Disease reservoirs: from conceptual frameworks to applicable criteria

Abstract

Central to the One Health approach and any disease eradication program is the question of whether a pathogen has a non-human reservoir. Despite well-established conceptual frameworks that define a reservoir of infection, empirical characterization of reservoirs often remains controversial, challenging and sometimes misleading. What is essentially missing are applicable requirements that standardize the use of the term 'reservoir of infection' across multiple disciplines. We propose an empirical framework, considering maintenance and feasible transmission of a pathogen, to standardize the acceptance of a disease reservoir across multiple disciplines. We demonstrate the intended use of these requirements by applying them to different diseases that are known to infect both humans and animals.

Figure 1

Three scenarios describing the dynamics of a simple reservoir system. (A) Pathogen maintenance in the non-target population and feasible transmission route towards the target population. Only this constellation fulfills the requirements of a functional reservoir system. (B) Pathogen maintenance in the non-target but no feasible transmission route towards the target population. This is a likely situation whether contact rates between the non-target and target populations are below the threshold. (C) No pathogen maintenance in the non-target, but a feasible transmission route exists. An example of the effect of a vaccination strategy in the non-target population. The dynamic of the system is indicated by arrows associated with a ‘t’ (time factor). ^#Maintenance, *feasible transmission, solid arrows=obligatory, broken line=optional.

Figure 2

The simplicity and functional orientation of the presented framework allows for straightforward application but does not negate more complex populations. The same principles apply to multi-species systems and metapopulations. The defined target group may be adjusted based on interest and may therefore include metapopulations (targets 1 and 2). The non-target group increases in complexity due to the inclusion of multiple populations (a–d). (A) Similarly, to a simple reservoir system, all susceptible populations that connect to the target either (a) directly or (b–d) indirectly are part of the non-target population. (B) Temporal shifts in the ecological landscape of the non-target population may lead to the (d) exclusion of populations either due to lack of connectivity or susceptibility. The dynamic of the system is indicated by arrows associated with a ‘t’ (time factor). ^#Maintenance, *feasible transmission, solid arrows=obligatory, broken line=optional.