Risk assessment

Risk assessment is a tool for the objective evaluation of risk, and is commonly performed by veterinary epidemiologists. Its use in the setting of veterinary epidemiology has increased in recent years, particularly as a tool for the objective consideration of the risk of movement of pathogens through international trade in animals and animal products. Risk assessment is only one component in an overarching risk analysis process, which also incorporates risk management (the process whereby procedures are implemented in order to reduce the risk) and risk communication (which involves the ongoing dissemination of relevant information to stakeholders). However, these other components of the risk analysis process will not be covered in further detail here as they are predominantly the responsibility of risk managers and policy makers.

What is risk?

In the context of risk assessment, the concept of risk can be viewed as the product of the likelihood of an event occurring and the consequences of that event (which may be biological, economic or environmental consequences). Additionally, the perception of the event is an important consideration, which can have a large impact upon risk. One example of a hazard which is generally perceived to be risky is radiation from nuclear power plants, due to a number of characteristics such as invisibility, association with long-term human health risks (including cancers), not viewed as being necessary, and being created from a process which many people do not understand. Therefore, when deciding where to site a nuclear power plant, consideration of the risk should include the likelihood of radiation release (which will generally be very low/negligible), the consequences of this (which are very high) and the perception of the risk by people who would live near to the power station.

How do we measure risk?

There are two main approaches to risk assessment, known as qualitative and 'quantitative risk assessment. Qualitative risk assessment is a subjective process which denotes risk using words, and is commonly used in the early stages of a risk assessment, before comprehensive data have been collected. Quantitative risk assessment is also a predominantly subjective process, but the risk is denoted using probabilities (either as point estimates or as probability distributions). The results of a quantitative risk assessment are often more easy to convey to stakeholders, but care must be taken to not give the illusion of perfect knowledge when these are presented. In the case of both qualitative and quantitative risk assessment, the framework adopted should be clearly explained, all data sources should be appropriately referenced and the whole process should be transparent.

Stages of a risk assessment process

Although the general process adopted during risk assessment is not affected by the intended application, a number of different risk assessment strategies have been developed. Of interest to veterinary epidemiologists are the OIE framework and the Codex Alimentarius Commission framework. The OIE approach is intended to estimate the risk of events (with associated consequences) occurring, and therefore is the framework used when investigating the movement of pathogens through animal trade. The CAC framework is based upon a model devised by the National Academy of Sciences National Research Council (NAS-NRC) in order to investigate the levels of chemicals of interest, and therefore devise maximum acceptable limits of these. As such, it is used in order to investigate the levels of pathogens and other contaminants in food.

As the OIE framework is most commonly used by veterinary epidemiologists investigating diseases of animals, this framework will be described in more detail here. The framework consists of three consecutive steps, all linked through risk communication in order to create an iterative, repeating process. These steps are hazard identification, risk assessment and risk management, and the first two of these will be covered in more detail below.

Hazard identification

Before a risk assessment can be conducted, the hazard(s) of interest must be identified. It is vital that all hazards of interest are identified, as if they are not identified in this stage, they will not be accounted for in the final risk assessment. In the context of veterinary risk assessment, hazards are usually pathogenic agents such as bacteria or viruses. The selection of which pathogens are of importance will depend on the risk question to be asked, which may specify only one pathogen or may include all pathogens present in the exporting country (in which case, a comprehensive investigation of which pathogens are present in the exporting country, and whether these are a hazard for the importing country, will need to be conducted). All relevant information about the hazard(s), including epidemiology, control measures, and general status in the exporting and importing countries) should then be compiled in preparation for the risk assessment stage. In the case of multiple hazards being considered, each should be investigated separately.

Risk assessment

This process aims to investigate the likelihood and consequences of entry of the pathogen into the country and the likelihood and consequences of pathogen spread within the country. It is divided into three steps following a logical progression (release, exposure, consequence), along with a step which brings these three together to produce a final estimate (estimation).

Release assessment

This stage requires the investigation of how animals to be imported may become contaminated or infected with the pathogen of interest (both within the exporting country and en route to the importing country), as well as whether they will remain infected by the time they arrive in the importing country (including pre-movement testing of live animals, quarantine practices, or slaughter and processing pathways for animal products). This is then used to estimate the likelihood of infected animals being imported. A useful way of visualising this process is through the creation of a scenario tree detailing the steps by which the pathogen can enter the country.

Exposure assessment

This stage requires the investigation of the likelihood of exposure of animals in the importing country to the pathogen of interest from infected imported animals. Note that this does not necessarily include infection (which is treated as a consequence of exposure). This will depend largely on the mechanisms by which the pathogen can be transmitted, as well as relevant demographic and management practices in the importing country.

Consequence assessment

This stage involves the description of the consequences of exposure and the probabilities of these consequences occurring. These consequences can be classified into those directly associated with the pathogen (such as the effects of disease and death amongst domestic animals, humans and/or wild animals) and those which are indirectly associated with the pathogen (such as trade restrictions, loss of consumer confidence, control/monitoring costs and effects on tourism. The scale of these consequences (for example, at the individual farm, local or national scale) should also be considered. Of course, the consequences could range from minimal to considerable, and therefore a number of scenarios should be considered (ranging from individual cases without establishment of pathogen in the population, up to the pathogen becoming endemic within the population).

Risk estimation

This is the final stage of the risk assessment process, and involves the combination of the results of the previous three stages in order to summarise the risk of pathogen entry. It is important that this stage remains as objective and transparent as possible, in order to ensure that risk managers have all the required information available to them in order to make decisions (i.e. it is not the risk assessors' responsibility to advise upon the best action to be taken).