Transmission dynamics: critical questions and challenges

Abstract

This article overviews the dynamics of disease transmission in one-host-one-parasite systems. Transmission is the result of interacting host and pathogen processes, encapsulated with the environment in a 'transmission triangle'. Multiple transmission modes and their epidemiological consequences are often not understood because the direct measurement of transmission is difficult. However, its different components can be analysed using nonlinear transmission functions, contact matrices and networks. A particular challenge is to develop such functions for spatially extended systems. This is illustrated for vector transmission where a 'perception kernel' approach is developed that incorporates vector behaviour in response to host spacing. A major challenge is understanding the relative merits of the large number of approaches to quantifying transmission. The evolution of transmission mode itself has been a rather neglected topic, but is important in the context of understanding disease emergence and genetic variation in pathogens. Disease impacts many biological processes such as community stability, the evolution of sex and speciation, yet the importance of different transmission modes in these processes is not understood. Broader approaches and ideas to disease transmission are important in the public health realm for combating newly emerging infections.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Keywords: contact matrix; force of infection; perception kernel; transmission triangle; vector.

Figure 1.

‘The curse of beta’: diagram of the disease cycle illustrating the complex nature of the transmission process, that often is summarized by one parameter β. (Online version in colour.)

Figure 2.

Transmission is the beating heart of the disease triangle: the idea that host and pathogen genotype and environment contribute to disease expression is equally applicable to disease transmission. (Online version in colour.)

Figure 3.

(a) Skipper butterfly (Hesperiidae) visiting Dianthus pavonius diseased with anther smut. (b) Healthy flower. (c) Diseased flower. Photos: Michael E. Hood.

Figure 4.

Transmission (fraction of healthy hosts with spores) as a function of density for a vector transmitted disease. Points are each means of 20 runs from a simulation of the ‘perception model’ using parameters approximating spore deposition in the anther-smut system (p = 3, k = 4.93, v = 100, with disease prevalence of 40% as is seen in the field). Units for density represent numbers per 100 m². Line is best fit of theoretical expectations based on Antonovics et al. [20] for vector transmission y = aN/(1 + bN)², where N = density, a = 0.0128, b = 0.113.

Figure 5.

Dispersal kernel for a vector visiting a high-density population (N = 200, red) and low-density population (N = 10, blue). Parameters for the simulation, as in figure 1. Units for distance in the anther-smut system are metres.