Climate and risk of vector borne zoonotic disease emergence: examples of Rift Valley and West Nile fevers. [P-1121-05]

Vector borne diseases have a major impact on human and animal health, but also on society economy. Due to their transmission routes, zoonotic or not, these diseases are very sensitive to climatic changes. Actors, conditions and processes requested for disease transmission are part of a complex and dynamic system whose behavior, influenced by climate but also by other environmental and socio-economic components, drive potential for pathogen transmission and outbreak occurrence. We illustrate this complexity through two examples, ie Rift Valley and West Nile fevers. Rift Valley fever (RVF) is one of the most important viral zoonoses in Africa. Transmitted by mosquitoes and direct contact, RVF affects both livestock and humans. Due to global changes, RVF also threats Northern Africa and Southern Europe (Chevalier, Pépin et al. 2010). Depending on areas, the influence of climate may differ, ranging from determining to insignificant. In Kenyan regions characterized by large depression areas called " Dambos " and a succession of dry and rainy seasons, RVF outbreaks occur every 5-10 years: a strong correlation was shown between outbreak occurrence and heavy rainfall events that favor both massive hatch and development of Aedes mcintoshi and Culex sp mosquitoes that are the main vectors of RVF in that region. Similarly, the Ferlo area located in Northern Senegal is characterized by a Sahelian climate and temporary ponds (small water bodies that are filled during the rainy season and then dry). But in this region, no correlation between outbreaks and extreme rainfall events has ever been demonstrated. However Soti et al (2012) showed that these outbreaks occurred when the abundance of the two main mosquito vectors in this region, namely, Culex poicilipes and Aedes vexans arabiensis, was higher than average: these abundances are directly linked to specific rainfall rhythms now well described (Soti, Tran et al. 2012). Nevertheless, herd management, herd renewal rates and nomadic herd movements that influence the immunologic coverage of herds, are probably also involved. These variables will have to be incorporated in existing models to refine them and allow for an accurate surveillance and prediction of outbreaks. In other countries that recently experienced RVF outbreaks, such as Madagascar or South Africa, heavy climatic perturbation could not be incriminated in RVF occurrence (Anyamba, Linthicum et al. 2010). In Egypt and Yemen, the role of socio-economic factors, along with climatic factors, was found to be determinant in RVF emergence and outbreak severity(Abdo-Salem, Tran et al. 2011; Xiao, Beier et al. 2015). Also transmitted by mosquitoes, mostly from Culex genus, West Nile fever (WNF) is caused by a Flavivirus. Reservoir hosts are wild birds, mostly passerines. Human and horses are dead-end hosts. Introduced in New-World in 1999, the virus spread throughout the USA in few years. It is now endemic and transmitted till South of Argentina. Between 1999 and 2010, nearly 1.8 million people were infected; more than 12 000 neurological cases were recorded, from which 1308 were fatal (Kilpatrick 2011). In Europe, the virus has been recorded in the Mediterranean Basin since the sixties without any human or animal health consequences. However, the incidence of human and equine neurological cases suddenly increased, in particular since 2010. Two recent studies showed that an above normal temperature during the preceding months of outbreaks was strongly linked to this occurrence : this high temperature increases the vector competency of mosquitoes and provokes a an increase of the mosquito population densities (Tran, Sudre et al. 2014). Ecological disruptions induced by climatic variations, but also by landscape transformation by human or socioeconomical disturbances, are the main component of disease emergence. The understanding of mechanisms and conditions that underlie these processes is part of the major challenges that scientist and health policy makers will have to face to in the coming years. (Texte intégral