Spatial scales in human movement between reservoirs of infection

The life cycle of parasitic organisms that are the cause of much morbidity in humans often depend on reservoirs of infection for transmission into their hosts. Understanding the daily, monthly and yearly movement patterns of individuals between reservoirs is therefore of great importance to implementers of control policies seeking to eliminate various parasitic diseases as a public health problem. This is due to the fact that the underlying spatial extent of the reservoir of infection, which drives transmission, can be strongly affected by inputs from undefined external sources. In order to study the importance of these effects, we build and examine a novel theoretical model of human movement between spatially defined reservoirs of infection. Using our model we demonstrate the potential for the reservoir of infection to vary in spatial extent and temporal stability - effects which can both strongly influence the local transmission dynamics and response to control measures, e.g., mass drug administration (MDA). Considering the helminth parasites as our main example, by varying the spatial scale at which locations are defined, we demonstrate that a critical scale exists for an evaluation unit at which the migration rate into the associated reservoir of infection can be neglected for practical purposes. This scale varies by species and geographic region. Our model is designed to be applicable to a very general pattern of infectious disease spread induced by the migration of infected individuals between clustered communities. For example, it may be readily adapted to study the spatial structure of hosts for macroparasites such as the soil-transmitted helminths, schistosomes and filarial worms.