Pathocénose et émergence des maladies transmissibles : un concept unificateur mis à l'épreuve sur des pathologies exemplaires – ANR-05-SEST-0013

The emergence of new infectious diseases is a public health problem of increasing acuity, which touches both developed and emerging countries. Several factors, which are known to support the emergence of infections, often of anthropic origin, lead to environmental modifications. However, the link between these factors and the risk of emergence are generally limited to a simple correlation. They don’t allow for the understanding of the intimate mechanism, i.e. the cascade of consequences affecting the dynamics of the host - parasite system, which brings this environmental factor to emergence. Without the identification of these precise mechanisms, it is very difficult to set up emergency plans that effectively diminish the risk of emergence of the disease. Our objective is to find the risk factors associated with an emerging disease and identify how these factors lead to either 1) the appearance of a more (or less) severe form of a preexistent pathology, 2) or the appearance of a new disease in human beings because of a contamination by a pathogen, which had previously infected only animals. To this aim, we propose to break with the vertical approach that isolates the microparasite - reservoir(s)/vector(s) species (s) - human system. On the contrary, we consider the emergence of a pathology as the deterioration of a preexistent equilibrium state between diseases that circulate in a given population at any moment in time. The concept of pathocenosis provides a framework adapted to this objective. This concept makes it possible to establish the link between all the pathogens simultaneously present within a host population. It is important to note that pathocenosis can predict both the link between pathogens and the interactions between a pathogen and its eco-epidemiological environment. Pathocenosis implies that the distribution of each disease must be considered in interaction with the other present pathologies. In a stable ecological situation, it tends towards an equilibrium characterized by all the observed diseases. Reciprocally, a disturbance of the environmental conditions can involve a deterioration of pathocenosis, possibly through its interactions with the pathocenosis of other species and the emergence of other diseases. The framework of pathocenosis unifies all the mechanisms of emergence and allows a synthetic study of the role of the factors involved in emergence. In order to study a diversity of risk factors and mechanisms, we base our study on a comparative approach of contrasted systems. After having understood these mechanisms, we will be able to circumscribe the risk zone and to define action plans against the effects of the pathogen, which does not usually imply the fight against the pathogen itself. Mathematical modeling associated with data collection will constitute the transverse directing clue to all the studied systems. The collaboration among researchers of different disciplines will help to formalize questions about emergence within the broader framework of pathocenosis. In addition, thoroughly chosen exemplary diseases and a variety of field studies will provide an essential comparative approach.