results showed that RT-PCR is a specific, sensitive tool for RVF diagnosis in the early phase of the disease and that its results do not differ significantly from those obtained by VI. Moreover, the combined results of RT-PCR and IgM antibody detection were in 100% concordance with the results of VI.

Arbovirus surveillance from 1990 to 1995 in the Barkedji area (Ferlo) of Senegal, a possible natural focus of Rift Valley fever virus.

Surveillance for mosquito-borne viruses was conducted in Barkedji area from 1990 to 1995, following an outbreak of Rift Valley fever (RVF) virus in southern Mauritania. Mosquitoes, sand flies, and midges were collected from human bait and trapped by solid-state U.S. Army battery-powered CDC miniature light traps baited with dry ice or animals (sheep or chickens) at four ponds. Overall, 237,091 male and female mosquitoes representing 52 species in eight genera, 214,967 Phlebotomine sand flies, and 2,527 Culicoides were collected, identified, and tested for arboviruses in 9,490 pools (7,050 pools of female and 331 of male mosquitoes, 2,059 pools of sand flies and 50 pools of Culicoides). Viruses isolated included one Alphavirus, Babanki (BBK); six Flaviviruses, Bagaza (BAG), Ar D 65239, Wesselsbron (WSL), West Nile (WN), Koutango (KOU), Saboya (SAB); two Bunyavirus, Bunyamwera (BUN) and Ngari (NRI); two Phleboviruses, Rift Valley fever (RVF) and Gabek Forest (GF); one Orbivirus, Ar D 66707 (Sanar); one Rhabdovirus, Chandipura (CHP); and one unclassified virus, Ar D 95537. Based on repeated isolations, high field infection rates and abundance, Culex appeared to be the vectors of BAG, BBK, Ar D 65239 (BAG-like), and WN viruses, Ae. vexans and Ae. ochraceus of RVF virus, Mansonia of WN and BAG viruses, Mimomyia of WN and BAG viruses, and Phlebotomine of SAB, CHP, Ar D 95537, and GF viruses. Our data indicate that RVF virus circulated repeatedly in the Barkedji area.

First isolation of the Rift Valley fever virus from Culex poicilipes (Diptera: Culicidae) in nature.

Following the reemergence of Rift Valley fever (RVF) virus in southeastern Mauritania in 1998, an entomological survey was undertaken in the boundary area in Senegal to assess the extent of the virus circulation. During this study, RVF virus (36 strains) was isolated for the first time from Culex poicilipes in nature. The possible role of Cx. poicilipes as an RVF vector is discussed regarding its biology and ecology.

Yellow fever outbreak in Kaffrine, Senegal 1996: epidemiological and entomological findings.

In November 1996 a yellow fever (YF) outbreak occurred near Kaffrine in the central part of Senegal. Thirty-six deaths were notified, all children under 15 years of age. The YF diagnosis was confirmed by MAC-ELISA or by virus isolation. The immune status against YF virus of a sample population of 449 individuals was determined, and 31 confirmed cases and 69 asymptomatic cases were reported. Distribution of YF cases and incidence rate decreased with age, while the attack rate was stable in all age groups. Larva indices were high and Aedes aegypti was common in all villages, causing man-to-man transmission. The greatest risk of YF disease was lack of immunity, especially in individuals <20 years of age. The outbreak was rapidly controlled by an emergency immunization campaign. YF epidemics occurred in Senegal over two consecutive years. The last outbreak reached the main road to Dakar and the risk of spread to urban areas has increased.

First evidence of natural vertical transmission of yellow fever virus in Aedes aegypti, its epidemic vector.

Entomological investigations were conducted in 1995 in Senegal, following a yellow fever (YF) outbreak. A total of 1125 mosquitoes collected in the field, including males, females and 12-48 h old newly emerged adults reared from wild-caught larvae, were tested for YF virus. Among the 22 species captured, Aedes aegypti was the most common. 'Wild' vectors of YF were also captured, including A. furcifer, A. metallicus and A. luteocephalus. In all, 28 YF virus isolations were made: 19 from A. aegypti females, including 2 from newly emerged specimens; 5 were obtained from A. aegypti males, including one from a pool of newly emerged specimens, 2 from A. furcifer females, and one each from a female A. metallicus and a female A. luteocephalus. The true infection rates (TIRs) were much higher in adult A. aegypti than in specimens reared from larvae--8.2% and 31.4% for female and male A. aegypti captured on human volunteers, respectively (P < 0.0001). The TIRs for A. aegypti reared from larvae were 1.4% and 0.5% for females and males, respectively (P > 0.05). This outbreak was an intermediate YF epidemic, involving 4 vector species. Our data provide the first evidence of vertical transmission of YF virus in nature by A. aegypti, its main vector to humans, and strongly suggest that vertical transmission played a major role in the spread of the epidemic.

Surveillance for yellow fever virus in eastern Senegal during 1993.

During the 1993 rainy season, 15,806 mosquitoes, including 14,304 Aedes ssp., were collected and tested for virus infection in 702 and 547 pools, respectively. Aedes furcifer (Edwards) was the most abundant species collected throughout the survey period. Yellow fever (YF) virus was detected in 187 pools: Ae. furcifer (123 isolates), Ae. taylori (Edwards) (41 isolates), and Ae. luteocephalus (Newstead) (23 isolates). A high prevalence of immunoglobulin (IgG) antibodies was found in human and simian populations. Results clearly indicated that increased sylvatic YF activity in eastern Senegal has the increased the risk of YF transmission among rural populations in West Africa. Our results showed that a minimal survey period may be effective in detecting the circulation of YF in the Kedougou area.

Short report: Rift Valley fever in western Africa: isolations from Aedes mosquitoes during an interepizootic period.

Thirteen strains of Rift Valley fever virus were isolated from Aedes vexans and Ae. ochraceus mosquitoes collected in October and November 1993 in northern Senegal. Entomologic and serologic data show that the risk of a new epizootic is increasing in this region.

Isolations of West Nile and Bagaza viruses from mosquitoes (Diptera: Culicidae) in central Senegal (Ferlo).

During October-November 1990, 31,497 mosquitoes consisting of 25 different species were collected in Barkedji, Ferlo area (Senegal), and tested for virus infection. Viruse were isolated from 55 of 407 pools. Eighteen pools were found positive for both Bagaza virus (BGA) and West Nile virus (WN). One alphavirus (Babanki [BBK] and 72 flaviviruses (19 BGA, 53 WN) were isolated from Culex poicilipes Theobald (29 WN, 8 BGA), C. neavei Theobald (3 WN, 1 BGA), Mimomyia hispida Theobald (8 WN, 6 BGA, and 1 BBK), M. lacustris Edwards (4 WN, 1 BGA), M. splendens Theobald (6 WN, 2 BGA), Mimomyia. spp. (2 WN), and Aedeomyia africana Neveu-Lemaire (1 WN). These were the first isolations of arboviruses from A. africana and Mimomyia species. C. poicilipes and possibly Mimomyia spp. may be involved in an avian-mosquito cycle of West Nile virus transmission in Senegal.

Dengue 2 outbreak in southeastern Senegal during 1990: virus isolations from mosquitoes (Diptera: Culicidae).

During 1990, Dengue-2 (DEN-2) virus was isolated for the first time from mosquitoes (Aedes furcifer, six isolates; Ae. taylori, six isolates; Ae. luteocephalus, seven isolates) collected during an epidemic in which DEN-2 virus also was isolated from humans. Numerous isolations have been made previously from mosquitoes in the absence of human infection. In Senegal, DEN-2 virus appears to be maintained in an enzootic cycle and, therefore, plays an expanding role in human disease and increases the need for effective surveillance in mosquito populations.

First isolations of arboviruses from phlebotomine sand flies in West Africa.

For the first time in West Africa, arboviruses were isolated from phlebotomine sand fly pools. One strain of Chandipura virus (a Vesiculovirus), four strains of Saboya virus (a Flavivirus), and one strain of a not yet identified virus were isolated. Three hundred twenty-two pools were established from a population of 33,917 sand flies caught in CO2 light traps in the Ferlo Sahelian region of Senegal from November 1991 to December 1992. This is the first isolation of Chandipura virus from any arthropod in Africa. Saboya virus has already been isolated from small rodents in Senegal; thus, its transmission cycle probably involves rodentophilic sand flies. No strain of Rift Valley fever phlebovirus, which caused an epizootic in this region in 1987, was isolated. During the same time at the same site, 11 sand fly species were identified from 4,191 specimens caught on sticky traps, including Phlebotomus duboscqi, a leishmaniasis vector.

[Surveillance of the circulation of arbovirus of medical interest in the region of eastern Senegal]. / Surveillance de la circulation des arbovirus d'intérêt médical dans la région du Sénégal oriental (1988-1991).

A study about the circulation of arboviruses of medical interest in southeastern Senegal was conducted from 1988 to 1991, during and around the periods of transmission. Specific IgM antibodies were detected by ELISA test in human sera, as a sign of a recent infection within 2 to 5 months. The comparison of the serological IgM results from human surveys in different villages, and the isolations of arboviruses from mosquitoes during the same period of time permitted a rapid and global evaluation of the circulation of these viruses. A low level of yellow fever virus activity was detected both in humans and mosquitoes in 1988 to 1990. A dengue 2 epizootic occurred in 1989-1990. Dengue 2 virus was isolated from humans and mosquitoes in 1990. Some dengue 2 outbreak may occur in the upcoming years. A Zika virus epizootic outbreak was observed each year. A human strain was isolated in 1990. The other flaviviruses (West-Nile, Kedougou, Wesselsbron), Chikungunya virus, Rift Valley Fever virus and Crimean-Congo Hemorrhagic Fever virus did not seem to present a major public health concern in southeastern Senegal.

Continuous cell lines and immune ascitic fluid pools in arbovirus detection.

Successive experiments led us to use two cellular systems, MOS61 (Aedes pseudoscutellaris cells) and Vero cells, among the continuous cell lines recommended by the WHO Collaborating Center for systematic research and isolation of arboviruses. Virus detection in cell cultures is carried out with 7 mixtures containing 10 hyperimmune ascitic fluids made with the reference viruses. This technique enables the detection of 70 of the 80 arboviruses transmitted by mosquitoes in Africa and very easily detects arbovirus associations by using either monospecific or monoclonal immune ascitic fluids (dengue-1-2-3-4 and yellow fever viruses) used in the indirect immunofluorescence technique.

[Neurologic and neurosensory forms of Rift Valley fever in Mauritania]. / Les formes neurologiques et neurosensorielles de la fièvre de la Vallée du Rift en Mauritanie.

During and after a Rift Valley fever epidemic in Southern Mauritania, we observed 348 patients infected by RVF virus. 17 of them had encephalitis. These belonged to 2 groups, acute febrile forms with short duration and possibility of death, and sub-acute forms, with a longer duration and with sequelae. They were pure encephalitis, without clinical or biological meningeal signs. We also noticed 5 brutal ocular attacks, running very slowly, with sequelae.

[Hemorrhagic forms of Rift Valley fever in Mauritania]. / Les formes hémorragiques de la fièvre de la Vallée du Rift en Mauritanie.

During and after a Rift Valley fever (RVF) epidemic in Southern Mauritania, we collected 600 clinical observations. 348 were confirmed to be RVF cases. Among the 5 clinical forms we observed, some are benign but others, especially those with hemorrhagic signs are serious. We observed 48 icterohemorrhagic forms with 25 deaths. An icterus was associated with hemorrhagic signs, varying from gingivorrhagia to abundant bleeding. Biological hepato-nephritis was always present in major hemorrhagic forms. Fulminant forms, spectacular and characteristic are excellent markers for epidemiological studies in the field.

[Mild clinical forms of Rift Valley fever during the epidemic in Mauritania]. / Les formes cliniques bénignes de la fièvre de la Vallée du Rift pendant l'épidémie de Mauritanie.

During and after a Rift Valley fever (RVF) epidemic in Southern Mauritania we collected 600 clinical observations. 348 were confirmed to be RVF cases. We described 5 major clinical aspects: mild, icteric, icterohemorrhagic, hemorrhagic and neurological forms. The first one is the most frequently seen with 42.8% of the cases at admission. Fever was associated with various pains (cephalalgia, myalgia, arthralgia) and an important asthenia. Inconsistently this syndrome was accompanied by epistaxis and conjunctival hyperemia. The icteric form, never described before, is an icterus occurring during evolution of a mild form. It represents 28.5% of total cases at admission. The great number of theses mild forms implies that they could be used as excellent markers for an epidemic emergence.