[The idea of natural reservoir in arbovirology]

Aboviruses usually have extremely complex epidemiological cycles since their circulation involves successively a susceptible vertebrate and an hematophagous arthropod acting as vector. Natural reservoirs for arboviruses can belong to two different categories of organisms: Vertebrate reservoirs, with as main characteristics: a susceptible population but showing no symptoms of disease or only a benign one, an abundant and prolific population, the infected animal must show an efficient viremia, a population with strong and frequent contacts with vector populations. Two factors tend to oppose these characteristics: the pathogenicity of the virus and infection-induced immunity. Many groups of wild vertebrates (birds and mammals) act as usual reservoirs for arboviruses. Domestic or commensal mammals, as well as human beings, are rarely involved as reservoirs. Invertebrate reservoirs: arthropods are involved in the maintenance of many arboviruses for two main reasons: their longevity and the possibility of vertical transmission of the virus. An infected arthropod remains infected throughout its life, in spite of successive moultings. This lifetime can be long, especially for ticks. Vertical transmission is known in some virus-vector systems, but its frequency appears to be very variable. The epidemiological importance of the phenomenon is specially high in the case of cold-resistant or dry-resistant eggs. These facts lead to a large variety of epidemiological situations according to virus and region. Studying epidemiological situations also shows up the evolution of these systems, as a response to environmental modifications, often resulting from human activities. Furthermore, the repeated passages of an arbovirus in reservoir hosts could induce modifications of the viral genome, for example by facilitating the emergence of reassortants during coinfections. Finally, for many different reasons, it seems difficult, for the moment, to imagine efficient preventive measures levelled at any kind of natural reservoirs.