Animal Waste, Water Quality and Human Health

Exposure 259main components influence the probability of human infection by pathogenicorganisms: the environment, the pathogen and the host. The principle behindeach component and the considerations for contributing to exposures are thefollowing:• Environment: The environment must be favourable to permit the pathogen’ssurvival and pathogen-host interaction.• Pathogen: The pathogen must be present and possess the specific virulencefactors required for it to cause infection; and these factors must besuccessfully expressed in the host environment.• Host: The host must take up the pathogen and be susceptible to infection.If a pathogen is to be successful in initiating infection, all three of thesecomponents must be satisfied. Should infection be established, the resultingdisease can range in severity from asymptomatic, through mild and severe to lethal.The interaction between these factors is depicted in a simplified illustration inFigure 7.1. The full impact of the pathogen on the risk of exposure and illnesscan be expected to occur when environmental interaction, host susceptibility andpathogen virulence are considered high. Similarly, the pathogen’s impact isreduced if one or more of these determinants are lowered.Figure 7.1 Risk triangle: Factors influencing exposure, infection and illness fromzoonotic pathogens.