Options for the Control of Disease 1: Targeting the Infectious or Parasitic Agent

There are three basic approaches to managing diseases: directly reduce the reproductive rate of the pathogen, reduce host (or infected host) density, or manipulate the environment to reduce contact between diseased and susceptible animals. In this chapter we will look at the first of these approaches. Since disease transmission results from direct or indirect contact between infectious and susceptible individuals, there are two ways to target an infectious agent: either limit the number of susceptible individuals by vaccinating them, or treat infected individuals in order to reduce the duration or intensity of the infectious period and the number of infectious individuals present at any given time. The overall aim of this chapter is to consider the conditions under which vaccination and treatment may make a valuable contribution to the control of infectious diseases in wild mammal populations. Both field research and mathematical modelling approaches have been used to address this question. For vaccination, early mathematical models of infectious disease dynamics suggested a simple answer: vaccination is useful as soon as the rate of control ensures that a sufficient proportion of the population is immune for a sufficient period of time (Bailey 1957). At the individual level, this herd immunity means that any given infectious individual has a low probability of encountering a susceptible animal. If the disease is introduced into a vaccinated population, the mean number of secondary infections caused by each infected case will be lower than unity, thus preventing further outbreaks from occurring (R <: see Chapter 3). However, this generalised scenario may be considered overly simplistic, as the practicalities of vaccination campaigns often complicate matters. For example, modelling studies often include assumptions about perfect vaccine efficacy, and the efficiency of delivering the vaccine to a population that may or may not reflect the situation in the field.