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After bluetongue (BT) appeared in northern Europe in August 2006 entomological studies were implemented in all five affected Member States (MSs) to establish which species of Culicoides had acted as vectors. The findings can be summarised as follows: (i) C. imicola the principal southern European/African vector of BTV has not penetrated into northern Europe, (ii) three pools of C. obsoletus/C. scoticus and one of C. dewulfi assayed RT-PCR-positive to BTV-8, (iii) in support of these results it was found that both potential vectors had also high parity rates (approximately 40%) indicating increased longevity favouring BTV virogenesis and transmission, (iv) furthermore, C. obsoletus/C. scoticus and C. dewulfi occurred also widely and abundantly on sheep and cattle holdings across the entire affected region, (v) and during the latter part of the season showed strong endophily readily entering livestock buildings in significant numbers to bite the animals inside (endophagy), (vi) which demonstrates that housing at best offers only limited protection to livestock from Culicoides attacks, (vii) in contrast the potential vector C. pulicaris sensu stricto was restricted geographically, was captured rarely, had a low parity rate (10%) and was exophilic indicating it played no role in the outbreak of BT, (viii) the incrimination of C. dewulfi as a novel vector is significant because it breeds in cattle and horse dung this close association raising its vectorial potential, but (ix) problems with its taxonomy (and that of the Obsoletus and Pulicaris species complexes) illustrates the need for morphological and molecular techniques to become more fully integrated to ensure progress in the accurate identification of vector Culicoides, (x) midge densities (as adjudged by light traps) were generally low indicating northern European Culicoides to have a high vector potential and/or that significant numbers of midges are going undetected because they are biting (and transmitting BTV) during the day when light traps are not effective, and (xi) the sporadic capture of Culicoides in the winter of 2007 invites re-examination of the current definition of a vector-free period. The re-emergence of BT over a wide front in 2007 raises anew questions as to precisely how the virus overwinters and asks also that we scrutinise our monitoring systems in terms of their sensitivity and early warning capability.

Culicoides imicola (Diptera: Ceratopogonidae) is the main vector of the bluetongue virus (btv), the causal agent of bluetongue, in southern Europe. This insect presents mainly an Afro-asian distribution, but it was first reported on mainland Europe in southern Spain ([Mellor and others 1983][1]) and

In August 2006, bluetongue virus (BTV) was detected in the Netherlands, Belgium, western Germany, Luxembourg and northern France for the first time. Consequently, a longitudinal entomological study was conducted in the affected region of northern France (Ardennes) throughout the autumn of 2006. Data on the spatio-temporal distribution of Culicoides (Diptera: Ceratopogonidae) associated with livestock were collected and an attempt was made to identify the vector(s) involved in BTV transmission by means of virus detection in wild-caught biting midges. Weekly sampling using standardized Onderstepoort-type blacklight traps were performed simultaneously both outdoors and indoors in one BTV-free and three BTV-affected farms between September and December 2006. Culicoides were sorted according to farm, location (outdoors vs. indoors), time point (in weeks), species and physiological stage. BTV detection was conducted by RT-PCR on monospecific pools of non-bloodfed parous female Culicoides. The principal results showed: (i) the absence of the Mediterranean vector, C. imicola, (ii) the relatively low abundance of C. dewulfi and C. pulicaris, (iii) the widespread occurrence and abundance of C. obsoletus/C. scoticus with longevity and behaviour compatible with BTV transmission, and (iv) all Culicoides pools tested for BTV were negative. In France, the very low levels of BTV-8 circulation were probably due to the limited introduction of the virus from affected neighbouring countries, and not due to the absence of local vector populations. A key finding has been the substantiation, for the first time, that Culicoides, and particularly the potential vectors C. obsoletus/C. scoticus and C. dewulfi, can be active at night inside livestock buildings and not only outside, as originally believed. The endophagic tendencies of members of the Obsoletus group are discussed in light of the prolonged period of BTV transmission during the autumn of 2006 and the risk of BTV overwintering and resurgence in the spring of 2007. Overall, there is an urgent need to improve our knowledge on the ecology of local Culicoides species before any clear, effective and reliable recommendations can be provided to the veterinary authorities in terms of prevention and control.

A longitudinal entomological study was carried out from 1999 to 2001 in Lena, a humid savannah village in the western region of Burkina Faso in order to establish malaria vector bionomics and the dynamics of malaria transmission. In the first year, malaria transmission was mainly due to An. gambiae s.s., but during the two later years was due to An. funestus, which were observed in high frequency towards the end of the rainy season. PCR identification of samples of An. gambiae s.l. showed 93% to be An. gambiae s.s. and 7% An. arabiensis. An. funestus constituting more than 60% of the vectors were identified in PCR as An. funestus s.s. The persistence of intense vectorial activity in this village was probably due to the road building in a swampy area creating a semi-permanent swamp that provided large sites for larval mosquitoes. These swampy sites seemed to be more favorable for An. funestus than for An. gambiae s.s. Thus, land development must be monitored and subjected to planning to minimize vector proliferation. Such a system of planning could lead to the restriction or even elimination of the swamp that is the source of larvae developing in the heart of the village. Journal of Vector Ecology 33 (1): 70-75. 2008.

The knowledge of the place where a disease is first introduced and from where it later spreads is a key element for the understanding of an epizootic. For a contagious disease, the main method is back tracing. For a vector-borne disease such as the Bluetongue virus serotype 8 epizootic that occurred in 2006 in North-Western Europe, the efficiency of tracing is limited because many infected animals are not showing clinical signs. In the present study, we propose to use a statistical approach, random walk, to model local spread in order to derive the Area of First Infection (AFI) and spread rate. Local spread is basically described by the random movements of infected insect vectors. Our model localised the AFI centre, origin of the infection, in the Netherlands, South of Maastricht. This location is consistent with the location of the farms where the disease was first notified in the three countries (Netherlands, Belgium, and Germany) and the farm where retrospectively the earliest clinical signs were found. The derived rate of spread of 10¿15 km/week is consistent with the rates observed in other Bluetongue epizootics. In another article Mintiens (2008), the AFI definition has then been used to investigate possible ways of introduction (upstream tracing) and to study the effect of animal movements from this area (downstream tracing).

Culicoides were captured at a BTV-infected dairy near Gulpen in the province of Limburg (south-east Netherlands) between 14 September and 4 October 2006. Onderstepoort-type blacklight traps were used to sample Culicoides both inside and outside a partially open shed housing 11 cattle. A total of 28 light trap collections were made at the shed and yielded: • 9371 Culicoides representing 11 species; >90% comprised five potential vectors of BTV and in order of abundance were Culicoides obsoletus and Culicoides scoticus (of the Obsoletus Complex), Culicoides dewulfi, Culicoides pulicaris and Culicoides chiopterus; Culicoides imicola, the principal Mediterranean (and African) vector of BTV, was absent. • 2339 Culicoides representing seven species were captured inside (endophily) the cattle shed; >95% comprised the Obsoletus Complex and C. dewulfi. Conversely, the Pulicaris Complex, represented by five species and including C. pulicaris, showed strong exophily with >97% captured outside the shed. • 7032 Culicoides were captured outside the shed, approximately threefold more than inside. This trend was reversed on an overcast day, when eightfold more Culicoides were captured inside; this indicates that when the light intensity outdoors is low Culicoides will attack (i) earlier in the day while cattle are still at pasture, and (ii) might follow cattle into the sheds in the late afternoon leading to elevated numbers of biting midges being trapped inside the shed during the subsequent hours of darkness. • Culicoides were captured inside the shed on all 14 sampling nights. On occasion up to 33% were freshly blood fed indicating they had avidly attacked the cattle inside (endophagy); because half the cattle had seroconverted to BTV, and because no cattle were left outdoors at night, the data indicate that (i) the housing of animals in partially open buildings does not interrupt the transmission of BTV, and/or (ii) BTV is being transmitted while cattle are grazing outdoors during the day. • The capture of partially engorged midges inside the shed shows they are being disturbed while feeding; this may lead to cattle being attacked repeatedly, and if these attacks include older parous BTV-infected Culicoides, may enhance virus dissemination (particularly in sheds where cattle stand close together). • Endo- and exophagy by potential vector Culicoides – coupled to increased adult longevity and multiple feeding events in single (potentially) infected midges – would ensure an R0 of >1, resulting in the continued maintenance and spread of BTV within local vertebrate populations. • Four light trap collections made additionally in a mature deciduous forest 70 m from the shed yielded a high proportion (48%) of gravid females amongst which 10% had incompletely digested blackened blood meals in their abdomens; the absence of this age category in Culicoides captured at the sheds indicates that all Culicoides, after engorgement, exit the buildings to undergo oogenesis elsewhere. In Europe, the blacklight trap is used widely for the nocturnal monitoring of Culicoides; a drawback to this approach is that this trap cannot be used to sample midges that are active during the day. Because diurnal biting in vector Culicoides may constitute a significant and underestimated component of BTV transmission a novel capture methodology will be required in future and is discussed briefly.

International audience; Since 1999, several serotypes of bluetongue virus (BTV) have been isolated in the western part of the Mediterranean basin, and since 2000, Corsica has been exposed to three different serotypes: BTv serotype 2 in 2000, BTv serotype 4 (BTV-4) in 2003 and BTv serotype 16 in 2004. In 2000 there were no surveillance systems for bluetongue, but in 2003, active surveillance of the circulation of BTv and its vector Culicoides species, aided by a raised level of awareness in farmers and veterinarians, made it possible to study the introduction Of BTV-4. The monitoring and analysis of the seroconversions of sentinel herds of goats, clinical signs and meteorological variables showed that the serotype had been present in the island since May that year, but clinical signs were first observed only in October. Moreover, the weather conditions and wind patterns were suitable for the transport of Culicoides species from Sardinia in May. These observations suggest that BTv had been transported on air currents from a southern infected area, and that it could have spread without causing clinical signs of disease for a few months.

SIR, — Zimmer and colleagues ( VR , January 26, 2008, vol 162, p 131) recently described maize silage residues as possible breeding sites for the sibling species Culicoides obsoletus/scoticus and confirmed that Culicoides dewulfi and Culicoides chiopterus breed in cattle dung. To complete the

In southern Europe, orbiviral diseases such as bluetongue (BT) have been assumed to have been largely transmitted by the classical Afro-Asian vector Culicoides imicola Kieffer (Diptera: Ceratopogonidae). Recent outbreaks have occurred in regions where C. imicola is normally absent, supporting the theory that other species belonging to the Obsoletus or Pulicaris complexes may play a role in BT virus transmission. Investigations of the ecology of the species within the former group are hampered by females of member species being extremely difficult to separate by classical morphology. To allow straightforward separation of these species in France, a multiplex polymerase chain reaction-based on internal transcribed spacer (ITS)-1 rDNA was developed to distinguish between Culicoides chiopterus Meigen, Culicoides dewulfi Goetghebuer, Culicoides montanus Shakirjanova, Culicoides obsoletus Meigen, and Culicoides scoticus Downes & Kettle. This tool will be useful in defining both the vector role and larval biotopes of these species in Europe.

The current microscopy method for identifying the Culicoides imicola Kieffer, 1913 species can be time and labour intensive. There is a need for the development of a rapid and quantitative tool to quantify the biting midges C. imicola ss in light trap catches. A reproducible and sensitive real-time polymerase chain reaction method that targets the internal transcribed spacer (ITS-1) of ribosomal DNA of C. imicola ss species was developed. This real-time PCR assay was first performed on 10-fold serial dilutions of purified plasmid DNA containing specific C. imicola ss ITS-1. It was then possible to construct standard curves with a high correlation coefficient (r2 = 0.99) in the range of 10−2–10−8 ng of purified DNA. The performances of this PCR were evaluated in comparison with morphological determination on Culicoides trapped along the Mediterranean coastal mainland France. ROC statistical analysis was carried out using morphology as gold standard and the area under the ROC curve had a satisfactory value of 0.9752. The results indicated that this real-time PCR assay holds promise for monitoring C. imicola ss population in both surveillance and research programmes because of its good specificity (92%) and sensitivity (95%).

SIR, — In Belgium, the first case of blue-tongue was notified on August 18, 2006. Up to March 7, 2007, bluetongue has been diagnosed on 399 sheep and 296 cattle farms ( ) and the last clinical case was recorded on January 15, 2007. Following the emergence of blue-tongue in Belgium

International audience; The recent and rapid spread in the Mediterranean Basin of bluetongue, a viral disease of ruminants transmitted by some species of Culicoides (biting midges), highlights the necessity of determining the conditions of its emergence. This study uses high spatial resolution satellite imagery and methods from landscape ecology science to identify environmental parameters related to bluetongue occurrence in Corsica, a French Mediterranean island where the disease occurred for the first time in 2000. A set of environmental variables recorded in the neighborhood of 80 sheep farms were related to case occurrence through a logistic regression model computed within three subsequent buffer distances of 0.5, 1 and 2 km. The results reveal the role of landscape metrics, particularly those characterizing land-use units such as prairies and woodlands, as well as farm type, latitude and sunshine to explain the presence of bluetongue. Internal and external validation both indicate that the best results are obtained with the 1 km buffer size model (area under Receiver Operating Characteristic curve = 0.9 for internal validation and 0.81 for external validation). The results show that high spatial resolution remote sensing (i.e. 10 m pixels) and landscape ecology approaches contribute to improving the understanding of bluetongue epidemiology.

The unprecedented epidemic of Ebola in west Africa has exposed not only response failures of governments, international instit utions, and public health agencies, but also the ineff ectiveness of using disconnected top-down health messages during public health emergencies. Gilles Guerrier and colleagues’ letter (Aug 29, p 851) on the need to strengthen scientific journalism in Africa is spot on. Their observation follows on the heels of a recent declaration of the World Federation of Science Journalists (WFSJ), which stressed the need for rigorous scientifi c journalism in tackling the Ebola crisis, and a survey among 112 African journalists of the Science journalism COOPeration (SjCOOP) mentoring project, which identified accessibility to credible scientific information as one of the most urgent issues in Ebola communication. To address the information crisis, the WFSJ and its partners—Fondation Hirondelle and African associations of science journalists—have launched a communication and training programme to strengthen independent science and health journalism in subSaharan Africa. The initiative, supported by Canada’s International Development Research Centre, aims to increase local access to health knowledge and sound evidence through better monitoring and reporting during disease outbreaks. The first phase, which launched in September, 2015, consisted of a series of workshops on the interplay between good scientifi c reporting and community engagement that were delivered in west African countries aff ected by Ebola. The second phase, launching in February, 2016, will extend the training to eastern and central Africa, focusing on networking and the development of a regional community of practice. By combining applied real-time research into communication barriers with dedicated training, we hope that this innovative training programme can promptly address some of Gilles Guerrier and colleagues’ concerns about scientifi c journalism in Africa.

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) play important roles in the transmission of viral diseases affecting wild and domestic ruminants and horses, including Bluetongue (BT) and African horse sickness (AHS) respectively. In southern Europe, BT has been largely transmitted by the classical Afro-Asian vector Culicoides imicola Kieffer. However, other species such as C. obsoletus Meigen, C. scoticus Downs & Kettle and C. pulicaris Linné may also be involved in BTV transmission. As a consequence of the discovery of C. imicola followed by BTV-2 outbreaks on the island of Corsica in October 2000, further studies on these biting midges have been carried out. To better characterize the evolution and phylogenetic relations of Culicoides, molecular analysis in parallel with a morphology-based taxonomic approach were performed. Phylogenetic analyses of French Culicoides species were undertaken using the ribosomal DNA (rDNA) internal transcribed spacer 1 (ITS1) as a molecular target. This region was shown to be useful in understanding evolutionary and genetic relationships between species. Construction of several trees showed that molecular phylogeny within the genus Culicoides correlates not only with morphological-based taxonomy but also with ecological patterns.

The M and S molecular forms of Anopheles gambiae sensu stricto Giles are thought to be reproductively isolated through premating barriers. However, the exact mechanisms of recognition of conspecific partners are unknown. Because mating in An. gambiae occurs in swarms, one might expect swarming behavior between the M and S forms to be different and that this probably reduces the risk of contact between males and females of the different forms in areas where they are sympatric. We report the occurrence of four mixed swarms, containing males of M and S forms, out of a total of 26 swarms sampled in Soumousso, a typical savannah village of Burkina Faso, West Africa. However, the frequency of mixed swarms was lower than that expected by chance. This observation suggests partial segregation between the swarms of the molecular forms, which may contribute to their isolation. Because the frequency of mixed swarms seems too high to explain the low frequency of cross-mating and hybrids, we suggest that mate recognition in a swarm is more important than swarm segregation.

Background The development of mosquito nets pre-treated with insecticide, Long Lasting Impregnated Nets (LLINs) that last the life span of the net, is a solution to the difficulty of the re-impregnation of conventional nets. Even if they showed a good efficacy in control conditions, their efficacy in the field, particularly in areas with resistance of Anopheles gambiae to pyrethroids, is not well documented. This study compares wide (Olyset®) and small (Permanet®) mesh LLINs in field conditions, using entomological parameters. Methods The two LLINs were tested in a rice-growing area of south-western Burkina Faso (West Africa) with year around high density of the main malaria vector An. gambiae s.s. In the study village (VK6), there is a mixed population of two molecular forms of An. gambiae, the S-form which dominates during the rainy season and the M-form which dominates the rest of the year. The two LLINs Olyset® and Permanet® were distributed in the village and 20 matched houses were selected for comparison with four houses without treated nets. Results Mosquito entrance rate was ten fold higher in control houses than in houses with LLINs and there was no difference between the two net types. Among mosquitoes found in the houses, 36 % were dead in LLIN houses compared to 0% in control houses. Blood feeding rate was 80 % in control houses compared to 43 % in LLIN houses. The type of net did not significantly impact any of these parameters. No mosquitoes were found inside Permanet®, whereas dead or dying mosquitoes were collected inside the Olyset®. More than 60% of mosquitoes found on top or inside the nets had had blood meals from cattle, as shown by ELISA analysis. Conclusion The percentage of blood-fed mosquitoes in a bed net study does not necessarily determine net success. The efficacy of the two types of LLINs was comparable, during a period when the S-form of An. gambiae was carrying the kdr gene. Significantly higher numbers of mosquitoes were collected in control houses compared to intervention houses, indicating that the LLINs provided an additional deterrent effect, which enhanced their expected prevention capacity.

The M and S molecular forms of Anopheles gambiae sensu stricto Giles are thought to be reproductively isolated through premating barriers. However, the exact mechanisms of recognition of conspecific partners are unknown. Because mating in An. gambiae occurs in swarms, one might expect swarming behavior between the M and S forms to be different and that this probably reduces the risk of contact between males and females of the different forms in areas where they are sympatric. We report the occurrence of four mixed swarms, containing males of M and S forms, out of a total of 26 swarms sampled in Soumousso, a typical savannah village of Burkina Faso, West Africa. However, the frequency of mixed swarms was lower than that expected by chance. This observation suggests partial segregation between the swarms of the molecular forms, which may contribute to their isolation. Because the frequency of mixed swarms seems too high to explain the low frequency of cross-mating and hybrids, we suggest that mate recognition in a swarm is more important than swarm segregation.

We compared the development of the molecular forms of Anopheles gambiae s.s. in different larval habitats. First stage larvae (Lls) of wild-caught females were placed into cages in natural habitats of the M form (rice fields) or the S form (puddles/quarries) . Each cage was covered with cloth, allowing exchange of water, solutes, and small particles, including microorganisms, and was seeded with 100 LI s of a single form (M or S) or by a mixture of 50:50 of M and S forms. Emergence success of both forms in puddles and quarries was three-fold higher than in the rice fields. The emergence rate of the S form was higher than that of the M form in both habitats, but the form X habitat interaction was not significant. In temporary larval sites such as puddles, emergence success of the M form was lower in mixed cages than in single form cages, whereas the reverse was true for the S form, suggesting competition between the forms. The median developmental time was not significantly different between forms. Although these findings demonstrate differences between forms, they do not suggest that their spatial segregation is determined by differences in their exploitation of the physical and chemical conditions in these environments. These results should be regarded with caution because small numbers of first stage larvae could pass through the cloth of the cages.

Over the past few decades, the geographical distribution of arthropod-borne zoonoses has dramatically expanded. The influence of human-induced or ecological changes on the risk of disease outbreaks is undeniable. However, few hypotheses have been proposed which address the re-emergence of these diseases, the spread of these viruses to previously uninfected areas and their establishment therein. Host and vector movements play an important role in the dissemination of pathogens, and the ability of these diseases to colonise previously uninfected areas may be explained by the diversity of hosts and vectors, the presence of favourable ecological conditions, and the successful adaptations of vectors or pathogens to new ecosystems. The objective of this paper is to describe the epidemiological processes of the vector-borne diseases Rift Valley fever, West Nile fever, Japanese encephalitis and Crimean-Congo haemorrhagic fever.

New contribution to the study of the Culicoides of Corsica. Update of the list of species, description of C. riebi n. sp. and redescription of C. paradisionensis Boorman, 1988 (Diptera, Ceratopogonidae). This paper presents the complementary list of Culicoides species collected in Corsica, from October 2001 to December 2003. It gives the description of C. riebi, a new species of biting midge, and a redescription of C. paradisionensis Boorman, 1988, the female of which is described and illustrated for the first time., Cet article présente la liste complémentaire des espèces de Culicoides récoltées en Corse d'octobre 2001 à décembre 2003. Il donne la description de C. riebi n. sp. et la redescription de C. paradisionensis Boorman, 1988, dont la femelle est ici décrite pour la première fois., Delécolle Jean-Claude, Mathieu Bruno, Baldet Thierry. Nouvelle contribution à l'étude des Culicoides de Corse. II. Mise à jour de la liste des espèces, description de C. riebi n. sp. et redescription de C. paradisionensis Boorman, 1988 (Diptera, Ceratopogonidae). In: Bulletin de la Société entomologique de France, volume 110 (1), mars 2005. pp. 69-76.

During extensive sampling in Burkina Faso and other African countries, the Leu-Phe mutation producing the kdr pyrethroid resistance phenotype was reported in both Anopheles gambiae ss and A. arabiensis. This mutation was widely distributed at high frequency in the molecular S form of A. gambiae while it has been observed at a very low frequency in both the molecular M form and A. arabiensis in Burkina Faso. While the mutation in the M form is inherited through an introgression from the S form, its occurrence is a new and independent mutation event in A. arabiensis. Three nucleotides in the upstream intron of the kdr mutation differentiated A. arabiensis from A. gambiae ss and these specific nucleotides were associated with kdr mutation in A. arabiensis. Ecological divergences which facilitated the spread of the kdr mutation within the complex of A. gambiae ss in West Africa, are discussed.

The bioefficacy of cyfluthrin-impregnated bed-nets was evaluated in the agro-industrial town of Mbandjock (southern Cameroon). The objectives were to assess the knockdown and mortality rates, the protection against bloodfeeding mosquitoes, and the irritant effect of cyfluthrin (SOLFAC EW050)-impregnated bed-nets against a susceptible strain of Anopheles gambiae. Polyester bed-nets were impregnated and distributed to inhabitants of Mbandjock for use, then nets were retreated after 6 months. The uptake of active ingredient on nets ranged from 30.8 mg/m2 to 75.1 mg/m2 at the initial treatment and from 131.0 mg/m2 to 225.0 mg/m2 at retreatment. The susceptible Kisumu reference strain of Anopheles gambiae reared in our laboratory was used for bioassays. The knockdown rate on freshly treated nets (2 wk after treatment) ranged from 93 to 96% and the mortality rate ranged from 52 to 70%. During the 12-month trial, knockdown and mortality rates showed 2 peaks, respectively, in the 4th month (94-97% knockdown and 61-96% mortality) and 7th month (i.e., soon after retreatment; 89-98% knockdown and 86-100% mortality), separated by breakdowns on the 5th through 6th and 12th months (46-77% knockdown and 8-69% mortality). Knockdown and mortality rates decreased faster on the lower part of nets than on the top, suggesting that some external factors such as dirt and daily hand manipulation could impede the efficacy of treated nets. Exceedingly dirty nets were less effective than clean nets. The protective rate against bloodfeeding mosquitoes ranged from 60 to 100% during the 1st 4 months and decreased at 40-70% during the 5th and 6th months after the initial treatment. After retreatment, the protective rate ranged from 50 to 90% for the 1st 5 months and from 35 to 64% at the 6th month. More than 70% of mosquitoes that attempt to feed through treated or retreated nettings died within 24 h after contact. Cyfluthrin was found to be mildly irritant during the whole evaluation. This trial reveals that cyfluthrin EW050-treated nets were effective against a susceptible strain of An. gambiae.

Only pyrethroid insecticides have so far been recommended for the treatment of mosquito nets for malaria control. Increasing resistance of malaria vectors to pyrethroids threatens to reduce the potency of this important method of vector control. Among the strategies proposed for resistance management is to use a pyrethroid and a non-pyrethroid insecticide in combination on the same mosquito net, either separately or as a mixture. Mixtures are particularly promising if there is potentiation between the two insecticides as this would make it possible to lower the dosage of each, as has been demonstrated under laboratory conditions for a mixture of bifenthrin (pyrethroid) and carbosulfan (carbamate). The effect of these types of treatment were compared in experimental huts on wild populations of Anopheles gambiae Giles and the nuisance mosquito Culex quinquefasciatus Say, both of which are multi-resistant. Four treatments were evaluated in experimental huts over six months: the recommended dosage of 50 mg m−2 bifenthrin, 300 mg m−2 carbosulfan, a mosaic of 300 mg m−2 carbosulfan on the ceiling and 50 mg m−2 bifenthrin on the sides, and a mixture of 6.25 mg m−2 carbosulfan and 25 mg m−2 bifenthrin. The mixture and mosaic treatments did not differ significantly in effectiveness from carbosulfan and bifenthrin alone against anophelines in terms of deterrency, induced exophily, blood feeding inhibition and overall mortality, but were more effective than in earlier tests with deltamethrin. These results are considered encouraging, as the combination of different classes of insecticides might be a potential tool for resistance management. The mixture might have an advantage in terms of lower cost and toxicity.

International audience; Bluetongue (BT) and African Horse Sickness (AHS) are infectious arthropod-borne viral diseases affecting ruminants and horses, respectively. Culicoides imicola Kieffer, 1913, a biting midge, is the principal vector of these livestock diseases in Africa and Europe. Recently bluetongue disease has re-emerged in the Mediterranean Basin and has had a devastating effect on the sheep industry in Italy and on the islands of Sicily, Sardinia, Corsica and the Balearics, but fortunately, has not penetrated onto mainland France and Spain. To survey for the presence of C. imicola, an extensive light-trap network for the collection of Culicoides, was implemented in 2002 in southern mainland France. The morphological identification of Culicoides can be both tedious and time-consuming because its size ranges from 1.5 to 3 mm. Therefore, an ITS1 rDNA polymerase chain reaction (PCR)-based diagnostic assay was developed to rapidly and reliably identify Culicoides spp. and C. imicola. The aim of this work was to set up a rapid test for the detection of C. imicola amongst a pool of insects collected in areas at risk for BT. The sequence similarity of the rDNA (nuclear ribosomal DNA), which is greater within species than between species, is the foundation of its utilisation in species-diagnostic assays. The alignment of the 11 ITS1 sequences of Culicoides obtained from Genbank and EMBL databases helped us to identify one region in the 5' end and one in the 3' end that appear highly conserved. PCR primers were designed within these regions to amplify genus-specific fragments. In order to set up a C. imicola-specific PCR, another forward primer was designed and used in combination with the previously designed reverse primer. These primers proved to be highly specific and sensitive and permitted a rapid diagnostic separation of C. imicola from Culicoides spp.

This paper addresses the problems of mosquito control in urban areas of Burkina Faso. The main objectives are to examine relevant socio-cultural aspects in relation to a mosquito control intervention using a biolarvicide with main emphasis on local perceptions of mosquito nuisance and existing practices of mosquito control, including the cost of protective measures at household level. This is the report of an inter-disciplinary research project carried out in the two major towns of Burkina Faso, Bobo-Dioulasso and Ouagadougou, in 1999 and 2000, respectively. Both quantitative and qualitative methods were used in the ethnographic part of the study. Two questionnaire surveys were conducted in both study areas: one prior to the intervention (n=1083) and the other after the intervention of the treatments with biolarvicide (n=956). In addition, 70 in-depth interviews and 17 focus group discussions (FGDs) were conducted. The findings show that mosquitoes are considered an important problem in the urban areas, both as a nuisance and a health risk and that the local population is very active in applying mosquito control measures at the household level. The intervention project was received positively by the local population with a decline in the perceived level of annoyance. The causal relationship between mosquitoes and malaria is clear, but the explanatory framework of the relationship between mosquitoes and other diseases is still under debate. The most common prevention methods are mosquito coils and aerosol spray, even though bed nets are perceived to be the most efficient and effective method. The investments in coils and aerosol sprays alone would mean an increase of 40% in the national figures for health expenditure at household level. © 2004 Elsevier Ltd. All rights reserved.

A longitudinal study based on mosquitoes sampled by larvae prospecting and adult catches on humans was carried out during 1999 in the rice field area of the Kou Valley (South-West Burkina Faso) to evaluate the malaria transmission level. Two sites were studied: VK5 located in the rice field centre and VK7 in the periphery. Irrigation is sub-permanent and two crops are grown each year: from February to June during the dry season and from July to November during the rainy season. A man sleeping in VK5 without any protection is exposed to more than 60,000 mosquito bites/year. Two majors vectors are present: Anopheles gambiae sl. and An. funestus. The An. gambiae M form which breeds in rice fields constitutes the majority of the complex. At the periphery of the rice fields, we observed a mix of M and S forms during the rainy season, the latter form coming from classical breeding sites created by rainfall (residual puddles). An. arabiensis is rare in this environment while we can find it in sympathy with An. gambiae in the surrounding savannah. An. gambiae remains present throughout the year. Its dynamics depends closely on both season and rice cycle. There are two density peaks, in March (210 bites/man/night) and in July (306 b/m/n), corresponding to the moment when rice is planted. The latter peak is more important due to additional productivity of residual puddles created by rainfall. These large and very young populations can not transmit malaria. Growing rice reduces larvae productivity: transmission occurs when the adult population decreases and becomes older. Variations in parity rate and sporozoitic index are inversely related to mosquito density. Finally, the An. gambiae population is lowest during the dry season in January when irrigation stops. An. funestus does not develop in rice fields but rather in drains made of dirt, in addition to the classical natural breeding sites at the end of the rainy season. The density of An. funestus is 25 to 30 times smaller than that of A. gambiae, as observed during 8 months in VK5 and 10 months in VK7, with a peak of 16 b/m/n in January. Its mean parity rate and sporozoitic index is higher than those of An. gambiae. An. funestus plays an additional role in malaria transmission at the same period as An. gambiae but more shortly and less intensively. Finally, malaria transmission is due mainly to An. gambiae (90%) and, to a lesser extent, to An. funestus (10%). It occurs in VK5 during two periods of four month search : from May to August and from November to February. In VK7, transmission is also bimodal but it occurs more extensively throughout the year with only July and Septemberas exemptions of transmission. The annual inoculation rate amounts to 697 and 515 infected bites per man. This value is higher than those obtained in 1984/1985 : 50 and 60 infected bites per man per year. This increase is due to both the growth of aggressive vectorial density and the sporozoitic index. Growth of adult density can be explained by the environmental modification and/or better adaptation of vectorial species. The sporozoitic index of vectors has been obtained using Elisa, which is twice as sensitive as the microscopic observations used in 1984/1985. This can partly explain the increase in the sporozoitic index. Other hypotheses in relation to human parasitological inputs have to be evaluated more accurately. The level and rhythm of transmission in the rice field in 1999 are more important than those registered in the surrounding savannah the same year. However, this important transmission is not proportionally related to the high vectorial density found in rice fields. Probably, it does not imply more malaria diseases, but its intensity and rhythm could concentrate malaria cases in young people with an early acquisition of premunition. Pyrethroid impregnated bednets should be used to prevent malaria especially since high mosquito nuisance drives local populations to use it and since the An. gambiae M form is particularly susceptible to this class of insecticides.

We have studied the impact of large-scale treatment with ivermectin on the transmission of loiasis in a forest village in south Cameroon where loiasis was highly endemic, with a prevalence of 30%. After one year of parasitological and entomological surveillance without treatment, all consenting residents aged > 5 years received ivermectin 200 microg/kg every 3 months. For ethical reasons, treatment was interrupted after 2 years, but parasitological and entomological surveillance continued for 18 months after the end of treatment. The prevalence of loiasis was reduced to < 10% and the mean microfilaraemia decreased by 90% in 2 years. The prevalence and average intensity of infection remained stable during the 18 months after treatment ended. Two vector species were identified, #Chrysops dimidiata$ (representing about 90% of the fly population) and #C. silacea$. The infection rate (all stages) in #Chrysops$ decreased by 75% and the infective rate (percentage of #Chrysops$ harbouring third-stage larvae of #Loa loa$ in the head) decreased by 85% in #C. dimidiata$ and became zero in #C. silacea$. After the end of treatment, the infection and infective rates increased gradually. Large-scale treatment seemed an efficient method for the control of #L. loa$ transmission provided high drug coverage was achieved. Nevertheless, because of the high risk of adverse effects when using the current microfilaricidal drugs, such a strategy remains unacceptable. (Résumé d'auteur)

La fièvre catarrhale ovine (FCO) est une maladie virale qui est transmise par des moucherons piqueurs (Diptera : Ceratopogonidae) et émerge en Méditerranée depuis 1998. Cette émergence est reliée à l’expansion septentrionale de son vecteur principal Culicoides imicola Kieffer 1913. En France, C. imicola a été décelé dans le sud de la Corse en octobre 2000, quelques jours avant l’apparition de foyers de FCO sérotype 2. Un réseau de surveillance entomologique a été mis en place en 2002 : (i) en Corse, pour étudier les dynamiques de populations de C. imicola et des autres espèces de Culicoides associées aux élevages ; (ii) sur le continent, pour détecter l’introduction de C. imicola. Des captures nocturnes ont été réalisées toutes les trois semaines sur 12 sites en Corse et une fois par mois sur 19 sites sur le continent. En 2002, en Corse, un total de 180 nuits de captures ont été effectuées. C. imicola comptait en moyenne pour 18 p. 100 des effectifs totaux capturés et prédominait au sud de l’île. Les densités adultes ont atteint un pic en septembre. La présence de C. imicola sur tous les sites et durant huit mois de l’année a confirmé que le vecteur principal de la FCO passait l’hiver et était installé durablement dans l’île. En outre, en 2003, les populations de C. imicola étaient en pleine expansion dans le Nord, particulièrement en Balagne. Le risque d’invasion du continent devient ainsi plus important. Sur le continent, sur un total de 233 nuits de captures réalisées en 2002-2003, deux spécimens de C. imicola ont été détectés en 2003 sur deux sites différents. Des investigations poussées n’ont pu mettre en évidence des populations locales installées. Les dynamiques saisonnière et spatiale de C. imicola et des autres espèces d’intérêt sont discutées en relation avec leur bioécologie et les facteurs environnementaux. Cette « entomosurveillance » apparaît indispensable pour une meilleure compréhension de l’épidémiologie de la FCO mais aussi pour élaborer et valider des modèles de prédiction de zones à risque.

The tiger mosquito, Ae. albopictus, is emerging throughout the world as a public health hazard through the transmission of many human pathogens for which no effective antiviral agent or vaccine are available. Chemical insecticides remain the main tools to control the tiger mosquito populations, but the development of resistances is threatening their effectiveness. In this context, it is necessary to develop alternative control methods, one of which is the Sterile Insect Technique (SIT) consisting in releasing males that have been sterilized using ionizing radiation. Those males reduce the reproductive success of the encountered wild females, hence causing the target population to decline. Another method is the "boosted SIT", a SIT improvement, where sterile males are also vectors of a biocide transmitted to the females through mating. The biocides (e.g. growth regulators affecting larval development) can be then specifically dispersed to the breeding sites by the females. Yet, experimenting those practices across the relevant space and time scales is costly, as well potentially hazardous. In this context, mathematical models can provide useful tools to test and optimize such control strategies. We will present an age-structured population dynamics model of the vector Ae. albopictus, designed to explore the demographic effects of SIT control strategies. The model accounts for the spatiotemporal heterogeneity of environmental and meteorological conditions across La Réunion Island (Indian Ocean, France). The control strategies and their optimization will be analyzed and discussed.

National audience

The Sterile Insect Technique (SIT) is a demographic control procedure consisting in re-leasing males sterilized by ionizing radiations. A workaround technique, named boost-ed SIT, aims at contaminating the sterilized males with pyriproxyfen; the males would then transfer this insecticide to other compartments of the target population (females, breeding sites) through auto-dissemination. To anticipate the effect of SIT and boosted SIT on mosquito populations, they were included into an age-structured population model of Aedes albopictus in the geographical context of La Reunion Island. A sensitiv-ity analysis shows that timing and duration of the male release events, as well as the quantity of sterile males released, are of key importance for both control methods. The persistence of pyriproxyfen in larval habitat appears to be the major determinant for boosted SIT to be more efficient than SIT. While the model appears a powerful tool to investigate release strategies, our findings specifically call for lab experiments towards a better assessment of pyriproxyfen transfer rates and persistence in breeding sites.

International audience; Background: African horse sickness (AHS) is an equine disease endemic to Senegal. The African horse sickness virus (AHSV) is transmitted to the mammalian hosts by midges of the Culicoides Latreille genus. During the last epizootic outbreak of AHS in Senegal in 2007, 1,169 horses died from this disease entailing an estimated cost of 1.4 million euros. In spite of the serious animal health and economic implications of AHS, very little is known about determinants involved in transmission such as contact between horses and the Culicoides species suspected of being its vectors. Methods: The monthly variation in host/vector contact was determined in the Niayes area, Senegal, an area which was severely affected by the 2007 outbreak of AHS. A horse-baited trap and two suction light traps (OVI type) were set up at each of five sites for three consecutive nights every month for one year. Results: Of 254,338 Culicoides midges collected 209,543 (82.4%) were female and 44,795 (17.6%) male. Nineteen of the 41 species collected were new distribution records for Senegal. This increased the number of described Culicoides species found in Senegal to 53. Only 19 species, of the 41 species found in light trap, were collected in the horse-baited trap (23,669 specimens) largely dominated by Culicoides oxystoma (22,300 specimens, i.e. 94.2%) followed by Culicoides imicola (482 specimens, i.e. 2.0%) and Culicoides kingi (446 specimens, i.e. 1.9%). Conclusions: Culicoides oxystoma should be considered as a potential vector of AHSV in the Niayes area of Senegal due to its abundance on horses and its role in the transmission of other Culicoides-borne viruses.

One of the greatest challenges in reducing malaria transmission in sub-Saharan Africa is the high degree of plasticity in the behaviour and ecology of members of the Anopheles gambiae complex, which includes the most efficient malaria vectors globally. For example, in areas of West Africa, sympatric populations of the M and S molecular forms of An. gambiae s.s. have diverged into recognized species (An. coluzzii Coetzee & Wilkerson sp.n. and the nominative species, An. gambiae s.s., respectively) through ecological speciation. Whereas the main breeding sites for An. gambiae s.s. continue to be temporary water bodies, An. coluzzii has become adapted to breeding well in irrigated fields, which has led to increases in relative abundance and duration of breeding seasons, and, hence, an enhancement of malaria transmission throughout the year. Thus, an increase in malaria prevalence is associated with a process of speciation that gave rise relatively rapidly to a new vector species highly adapted to a new breeding habitat created by changes in agricultural practice. To face this challenge, it is important that we understand the mechanisms by which this kind of speciation occurs. There is good evidence that speciation in this case has involved reproductive isolation, and, therefore, a five year study of sympatric populations of the M and S molecular forms in the Bobo Dioulasso area of Burkina Faso was undertaken to identify mechanisms that limit cross-matings. Mating in the An. gambiae complex generally occurs in swarms consisting mainly of males, which form at specific times of day at consistent sites defined by physical features in the environment, e.g., over wells, piles of wood, clearings in vegetation. More than 1,000 mating swarms were documented; a measurable degree of reproductive isolation could be explained by significant differences between species in seasonal abundance, characteristics of swarm sites and daily timing (circadian rhythm) of swarming. Although these factors each contribute to a reduced likelihood that males and females of ‘opposite’ molecular form encounter each other at close range in a mating swarm, both species were occasionally found in mixed swarms. All of the females caught in mixed swarms, however, had been inseminated by a conspecific male. Hence, further research is required to establish the close-range mechanism(s) of species recognition that seem to occur within swarms, such as auditory communication, that lead to assortative mating.

Background The use of insecticide-treated nets (ITN) is an important tool in the Roll Back Malaria (RBM) strategy. For ITNs to be effective they need to be used correctly. Previous studies have shown that many factors, such as wealth, access to health care, education, ethnicity and gender, determine the ownership and use of ITNs. Some studies showed that free distribution and public awareness campaigns increased the rate of use. However, there have been no evaluations of the short- and long-term impact of such motivation campaigns. A study carried out in a malaria endemic area in south-western Burkina Faso indicated that this increased use declined after several months. The reasons were a combination of the community representation of malaria, the perception of the effectiveness and usefulness of ITNs and also the manner in which households are organized by day and by night. Methods PermaNet 2.0® and Olyset® were distributed in 455 compounds at the beginning of the rainy season. The community was educated on the effectiveness of nets in reducing malaria and on how to use them. To assess motivation, qualitative tools were used: one hundred people were interviewed, two hundred houses were observed directly and two houses were monitored monthly throughout one year. Results The motivation for the use of bednets decreased after less than a year. Inhabitants' conception of malaria and the inconvenience of using bednets in small houses were the major reasons. Acceptance that ITNs were useful in reducing malaria was moderated by the fact that mosquitoes were considered to be only one of several factors which caused malaria. The appropriate and routine use of ITNs was adversely affected by the functional organization of the houses, which changed as between day and night. Bednets were not used when the perceived benefits of reduction in mosquito nuisance and of malaria were considered not to be worth the inconvenience of daily use. Conclusion In order to bridge the gap between possession and use of bednets, concerted efforts are required to change behaviour by providing accurate information, most particularly by convincing people that mosquitoes are the only source of malaria, whilst recognising that there are other diseases with similar symptoms, caused in other ways. The medical message must underline the seriousness of malaria and the presence of the malaria vector in the dry season as well as the wet, in order to encourage the use of bednets whenever transmission can occur. Communities would benefit from impregnated bednets and other vector control measures being better adapted to their homes, thus reducing the inconvenience of their use.

Background: Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are biological vectors of internationally important arboviruses. To understand the role of Culicoides in the transmission of these viruses, it is essential to correctly identify the species involved. Within the western Palaearctic region, the main suspected vector species, C. obsoletus, C. scoticus, C. dewulfi and C. chiopterus, have similar wing patterns, which makes it difficult to separate and identify them correctly. Methods: In this study, designed as an inter-laboratory ring trial with twelve partners from Europe and North Africa, we assess four PCR-based assays which are used routinely to differentiate the four species of Culicoides listed above. The assays based on mitochondrial or ribosomal DNA or microarray hybridisation were tested using aliquots of Culicoides DNA (extracted using commercial kits), crude lysates of ground specimens and whole Culicoides (265 individuals), and non-Culicoides Ceratopogonidae (13 individuals) collected from across Europe. Results: A total of 800 molecular assays were implemented. The in-house assays functioned effectively, although specificity and sensitivity varied according to the molecular marker and DNA extraction method used. The Obsoletus group specificity was overall high (95-99%) while the sensitivity varied greatly (59.6-100%). DNA extraction methods impacted the sensitivity of the assays as well as the type of sample used as template for the DNA extraction. Conclusions: The results are discussed in terms of current use of species diagnostic assays and the future development of molecular tools for the rapid differentiation of cryptic Culicoides species. Published

Background Insecticide resistance of the main malaria vector, Anopheles gambiae, has been reported in south-western Burkina Faso, West Africa. Cross-resistance to DDT and pyrethroids was conferred by alterations at site of action in the sodium channel, the Leu-Phe kdr mutation; resistance to organophosphates and carbamates resulted from a single point mutation in the oxyanion hole of the acetylcholinesterase enzyme designed as ace-1R. Methods An entomological survey was carried out during the rainy season of 2005 at Vallée du Kou, a rice growing area in south-western Burkina Faso. At the Vallée du Kou, both insecticide resistance mechanisms have been previously described in the M and S molecular forms of An. gambiae. This survey aimed i) to update the temporal dynamics and the circumsporozoite infection rate of the two molecular forms M and S of An. gambiae ii) to update the frequency of the Leu-Phe kdr mutation within these forms and finally iii) to investigate the occurrence of the ace-1R mutation. Mosquitoes collected by indoor residual collection and by human landing catches were counted and morphologically identified. Species and molecular forms of An. gambiae, ace-1R and Leu-Phe kdr mutations were determined using PCR techniques. The presence of the circumsporozoite protein of Plasmodium falciparum was determined using ELISA. Results Anopheles gambiae populations were dominated by the M form. However the S form occurred in relative important proportion towards the end of the rainy season with a maximum peak in October at 51%. Sporozoite rates were similar in both forms. The frequency of the Leu-Phe kdr mutation in the S form reached a fixation level while it is still spreading in the M form. Furthermore, the ace-1R mutation prevailed predominately in the S form and has just started spreading in the M form. The two mutations occurred concomitantly both in M and S populations. Conclusion These results showed that the Vallée du Kou, a rice growing area formerly occupied mainly by M susceptible populations, is progressively colonized by S resistant populations living in sympatry with the former. As a result, the distribution pattern of insecticide resistance mutations shows the occurrence of both resistance mechanisms concomitantly in the same populations. The impact of multiple resistance mechanisms in M and S populations of An. gambiae on vector control measures against malaria transmission, such as insecticide-treated nets (ITNs) and indoor residual spraying (IRS), in this area is discussed.

Although malaria disappeared from southern France more than 60 years ago, suspicions of recent autochthonous transmission in the French Mediterranean coast support the idea that the area could still be subject to malaria transmission. The main potential vector of malaria in the Camargue area, the largest river delta in southern France, is the mosquito Anopheles hyrcanus (Diptera: Culicidae). In the context of recent climatic and landscape changes, the evaluation of the risk of emergence or re-emergence of such a major disease is of great importance in Europe. When assessing the risk of emergence of vector-borne diseases, it is crucial to be able to characterize the arthropod vector's spatial distribution. Given that remote sensing techniques can describe some of the environmental parameters which drive this distribution, satellite imagery or aerial photographs could be used for vector mapping., Journal Article, Research Support, Non-U.S. Gov't, info:eu-repo/semantics/published

Background Ultra-low volume (ULV) insecticidal aerosols dispensed from vehicle-mounted cold-foggers are widely considered the method of choice for control of Aedes aegypti and Ae. albopictus during outbreaks of dengue and chikungunya and, more recently, Zika. Nevertheless, their effectiveness has been poorly studied, particularly in Europe. Nearly all published studies of ULV efficacy are bio-assays based on the mortality of caged mosquitoes. In our study we preferred to monitor the direct impact of treatments on the wild mosquito populations. This study was undertaken to evaluate the efficiency of the two widely used space spraying methods to control Ae. albopictus and Ae. aegypti. Methods We determined the susceptibility of local Ae. albopictus to deltamethrin by two methods: topical application and the “WHO Tube Test”. We used ovitraps baited with hay infusion and adult traps (B-G Sentinel) baited with a patented attractant to monitor the mosquitoes in four residential areas in Nice, southern France. The impact of deltamethrin applied from vehicle-mounted ULV fogging-machines was assessed by comparing trap results in treated vs untreated areas for 5 days before and 5 days after treatment. Four trials were conducted at the maximum permitted application rate (1 g.ha-1). We also made two small-scale tests of the impact of the same insecticide dispensed from a hand-held thermal fogger. Results Susceptibility to the insecticide was high but there was no discernable change in the oviposition rate or the catch of adult female mosquitoes, nor was there any change in the parous rate. In contrast, hand-held thermal foggers were highly effective, with more than 90% reduction of both laid eggs and females. Conclusions We believe that direct monitoring of the wild mosquito populations gives a realistic assessment of the impact of treatments and suggest that the lack of efficacy is due to lack of interaction between the target mosquitoes and the ULV aerosol. We discuss the factors that influence the effectiveness of both methods of spraying in the context of epidemic situations. Electronic supplementary material The online version of this article (doi:10.1186/s13071-016-1881-y) contains supplementary material, which is available to authorized users.

Background Rift Valley fever (RVF) is a vector-borne viral zoonosis of increasing global importance. RVF virus (RVFV) is transmitted either through exposure to infected animals or through bites from different species of infected mosquitoes, mainly of Aedes and Culex genera. These mosquitoes are very sensitive to environmental conditions, which may determine their presence, biology, and abundance. In East Africa, RVF outbreaks are known to be closely associated with heavy rainfall events, unlike in the semi-arid regions of West Africa where the drivers of RVF emergence remain poorly understood. The assumed importance of temporary ponds and rainfall temporal distribution therefore needs to be investigated. Methodology/Principal Findings A hydrological model is combined with a mosquito population model to predict the abundance of the two main mosquito species (Aedes vexans and Culex poicilipes) involved in RVFV transmission in Senegal. The study area is an agropastoral zone located in the Ferlo Valley, characterized by a dense network of temporary water ponds which constitute mosquito breeding sites. The hydrological model uses daily rainfall as input to simulate variations of pond surface areas. The mosquito population model is mechanistic, considers both aquatic and adult stages and is driven by pond dynamics. Once validated using hydrological and entomological field data, the model was used to simulate the abundance dynamics of the two mosquito species over a 43-year period (1961–2003). We analysed the predicted dynamics of mosquito populations with regards to the years of main outbreaks. The results showed that the main RVF outbreaks occurred during years with simultaneous high abundances of both species. Conclusion/Significance Our study provides for the first time a mechanistic insight on RVFV transmission in West Africa. It highlights the complementary roles of Aedes vexans and Culex poicilipes mosquitoes in virus transmission, and recommends the identification of rainfall patterns favourable for RVFV amplification., Author Summary Rift Valley fever (RVF) is a zoonotic disease that affects domestic livestock and humans. During inter-epizootic periods, the main infection mechanism is suspected to be through bites by infected mosquitoes, mainly of Aedes and Culex genera. In East Africa, RVF outbreaks are known to be closely associated with heavy rainfall events, unlike in the semi-arid regions of West Africa where the drivers of RVF emergence remain poorly understood. This study brings mechanistic insight to explain why reported RVF outbreaks in Northern Senegal cannot be correlated directly to rainfall. This is done through the use of a rainfall-driven model of RVF vector populations that combines a hydrological model to simulate daily water variations of mosquito breeding sites, with mosquito population models capable of reproducing the major trends in population dynamics of the two main vectors of RVF virus in Senegal, Ae. vexans and Cx. poicilipes. Results show that RVF occurs during years when both species are present simultaneously in high densities. Simulations of inter-annual variations in mosquito populations successfully explained the dates of RVF outbreaks observed between 1961 and 2003.