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Chikungunya and dengue viruses emerged in Gabon in 2007, with large outbreaks primarily affecting the capital Libreville and several northern towns. Both viruses subsequently spread to the south-east of the country, with new outbreaks occurring in 2010. The mosquito species Aedes albopictus, that was known as a secondary vector for both viruses, recently invaded the country and was the primary vector involved in the Gabonese outbreaks. We conducted a retrospective study of human sera and mosquitoes collected in Gabon from 2007 to 2010, in order to identify other circulating arboviruses. Sample collections, including 4312 sera from patients presenting with painful febrile disease, and 4665 mosquitoes belonging to 9 species, split into 247 pools (including 137 pools of Aedes albopictus), were screened with molecular biology methods. Five human sera and two Aedes albopictus pools, all sampled in an urban setting during the 2007 outbreak, were positive for the flavivirus Zika (ZIKV). The ratio of Aedes albopictus pools positive for ZIKV was similar to that positive for dengue virus during the concomitant dengue outbreak suggesting similar mosquito infection rates and, presumably, underlying a human ZIKV outbreak. ZIKV sequences from the envelope and NS3 genes were amplified from a human serum sample. Phylogenetic analysis placed the Gabonese ZIKV at a basal position in the African lineage, pointing to ancestral genetic diversification and spread. We provide the first direct evidence of human ZIKV infections in Gabon, and its first occurrence in the Asian tiger mosquito, Aedes albopictus. These data reveal an unusual natural life cycle for this virus, occurring in an urban environment, and potentially representing a new emerging threat due to this novel association with a highly invasive vector whose geographic range is still expanding across the globe.

Only few data exist in Cambodia on mosquito diversity and their potential role as vectors. Many arboviruses, such as dengue and Japanese encephalitis, are endemic and mostly affect children in the country. This research sets out to evaluate vector relative abundance and diversity in primary schools in Cambodia in an attempt to explain the apparent burden of dengue fever, severe dengue (DEN), Japanese encephalitis (JE), other arboviral diseases and malaria among children, 15 years and under, attending selected primary schools through vector surveys. Entomological surveys were implemented in primary schools in two provinces of Cambodia to assess the potential risk of exposure of schoolchildren to mosquito vector species. Light traps and BG traps were used to collect adult mosquitoes in 24 schools during the rainy and dry seasons of 2017 and 2018 in Kampong Cham and Tboung Khmum provinces. A total of 61 species were described, including Aedes, Culex and Anopheles species. The relative abundance and biodiversity of mosquito species were dependent on the month and school. Of the 37,725 mosquitoes caught during the study, three species accounted for three-quarters of the relative abundance: Culex vishnui, Anopheles indefinitus and Culex quinquefasciatus. More importantly, nearly 90% of the mosquitoes caught in the schools were identified as potential vectors of pathogens including Japanese encephalitis, dengue, and malaria parasites. Our results showed that schools in Cambodia represent a risk for vector-borne disease transmission and highlight the importance of implementing vector control in schools in Cambodia to decrease the risk of transmission.

Between 2016 and 2020, the Medical and Veterinary Entomology unit of the Institut Pasteur du Cambodge collected over 230,000 mosquitoes. Based on this sampling effort, a checklist of 290 mosquito species in Cambodia is presented. This is the first attempt to list the Culicidae fauna of the country. We report 49 species for the first time in Cambodia. The 290 species belong to 20 genera: Aedeomyia (1 sp.), Aedes (55 spp.), Anopheles (53 spp.), Armigeres (26 spp.), Coquillettidia (3 spp.), Culex (57 spp.), Culiseta (1 sp.), Ficalbia (1 sp.), Heizmannia (10 spp.), Hodgesia (3 spp.), Lutzia (3 spp.), Malaya (2 spp.), Mansonia (5 spp.), Mimomyia (7 spp.), Orthopodomyia (3 spp.), Topomyia (4 spp.), Toxorhynchites (4 spp.), Tripteroides (6 spp.), Uranotaenia (27 spp.), and Verrallina (19 spp.). The Cambodian Culicidae fauna is discussed in its Southeast Asian context. Forty-three species are reported to be of medical importance, and are involved in the transmission of pathogens. Entre 2016 et 2020, plus de 230 000 moustiques ont été capturés par l’Unité d’Entomologie Médicale et Vétérinaire de l’Institut Pasteur du Cambodge. Sur la base de ces collectes, une liste des 290 espèces de moustiques présentes au Cambodge est présentée. Il s’agit de la première tentative de répertorier la faune Culicidae du pays. Quarante-neuf espèces sont signalées pour la première fois au Cambodge. Les 290 espèces appartiennent à vingt genres : Aedeomyia (1 sp.), Aedes (55 spp.), Anopheles (53 spp.), Armigeres (26 spp.), Coquillettidia (3 spp.), Culex (57 spp.), Culiseta (1 sp.), Ficalbia (1 sp.), Heizmannia (10 spp.), Hodgesia (3 spp.), Lutzia (3 spp.), Malaya (2 spp.), Mansonia (5 spp.), Mimomyia (7 spp.), Orthopodomyia (3 spp.), Topomyia (4 spp.), Toxorhynchites (4 spp.), Tripteroides (6 spp.), Uranotaenia (27 spp.) et Verrallina (19 spp.). La faune des Culicidés cambodgiens est discutée dans son contexte sud-est asiatique. Quarante-trois espèces sont d’importance médicale et impliquées dans la transmission de pathogènes.

Extreme precipitation events are increasing as a result of ongoing global warming, but controversy surrounds the relationship between flooding and mosquito-borne diseases. A common view among the scientific community and public health officers is that heavy rainfalls have a flushing effect on breeding sites, which negatively affects vector populations, thereby diminishing disease transmission. During 2014 in Montpellier, France, there were at least 11 autochthonous cases of chikungunya caused by the invasive tiger mosquito Aedes albopictus in the vicinity of an imported case. We show that an extreme rainfall event increased and extended the abundance of the disease vector Ae. albopictus, hence the period of autochthonous transmission of chikungunya. We report results from close monitoring of the adult and egg population of the chikungunya vector Ae. albopictus through weekly sampling over the entire mosquito breeding season, which revealed an unexpected pattern. Statistical analysis of the seasonal dynamics of female abundance in relation to climatic factors showed that these relationships changed after the heavy rainfall event. Before the inundations, accumulated temperatures are the most important variable predicting Ae. albopictus seasonal dynamics. However, after the inundations, accumulated rainfall over the 4 weeks prior to capture predicts the seasonal dynamics of this species and extension of the transmission period. Our empirical data suggests that heavy rainfall events did increase the risk of arbovirus transmission in Southern France in 2014 by favouring a rapid rise in abundance of vector mosquitoes. Further studies should now confirm these results in different ecological contexts, so that the impact of global change and extreme climatic events on mosquito population dynamics and the risk of disease transmission can be adequately understood.

Invasion of new territories by insect vector species that can transmit pathogens is one of the most important threats for human health. The spread of the mosquito Aedes albopictus in Europe is emblematic, because of its major role in the emergence and transmission of arboviruses such as dengue or chikungunya. Here, we modeled the spread of this mosquito species in France through a statistical framework taking advantage of a long-term surveillance dataset going back to the first observation of Ae. albopictus in the Metropolitan area. After validating the model, we show that human activities are especially important for mosquito dispersion while land use is a major factor for mosquito establishment. More importantly, we show that Ae. albopictus invasion is accelerating through time in this area, resulting in a geographic range extending further and further year after year. We also show that sporadic \"jump\" of Ae. albopictus in a new location far from the colonized area did not succeed in starting a new invasion front so far. Finally, we discuss on a potential adaptation to cooler climate and the risk of invasion into Northern latitudes.

A Chikungunya (CHIK) outbreak hit La Réunion Island in 2005-2006. The implicated vector was Aedes albopictus. Here, we present the first study on the susceptibility of Ae. albopictus populations to sympatric CHIKV isolates from La Réunion Island and compare it to other virus/vector combinations. We orally infected 8 Ae. albopictus collections from La Réunion and 3 from Mayotte collected in March 2006 with two Chikungunya virus (CHIKV) from La Réunion: (i) strain 05.115 collected in June 2005 with an Alanine at the position 226 of the glycoprotein E1 and (ii) strain 06.21 collected in November 2005 with a substitution A226V. Two other CHIKV isolates and four additional mosquito strains/species were also tested. The viral titer of the infectious blood-meal was 10(7) plaque forming units (pfu)/mL. Dissemination rates were assessed by immunofluorescent staining on head squashes of surviving females 14 days after infection. Rates were at least two times higher with CHIKV 06.21 compared to CHIKV 05.115. In addition, 10 individuals were analyzed every day by quantitative RT-PCR. Viral RNA was quantified on (i) whole females and (ii) midguts and salivary glands of infected females. When comparing profiles, CHIKV 06.21 produced nearly 2 log more viral RNA copies than CHIKV 05.115. Furthermore, females infected with CHIKV 05.115 could be divided in two categories: weakly susceptible or strongly susceptible, comparable to those infected by CHIKV 06.21. Histological analysis detected the presence of CHIKV in salivary glands two days after infection. In addition, Ae. albopictus from La Réunion was as efficient vector as Ae. aegypti and Ae. albopictus from Vietnam when infected with the CHIKV 06.21. Our findings support the hypothesis that the CHIK outbreak in La Réunion Island was due to a highly competent vector Ae. albopictus which allowed an efficient replication and dissemination of CHIKV 06.21.

Japanese Encephalitis (JE) is the most important cause of human encephalitis throughout Asia and the Pacific. Although JE is a vector-borne disease, it has been demonstrated experimentally that transmission between pigs can occur through direct contact. Whether pig-to-pig transmission plays a role in the natural epidemiological cycle of JE remains unknown. To assess whether direct transmission between pigs may occur under field conditions, we built two mathematical models of JE transmission incorporating vector-borne transmission alone or a combination of vector-borne and direct transmission. We used Markov Chain Monte Carlo (MCMC) techniques to estimate the parameters of the models. We fitted the models to (i) two serological datasets collected longitudinally from two pig cohorts (C1 and C2) during two periods of four months on a farm on the outskirts of Phnom-Penh, Cambodia and to (ii) a cross-sectional (CS) serological survey dataset collected from 505 swine coming from eight different provinces of Cambodia. In both cases, the model incorporating both vector-borne and direct transmission better explained the data. We computed the value of the basic reproduction number R0 (2.93 for C1, 2.66 for C2 and 2.27 for CS), as well as the vector-borne reproduction number Rpv and the direct transmission reproduction number Rpp. We then determined the contribution of direct transmission on R0 (11.90% for C1, 11.62% for C2 and 7.51% for CS). According to our results, the existence of pig-to-pig transmission is consistent with our swine serological data. Thus, direct transmission may contribute to the epidemiological cycle of JE in Cambodia. These results need to be confirmed in other eco-climatic settings, in particular in temperate areas where pig-to-pig transmission may facilitate the persistence of JE virus (JEV) during cold seasons when there are no or few mosquitoes.

Anthropogenic habitat disturbance is a prime cause in the current trend of the Earth's reduction in biodiversity. Here we show that the human footprint on the Central African rainforest, which is resulting in deforestation and growth of densely populated urban agglomerates, is associated to ecological divergence and cryptic speciation leading to adaptive radiation within the major malaria mosquito Anopheles gambiae. In southern Cameroon, the frequency of two molecular forms--M and S--among which reproductive isolation is strong but still incomplete, was correlated to an index of urbanisation extracted from remotely sensed data, expressed as the proportion of built-up surface in each sampling unit. The two forms markedly segregated along an urbanisation gradient forming a bimodal cline of ∼6-km width: the S form was exclusive to the rural habitat, whereas only the M form was present in the core of densely urbanised settings, co-occurring at times in the same polluted larval habitats of the southern house mosquito Culex quinquefasciatus--a species association that was not historically recorded before. Our results indicate that when humans create novel habitats and ecological heterogeneities, they can provide evolutionary opportunities for rapid adaptive niche shifts associated with lineage divergence, whose consequences upon malaria transmission might be significant.

The past few decades have seen the emergence of several worldwide arbovirus epidemics (chikungunya, Zika), the expansion or recrudescence of historical arboviruses (dengue, yellow fever), and the modification of the distribution area of major vector mosquitoes such as Aedes aegypti and Ae. albopictus, raising questions about the risk of appearance of new vectors and new epidemics. In this opinion piece, we review the factors that led to the emergence of yellow fever in the Americas, define the conditions for a mosquito to become a vector, analyse the recent example of the new status of Aedes albopictus from neglected mosquito to major vector, and propose some scenarios for the future.

Aedes albopictus (Skuse, 1884) (Diptera: Culicidae), a mosquito native to Asia, has recently invaded all five continents. In Central Africa it was first reported in the early 2000s, and has since been implicated in the emergence of arboviruses such as dengue and chikungunya in this region. Recent genetic studies of invasive species have shown that multiple introductions are a key factor for successful expansion in new areas. As a result, phenotypic characters such as vector competence and insecticide susceptibility may vary within invasive pest species, potentially affecting vector efficiency and pest management. Here we assessed the genetic variability and population genetics of Ae. albopictus isolates in Cameroon (Central Africa), thereby deducing their likely geographic origin. Mosquitoes were sampled in 2007 in 12 localities in southern Cameroon and analyzed for polymorphism at six microsatellite loci and in two mitochondrial DNA regions (ND5 and COI). All the microsatellite markers were successfully amplified and were polymorphic, showing moderate genetic structureamong geographic populations (F(ST)  = 0.068, P < 0.0001). Analysis of mtDNA sequences revealed four haplotypes each for the COI and ND5 genes, with a dominant haplotype shared by all Cameroonian samples. The weak genetic variation estimated from the mtDNA genes is consistent with the recent arrival of Ae. albopictus in Cameroon. Phylogeographic analysis based on COI polymorphism indicated that Ae. albopictus populations from Cameroon are related to tropical rather than temperate or subtropical outgroups. The moderate genetic diversity observed among Cameroonian Ae. albopictus isolates is in keeping with recent introduction and spread in this country. The genetic structure of natural populations points to multiple introductions from tropical regions.