Cattle trade network in Madagascar highlands and Rift Valley fever virus circulation

In 2008-2009 a RVF outbreak occurred in the Anjozorobe area, a temperate and mountainous area of Madagascar highlands. A serological study conducted there in 2009 showed an IgG seroprevalence rate of 28%. Data analysis suggested a recurrent circulation of RVFV. The objectives of this study were to describe the cattle trade network in this area and analyze the link between network structure and RVFV circulation. Questionnaire survey among 386 breeders from 47 villages was carried out to collect trade data. Yearly village-level seroconversion rate was estimated in 2010 by testing 516 cattle negative in 2009. Association between the occurrence of seroconversion and network centrality parameters, distance to the nearest water point and 2009 seroprevalence level was tested. Due to the non-independence of the centrality parameters, a bootstrap procedure was used to assess the effects of the independent variables. Average village-level seroconversion was 7%, ranging from 0-20%. Two types of trading practice were observed: exchanges and buy/sale. The corresponding networks appeared both scale-free, and a significant but low correlation was observed between them. A negative association was observed between the occurrence of seroconversion in the village and the 2009 seroprevalence level, as well as the distance to the nearest water point. After RVFV introduction, vector-based transmission may support the within-village circulation. The node degree in the exchange network was positively linked with the occurrence of seroconversion. It was not the case for the buy/sale network. Both networks could thus have distinct roles in RVFV circulation. The exchanges network could be the support for RVFV introduction in villages, the buy/sale network being probably rather implicated in the introduction of RVFV in the area, from other parts of Madagascar. (Texte intégral

Geographic variations of the bird-borne structural risk of West Nile virus circulation in Europe

The structural risk of West Nile Disease results from the usual functioning of the socio-ecological system, which may favour the introduction of the pathogen, its circulation and the occurrence of disease cases. Its geographic variations result from the local interactions between three components: (i) reservoir hosts, (ii) vectors, both characterized by their diversity, abundance and competence, (iii) and the socio-economic context that impacts the exposure of human to infectious bites. We developed a model of bird-borne structural risk of West Nile Virus (WNV) circulation in Europe, and analysed the association between the geographic variations of this risk and the occurrence of WND human cases between 2002 and 2014. A meta-analysis of WNV serosurveys conducted in wild bird populations was performed to elaborate a model of WNV seropositivity in European bird species, considered a proxy for bird exposure to WNV. Several eco-ethological traits of bird species were linked to seropositivity and the statistical model adequately fitted species-specific seropositivity data (area under the ROC curve: 0.85). Combined with species distribution maps, this model allowed deriving geographic variations of the bird-borne structural risk of WNV circulation. The association between this risk, and the occurrence of WND human cases across the European Union was assessed. Geographic risk variations of bird-borne structural risk allowed predicting WND case occurrence in administrative districts of the EU with a sensitivity of 86% (95% CI: 0.79–0.92), and a specificity of 68% (95% CI: 0.66–0.71). Disentangling structural and conjectural health risks is important for public health managers as risk mitigation procedures differ according to risk type. The results obtained show promise for the prevention of WND in Europe. Combined with analyses of vector-borne structural risk, they should allow designing efficient and targeted prevention measures

Rift Valley fever in a temperate and mountaneous area of Madagascar

A serological study was performed in 2009 in Madagascar highlands to evaluate the point prevalence rate of RVF antibodies in ruminants and identify environmental and trade factors potentially linked to RVF persistence and transmission. Nine hundred ruminants were randomly selected among 51 villages and sampled. Sera were analysed using a commercial validated ELISA test. For each ruminant, the following variables were recorded: age, location of the night pen, minimum distance from the pen to the closest water point and the forest, water point type, and the way owners use to renewal their herd: auto-renewal vs purchase. Serological data were analysed using a generalized linear mixed model, the individual serological status being the binomial response, and the above-mentioned variables the explicative factors, and the breeder as random effect. The overall IgG seroprevalence rate was 28% [IC95% 25-31]. The main effect on prevalence was due to age (P=10-4) suggesting an endemic RVF circulation in this area. The distance to the closest water point was a protective factor (P=5×10-3) but the type of water point had no effect on the prevalence rate: a substantial part of the transmission is due to vectors and several mosquito species are probably involved. Ruminants belonging to owners renewing their herd by purchase were significantly more infected than others (P=0.04), suggesting that cattle trade may contribute to the circulation of the virus in this area. This is the first evidence of a recurrent transmission of RVFV in such an ecosystem which associates a wet, temperate climate, high altitude, paddy fields, and vicinity to a dense rain forest. Persistence mechanisms need to be further investigated. (Texte intégral

Description et analyses du reseau commercial des bovins sur les Hautes Terres de Madagascar et rôle potentiel dans la diffusion de la Fièvre de la Vallée du Rift

La Fièvre de la Vallée du Rift, isolée pour la première fois en 1979 sur les hauts plateaux malgaches, a causée des épidémies en 1991 et 2008. En 2008, des cas humains et animaux ont été identifiés dans le district d'Anjozorobe. Une étude sérologique des bovins identifiés de cette zone a montré un taux de séroprévalence de 28% pour cette même année. L'analyse des données suggère une circulation récurrente du virus de la FVR dans cette zone, favorisée par les mouvements commerciaux de bovins. Le rôle des mouvements de bétails ayant déjà été mis en évidence dans la diffusion des maladies animales, l'objectif de cette étude était de décrire et analyser le réseau commercial des bovins de la zone d'Anjozorobe et de déterminer le lien potentiel entre la structure du réseau et la circulation du virus dans la zone. Une enquête auprès des éleveurs de la zone a été menée de Février à Juillet 2009 afin de collecter les données commerciales. Ces données ont été utilisées pour construire le réseau commercial, les villages étant définis comme noeuds et les mouvements d'animaux comme liens. Dans le même temps, les bovins qui étaient séronégatifs en 2008 ont été rééchantillonnés en 2009 afin de détecter des séroconversions. Un taux d'incidence intra-village annuel a été estimé. Une analyse multivariée a été menée pour étudier l'association entre l'occurrence de séroconversion dans chaque village (variable binaire), et les paramètres de centralité (degrés d'échanges et d'achat/vente) ainsi qu'une variable environnementale qui avait précédemment été liée au risque de transmission (distance du village au point d'eau le plus proche) et le taux de séroprévalence en 2008 (i.e., niveau de couverture immunitaire du cheptel bovin). Du fait de la non-dépendance des observations (les villages sont liés par le commerce d'animaux), une procédure bootstrap a été utilisée pour estimer l'effet des variables explicatives. L'enquête incluait 777 animaux et 386 éleveurs de 41 villages parmi 52 existants dans la zone d'étude. Le taux d'incidence intra-village s'étalait de 0% à 20%. Deux types de mouvements commerciaux de bovins ont été observés : échanges et achats/ventes. Deux différents réseaux ont donc été construit et analysés. La distribution des degrés suggère que les deux réseaux sont libres d'échelle avec un fort degré d'hétérogénéité. Une significative mais faible corrélation a été observée entre les deux réseaux (0.16, p<0.0001). Le modèle multivarié impliquait le degré freeman de chaque réseau (échanges et achats/ventes), le niveau de séroprévalence en 2008 pour chaque village et la distance moyenne entre chaque village et le point d'eau le plus proche. Comme attendu, une association négative a été observée entre l'occurrence de séroconversion dans les villages et le niveau de couverture immunitaire de chaque village. Il en a été de même pour la distance au point d'eau le plus proche. Les degrés du réseau d'échanges étaient positivement liés à l'occurrence de séroconversions, alors qu'aucune association n'a été observée entre la variable de réponse et les degrés du réseau d'achats/ventes. Les deux types de mouvements observés (échanges et achats/ventes) créent deux différents réseaux. Les deux sont libres d'échelle et pourraient être formés par attachement préférentiel, du fait de la meilleure réputation de quelques éleveurs ou villages. La faible corrélation des réseaux d'échanges et d'achats/ventes suggère que le critère utilisé pour établir une telle réputation est différent pour les éleveurs et les commerçants. Les résultats montrent que le réseau d'échanges pourrait être le support de l'introduction du RVFV dans les villages, alors que le réseau d'achats/ventes serait probablement plus impliqué dans l'introduction du RVFV dans la zone depuis d'autres parties de Madagascar. Finalement, l'effet négatif de la distance du point d'eau le plus proche suggère qu'après que le virus ait été introduit dans le village, la transmission vectorielle soit l

Identification of hotspots in the European Union for the introduction of four zoonotic arboviroses by live animal trade

Live animal trade is considered a major mode of introduction of viruses from enzootic foci into disease-free areas. Due to societal and behavioural changes, some wild animal species may nowadays be considered as pet species. The species diversity of animals involved in international trade is thus increasing. This could benefit pathogens that have a broad host range such as arboviruses. The objective of this study was to analyze the risk posed by live animal imports for the introduction, in the European Union (EU), of four arboviruses that affect human and horses: Eastern and Western equine encephalomyelitis, Venezuelan equine encephalitis and Japanese encephalitis. Importation data for a five-years period (2005-2009, extracted from the EU TRACES database), environmental data (used as a proxy for the presence of vectors) and horses and human population density data (impacting the occurrence of clinical cases) were combined to derive spatially explicit risk indicators for virus introduction and for the potential consequences of such introductions. Results showed the existence of hotspots where the introduction risk was the highest in Belgium, in the Netherlands and in the north of Italy. This risk was higher for Eastern equine encephalomyelitis (EEE) than for the three other diseases. It was mainly attributed to exotic pet species such as rodents, reptiles or cage birds, imported in small-sized containments from a wide variety of geographic origins. The increasing species and origin diversity of these animals may have in the future a strong impact on the risk of introduction of arboviruses in the EU. (Résumé d'auteur

Maintenance mechanisms of Rift Valley fever virus in a temperate and mountainous ecosystem of Madagascar: dynamic and spatial modelling based on field data. [262]

Purpose: Rift Valley fever virus (RVFV) is a mosquito-borne zoonosis endemic in Africa. Since 1930, outbreaks have occurred in tropical, hot-irrigated or arid ecosystems. In 2008-2009, an outbreak occurred in a temperate and mountainous area of Madagascar highlands. We conducted a 3-year serological follow-up in cattle in a pilot area of these highlands (894 bovine in 2009, 516 in 2010 and 210 in 2011). Cattle exchange and vector population dynamics were investigated. Despite unfavourable climatic conditions and absence of Aedes mosquitoes (main vectors in Africa), results showed that the virus kept circulating till 2011 in absence of clinical cases. The objective of this study was to understand the mechanisms allowing the virus to circulate in this unfavourable ecosystem. We propose a model coupling cattle exchange practices and vector-borne transmission to explain the RVFV spread and persistence in this area. Methods: The model is parameterized to reproduce the local conditions of Madagascar highlands, using observational data collected in the area: villages/ rice field locations, number of cattle per village, cattle birth/death rates, cattle exchange networks built using SNA methodology, monthly abundance of mosquitoes, monthly variations of the mosquito parity rate. Non-observable parameters such as transmission parameters were estimated using collected serological data. The model was used to analyse 3 potential mechanisms that could explain the recurrent circulation of RVFV in the area: (i) recurrent introductions from other regions of Madagascar, (ii) RVFV direct transmission between cattle during calving period, (iii) a low level vector-based circulation during winter thanks to a residual vector population. All possible combinations were tested. Results: Predictions satisfactorily reproduced field observations. Results appeared robust according to the sensitivity analysis. Conclusions/ Relevance: Interweaving between agricultural works in rice fields, seasonality of vector proliferation, cattle exchange and traditional practices (socio-economic practices) could be a key element for understanding RVFV circulation in this area of Madagascar highlands. (Texte intégral

Identification de zones à risque d'introduction de quatre arboviroses zoonotiques dans l'Union européenne par le commerce des animaux vivants

Le commerce des animaux vivants est l'un des principaux modes d'introduction de virus dans les zones indemnes à partir de leurs zones d'enzootie. Depuis quelques années, certains animaux sauvages sont considérés comme des animaux de compagnie, ce qui augmente la diversité des espèces d'animaux faisant l'objet d'un commerce international. Ceci pourrait permettre à des pathogènes présentant un large spectre d'hôtes, comme certains arbovirus, d'élargir leur aire de répartition. L'objectif de cette étude était d'analyser le risque posé par les importations d'animaux vivants pour l'introduction, dans l'Union européenne (UE), de quatre arbovirus affectant l'Homme et le cheval : les virus des encéphalites équines de l'est (EEEV) et de l'ouest (WEEV), de l'encéphalite vénézuélienne (VEEV) et de l'encéphalite japonaise (JEV). Des données d'importations, environnementales et des données de densité de population humaine et équine ont été combinées pour calculer des indicateurs spatialisés du risque d'introduction de ces virus et des conséquences potentielles de telles introductions. Les résultats montrent l'existence de zones à risque en Belgique, aux Pays-Bas et dans le nord de l'Italie, avec un niveau de risque plus élevé pour l'EEEV que pour les trois autres virus, risque principalement attribuable au commerce des animaux de compagnie importés par lots de petite taille d'origines variées. La diversité croissante des espèces et des origines de ces animaux pourrait avoir dans le futur un impact important sur le risque d'introduction d'arbovirus dans l'UE

A meta-population model to explain an endemic Rift Valley fever transmission in Northern Senegal. [P260]

Purpose: Certain Palaearctic biting midges have been implicated as vectors of bluetongue virus in northern Europe. Separation of two species (Culicoides obsoletus and C. scoticus) is considered difficult morphologically, with females often grouped together in entomological studies. Species specific identification is desirable to assess their roles in disease transmission or measure abundance during arboviral outbreaks. Our aim is to investigate whether morphometric identification techniques can be applied to female C. obsoletus and C. scoticus individuals trapped in different geographical regions and time periods during the year. Methods: Using light-suction traps, female C. obsoletus and C. scoticus were sampled from two locations in the UK, France and Spain. A total of 759 individuals were identified with a molecular assay using the cytochrome oxidase I gene. Fifteen morphometric measurements were then taken from the head, wings and abdomen of slide-mounted specimens. Multivariate analyses investigated whether a combination of these could lead to accurate species identification. Results: Principal component analyses revealed that the length and width of the smaller and larger spermathecae, and the length of, and width between, the chitinous plates can differentiate the species. These are all abdominal characteristics. Seasonal and geographic variation was observed for head and wing measurements, but not for those from the abdomen. Conclusions: Our results suggest that female C. obsoletus and C. scoticus individuals can be separated under a stereomicroscope using abdominal measurements. Although we show that morphometrics can be used to differentiate the species, this can be time-consuming and we recommend undertaking this using standardized subsampling of large catches. Relevance: This work highlights a new morphometric method of discriminating two of the main vector species of bluetongue virus. Such separations generally rely on molecular techniques, which can be expensive. Morphometric identifications may prove useful in outbreak situations when they can be quickly undertaken on a subsample of individuals to determine the proportions of each species present. (Texte intégral

Modelling Japanese encephalitis virus transmission dynamics and human exposure in a Cambodian rural multi-host system

Japanese encephalitis (JE) is a vector-borne zoonosis and the leading cause of human acute encephalitis in Asia. Its epidemiological cycle is usually described as involving wild birds as reservoirs and pigs as amplifying hosts. JE is endemic in Cambodia, where it circulates in areas with low pig densities (<70 pigs per km²), and could be maintained in a multi-host system composed of pigs, but also poultry as competent hosts, and dogs, cattle and humans as non-competent hosts. We built and used a mathematical model representing Japanese Encephalitis virus (JEV) transmission in a traditional Cambodian village (calibrated with field data collected in 3 districts of Kandal province), to assess the capacity of the epidemiological system to sustain JEV transmission in villages in the 3 districts (based on R0), and to quantify human exposure. Changes in farm density and agricultural practices, or epizootics (e.g., African swine fever), can profoundly alter the composition of host communities, which could affect JEV transmission and its impact on human health: we used the model to analyze how host community composition affected R0 and human exposure. Lastly, we evaluated the potential use of dog JE seroprevalence as an indicator of human exposure to JEV. In the modeled villages, the calculated R0 ranged from 1.07 to 1.38. Predicted annual probability of human exposure ranged from 9% to 47%, and predicted average age at infection was low, between 2 and 11 years old, highlighting the risk of severe forms of JEV infection and the need to implement vaccination of children. According to the model, increasing the proportion of competent hosts induced a decrease in age at infection. Finally, the annual human exposure probability appeared linearly correlated with dog seroprevalence, suggesting that in our specific study area, dog seroprevalence would be a good proxy for human exposure