Episystem Approach is a concept to guide Peste des petits ruminants (PPR) eradication strategy. Viral episystem is a set of interconnected host populations capable of maintaining circulation and transmission of the virus indefinitely. It includes the biological, epidemiological, environmental, and anthropogenic aspects of disease maintenance on a defined geographic and temporal scale. This defines the interactions between viruses, hosts, vectors, and environment, Dr. Simon Kihu from the WOAH described in his presentation at the Regional Advisory Group Meeting on FMD and PPR in Baku.
An episystem is not static. Context determines component importance and, working definitions must be continuously re-evaluated. The core of an episystem is the population component capable of sustaining PPR virus transmission over prolonged periods i.e., domestic small ruminants that is sheep & goats.
Not all susceptible species have been shown to transmit PPR to the extent that they have a practical role in enabling virus maintenance, i.e., cattle, camels, pigs & wildlife. These are considered dead-end hosts since there is no scientific evidence to prove that they contribute to virus maintenance.
An episystem is more than geography and requires empirical data to be constructed and described. A key source of the data is conventional epidemiology: host contact patterns, disease surveillance and modelling. Participatory epidemiology is also a great source of empirical data: host population density/dynamics, herd management practices, animal movement/trade, value chain identification and participatory disease maps. The remote monitoring is also required to provide health surveillance systems with environmental data to identify the moments when they are conducive to the emergence of diseases. Scientists and veterinary practitioners should not forget a key and important source of empirical data; the viral genomics. Molecular epidemiology can show the relationships of viruses between populations, the direction of virus flows and the origin of strains.
An episystem may consist of one large population but more commonly consists of several interlinked sub-populations. Small ruminant holdings of an ethnic community often share common herding and contact patterns and can be considered a population. Movement and transhumance over a geographical area is often a feature that must be considered.
The structure of episystems may include value chains or marketing chains. It can be a network of populations at different locations and not necessarily in a contiguous zone. PPR outbreaks may not necessarily be a true representation of where the virus is of circulating (these could be incidental outbreak areas). Outbreak reports might often be a consequence of efficient reporting of infection in areas with effective surveillance mechanisms, or relatively developed animal health infrastructure, whereas the virus may predominantly be circulating in areas with weak surveillance and/or weak animal health services deliveries (these represents the remote and hard-to-reach areas – virus sustenance areas). These remote and hard-to-reach areas may need a specific focus possibly through community-based animal health services delivery. Episystems typically transcend international borders and may involve more than one country or regional economic community. Once a PPR viral episystem is empirically identified, it becomes the target of the PPR eradication efforts through vaccination and constant surveillance to ensure the virus is eliminated.
Possible hypothesised PPR episystems are as follows:
