The transmission mechanism theory of disease dynamics: Its aims, assumptions and limitations

Abstract

Most of the progress in the development of single scale mathematical and computational models for the study of infectious disease dynamics which now span over a century is build on a body of knowledge that has been developed to address particular single scale descriptions of infectious disease dynamics based on understanding disease transmission process. Although this single scale understanding of infectious disease dynamics is now founded on a body of knowledge with a long history, dating back to over a century now, that knowledge has not yet been formalized into a scientific theory. In this article, we formalize this accumulated body of knowledge into a scientific theory called the transmission mechanism theory of disease dynamics which states that at every scale of organization of an infectious disease system, disease dynamics is determined by transmission as the main dynamic disease process. Therefore, the transmission mechanism theory of disease dynamics can be seen as formalizing knowledge that has been inherent in the study of infectious disease dynamics using single scale mathematical and computational models for over a century now. The objective of this article is to summarize this existing knowledge about single scale modelling of infectious dynamics by means of a scientific theory called the transmission mechanism theory of disease dynamics and highlight its aims, assumptions and limitations.

Keywords: Levels of organization of infectious disease system; Multiscale modelling of infectious disease dynamics; Scales of organization of infectious disease system; Single scale modelling of infectious disease dynamics; The replication-transmission relativity theory of disease dynamics; Transmission mechanism theory of disease dynamics.

Fig. 1

A schematic diagram of the seven main different single scales of organization of an infectious disease system based on the transmission mechanism theory which are: [a.] the cell scale, [b.] the tissue scale, [c.] the organ/microcommunity scale, [d.] the microecosystem scale, [e.] the whole organism scale, [f.] the macrocommunity scale, and [g.] the macroecosystem scale. These scales are hierarchically organized in both space and time such that as the spatial scale of the transmission process increases, so does the time scale of the same transmission process.

Fig. 2

A conceptual diagram of the new single scale model of malaria transmission dynamics (5.13) based on the transmission mechanism theory using the whole organism scale (whole human scale and whole mosquito scale) as the scale of observation and the macrocommunity scale as the scale of analysis.

Fig. 3

The evolution of [a.] population of infected humans I_H, [b.] population of infected mosquitoes I_V, [c.] community gametocytes load G_H, and [d.] community sporozoite load P_V for different values of α_h: α_h = 0.4, α_h = 0.6, α_h = 0.8.

Fig. 4

The evolution of [a.] population of infected humans I_H, [b.] population of infected mosquitoes I_V, [c.] community gametocytes load G_H, and [d.] community sporozoite load P_V for different values of for different values of α_v: α_v = 0.25; α_v = 0.45 and α_v = 0.85.

Fig. 5

The evolution of [a.] population of infected humans I_H, [b.] population of infected mosquitoes I_V, [c.] community gametocytes load G_H, and [d.] community sporozoite load P_V for different values of for different values of β_H: β_H = 0.356, β_H = 0.456, β_H = 0.556.

Fig. 6

The evolution of [a.] population of infected humans I_H, [b.] population of infected mosquitoes I_V, [c.] community gametocytes load G_H, and [d.] community sporozoite load P_V for different values of β_V: β_V = 0.32135, β_V = 0.42135, β_V = 0.52135.

Fig. 7

A conceptual representation of the replication-transmission mechanism theory of infectious disease dynamics at [a.] the cell level - which consists of within-cell scale as the microscale and between-cell scale as the macroscale, [b.] the tissue level - which consists of within-tissue scale as the microscale and between-tissue scale as the macroscale, and [c.] the whole organism level - which consists of within-whole organism scale as the microscale and between-organism scale as the macroscale. The theory makes the point that in multiscale dynamics of infectious diseases, there is an interacting multiscale cycle of four processes which are [a.] infection/super-infection by pathogen process, [b.] pathogen replication process, [c.] pathogen shedding/excretion process, and [d.] pathogen transmission process, which is repeated sequentially at each level of organization of an infectious disease system.