Animal Waste, Water Quality and Human Health

364Animal Waste, Water Quality and Human Healthloads and composition of pathogens in sources of contamination, and factorsthat would increase or decrease the risk of exposure such as inactivation dueto UV irradiation, predation, or human interventions. It should also considerfactors that would alter exposure of different members of the population suchas age (children may swim more often, or ingest more water whenswimming), cultural and gender factors (e.g., women in developing nationsmay be more likely to be exposed due to their domestic responsibilities),type of water exposure (e.g., full immersion through swimming cf.accidental exposure through clothes-washing). In the context of water andsanitation this corresponds to consideration and characterization ofelements of load, transport and exposure (see: Chapters 2–7).• Risk Characterization – the systematic and scientific process of synthesizingall the relevant knowledge and information to produce estimates of risk, orrelative risk.10.1.2 Principles of risk assessmentIdeally, the model and the data and knowledge that it is based on will be clearlydocumented, or ‘transparent’. Usually, however, insufficient knowledge and dataare available to enable unambiguous assessment of (comparative) risk, ordecisions based on that risk, and a number of assumptions will need to be madein the development and application of the risk assessment model. Theseassumptions, and their potential consequences for the decisions based on them,should also be clearly articulated.10.1.3 Pathogen selectionMany zoonotic pathogens have been reported in the literature as the causal agentsof human infections or outbreaks of disease. The majority of these humaninfections were transmitted from animals to humans by food products (O’Brien2005). Several outbreaks have been transmitted by drinking-water and only asmall number have been transmitted by recreational water (Craun 2004).Outbreaks may, however, comprise only a minority of the actual cases suffered:“…probably the vast majority of waterborne disease burden arises outside ofdetected outbreaks” (Bartram 2003). The zoonotic bacterial pathogensmentioned most frequently are E. coli O157, Campylobacter, Salmonella,Leptospira and Listeria. Protozoan pathogens include Cryptosporidium, Giardiaand Toxoplasma (Schlundt et al. 2004, Pell 1996, Rosen 2000, Bicudo & Goyal2003). Although most of the zoonotic pathogens have been associated withsporadic and outbreak patterns of disease, a few have not been associated with