10 Comparative risk analysis Graham McBride, Tom Ross and Al Dufour 10.1 ESSENTIALS OF RISK ASSESSMENT Risk assessment is a systematic process to estimate the level of risk related to some specific action or activity. It is now commonly applied to a wide variety of human endeavours in which harm to people, the environment or economic interests might occur. In the context of public health, the process attempts to quantify the likelihood and severity of illness to individuals or populations from a specified hazard. The primary purpose of risk assessment is to provide support for decisions about managing risks associated with those specific actions or activities. This is done by systematically synthesizing information about the factors that contribute to the risk in a coherent framework that enables risk, or relative risk, to be inferred from knowledge of those risk-contributing factors in specific circumstances. Depending on the needs of the risk manager, the risk assessment may attempt to assess the relative effectiveness of proposed risk mitigations, or to estimate the © 2012 World Health Organization (WHO). Animal Waste, Water Quality and Human Health. Edited by Al Dufour, Jamie Bartram, Robert Bos and Victor Gannon. ISBN: 9781780401232. Published by IWA Publishing, London, UK. 362 Animal Waste, Water Quality and Human Health magnitude of the risk under different circumstances, or the risk to different populations or sub-groups within the population. There are many frameworks that describe the interaction between risk assessment1 and risk management, and a third component known as “risk communication” which involves understanding the interests, concerns and values of “stakeholders”, that is, those affected by the risk, so as to guide and optimize risk management decisions. One depiction of the interaction among these aspects of risk analysis is presented in Figure 10.1. RISK ASSESSMENT RISK MANAGEMENT Science-based Policy-based RISK COMMUNICATION Interactive exchange of opinion and information concerning risk Figure 10.1 A depiction of the interaction between risk assessors, risk managers and those affected by the risk (‘stakeholders’) within the risk analysis framework developed by the World Health Organization and the Food and Agriculture Organisation of the United Nations. Arrows indicate lines of communication, while separate circles are intended to depict discrete roles of those responsible for risk assessment, those responsible for making decisions about risk, and stakeholders (based on: WHO/FAO 2009). 1 While some organisations consider risk assessment to encompass risk management, risk communication and risk analysis, many organisations involved in environmental and public health risk assessment consider risk analysis to be the ‘umbrella’ activity that encompasses risk assessment, management and communication. For example, the Society for Risk Analysis (http:// www.sra.org) broadly defines risk analysis “…to include risk assessment, risk characterization, risk communication, risk management, and policy relating to risk.” We choose to adopt this convention here. Comparative risk analysis 363 Risk management involves a balance between the most effective risk mitigation action, based on cost or technical feasibility, and the interests and values of stakeholders. Therefore risk managers will also seek information on costs (compared to the benefits) of risk management options. The efficacy of various approaches and the factors that affect it may be considered in the risk assessment to support the decisions of risk managers. The combination of risk assessment, risk management and risk communication is, in some frameworks, collectively termed ‘risk analysis’. Microbiological risk assessment frameworks relevant to waterborne risk are discussed by Haas et al. (1999, Chapter 3); WHO (2003); Gale (2001a&b, 2003), Coffey et al. (2007) and Goss & Richards (2008). 10.1.1 Key elements The first step in risk assessment usually is the development of a conceptual model or framework that combines knowledge of risk-affecting factors and how they could interact to cause harm. In many cases this information can be expressed mathematically as a series of equations that, provided sufficient quantitative data and knowledge exist, enable quantification of the risk or, at least, the relative risk. In assessing the public health risk from exposure to microbial pathogens, the “risk assessment” task is often broken down into four discrete components: • Hazard Identification – which involves describing the hazard (pathogens), presenting the evidence that the hazard causes illness and that it can cause illness from the source of exposure being considered, and the type of illness caused. • Hazard Characterization – which presents information about characteristics of the organism that affect its ability to cause illness, such as virulence factors, physiological traits that affect its survival in the environment and, importantly, the severity of disease caused, including consideration of differences in susceptibility of different members of the population exposed and the probability of illness as a function of the dose ingested. In the context of water and sanitation this corresponds to consideration and characterization of elements of “pathogen virulence” and “host susceptibility”. This includes consideration of dose-response relationships, which is sometimes considered as a further discrete component (e.g., Haas et al. 1999). • Exposure Assessment – which attempts to estimate the exposure of the affected population(s) to the pathogen under scrutiny. In the context of public health risk from recreational/occupational exposure to water this could involve consideration of, for example: sources of contamination, 364 Animal Waste, Water Quality and Human Health loads and composition of pathogens in sources of contamination, and factors that would increase or decrease the risk of exposure such as inactivation due to UV irradiation, predation, or human interventions. It should also consider factors that would alter exposure of different members of the population such as age (children may swim more often, or ingest more water when swimming), cultural and gender factors (e.g., women in developing nations may 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 and sanitation this corresponds to consideration and characterization of elements of load, transport and exposure (see: Chapters 2–7). • Risk Characterization – the systematic and scientific process of synthesizing all the relevant knowledge and information to produce estimates of risk, or relative risk. 10.1.2 Principles of risk assessment Ideally, the model and the data and knowledge that it is based on will be clearly documented, or ‘transparent’. Usually, however, insufficient knowledge and data are available to enable unambiguous assessment of (comparative) risk, or decisions based on that risk, and a number of assumptions will need to be made in the development and application of the risk assessment model. These assumptions, and their potential consequences for the decisions based on them, should also be clearly articulated. 10.1.3 Pathogen selection Many zoonotic pathogens have been reported in the literature as the causal agents of human infections or outbreaks of disease. The majority of these human infections were transmitted from animals to humans by food products (O’Brien 2005). Several outbreaks have been transmitted by drinking-water and only a small 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 of detected outbreaks” (Bartram 2003). The zoonotic bacterial pathogens mentioned most frequently are E. coli O157, Campylobacter, Salmonella, Leptospira and Listeria. Protozoan pathogens include Cryptosporidium, Giardia and Toxoplasma (Schlundt et al. 2004, Pell 1996, Rosen 2000, Bicudo & Goyal 2003). Although most of the zoonotic pathogens have been associated with sporadic and outbreak patterns of disease, a few have not been associated with Comparative risk analysis 365 waterborne outbreaks or with faecal contamination of water. The following criteria have been adopted whereby pathogens from the list above will be selected for inclusion in our risk comparison process. • The pathogens are known to be carried by animal or bird species; • The pathogens are discharged in the faeces of animals or birds; • The pathogens have been isolated from surface waters; • The pathogens cause disease in humans. The zoonotic pathogens that meet these criteria are: E. coli O157 (more generally, EHEC) which are predominantly carried by ruminants including beef and dairy cattle and sheep (USEPA 2000a, Caprioli et al. 2005) and, to a lesser extent, monogastric animals (Chapman 2000, Chapman et al. 1997). This organism has been associated with outbreaks of disease that are related to recreational water (e.g. Keene et al. 1994) and has been isolated from surface waters. None of the outbreaks have been linked directly to direct contacts with animals. However, some have been associated with food and drinking-water. Campylobacter species are frequently found in surface waters and this organism is carried by poultry, cattle and sheep (Jones 2001). Campylobacter has frequently been linked to outbreaks transmitted