FAQ: Risk assessment

What is the high risk period (HRP)?

The time interval between first infection -in a country or region - and first detection.

What is risk analysis? What is the difference between risk and a hazard?

Risk analysis according to the World Organisation for Animal health (OIE) framework is the process of hazard identification, risk assessment, risk management and risk communication (Figure 1). Where hazard identification is the process of identifying the pathogenic agents which could potentially cause harm; risk management is the process of identifying, selecting and implementing measures that can be applied to reduce the level of risk; risk communication is the interactive exchange of information on risk among risk assessors, risk managers and other interested parties. Risk assessment is described in more detail in question 2. Risk is the likelihood of a specific occurrence and the likely magnitude of the resultant consequences whilst hazard refers to any condition or agent – biological, chemical or physical - with the potential to cause an adverse effect.

What is risk assessment and why is it useful? What did we do before formal risk assessments?

Risk assessment (RA) is one component of risk analysis and refers to the evaluation of the likelihood and the biological and economic consequences of a hazard spreading. Risk assessments can inform the animal and public health decision making process. In conducting a risk assessment information is collected and analysed in a transparent and systematic manner. Prior to formal animal health risk assessments, human and veterinary epidemiological information was consulted. However, there was a lack of consistency and transparency in the decision making process. The formal risk assessment process increases confidence in international animal trade, with the World Trade Organisation (WTO) recognising RAs’ role in members’ regulations. Stringent and formal safeguards are now required with the increased industrialisation of the food industry, risk assessments commissioned by the European Food Safety Authority (EFSA) have also assisted in this process.

What are the main steps required to conduct a risk assessment?

The main steps taken to complete a risk assessment are: 1. Define a risk question, which forms the scope of the Risk Assessment; 2. Construct the risk framework outlining the possible pathways required for the unwanted outcome to occur; 3. Data collection; 4. Model construction and parameterisation if assessment is quantitative; 5. Risk estimation; 6. Sensitivity analysis if risk assessment is quantitative; 7. and lastly a transparent report outlining all model assumptions, data, references and conclusions.

What framework would you use to conduct an import risk assessment?

The World Organisation for Animal Health (OIE) framework is useful, particularly in trade disputes. The system defines risk assessment as release, exposure and consequence assessment • release assessment – describing the routes a pathogen could be potentially released into a particular environment; • exposure assessment – describing the biological pathways within the importing country leading to exposure of either animals or humans, given release; • consequence assessment – describing the relationship between specified exposures and the potential consequences either to health or the environment. The release, exposure and consequence assessments are ultimately integrated to produce a risk estimate which gives an overall measure of the risk associated with the specific hazard.

What is uncertainty and what is variability in relation to model inputs?

Uncertainty refers to the lack of precise knowledge about the input values, which may be due to measurement error or to lack of knowledge. Uncertainty can be reduced with increased measurement and knowledge. Variability refers to real-world diversity in which the value varies e.g. height of people or bat forearm length. Variability can not be reduced, unlike uncertainty, by increased measurement as it is an inherent characteristic of the system.

What is the difference between risk assessment and risk management? What is an acceptable risk?

Risk assessment and risk management are two separate components of risk analysis. Risk assessment is the procedural evaluation of the likelihood and the biological and economic consequences of a hazard spreading whilst risk management is the process of identifying, selecting and implementing measures that can be applied to reduce the level of risk. An acceptable risk means a risk level judged by the relevant competent authorities to be compatible with health protection; whether that is animal, human or environmental health will be dependent upon the scope of the risk assessment and the remit of the competent authority.

What is the difference between quantitative and qualitative risk assessment?

A quantitative risk assessment gives a numerical estimate of the risk by combining numerical inputs for the parameters identified in the risk pathway. Risk assessments that use single point quantitative estimates of risk (typically the arithmetic mean or worst-case value) are called deterministic risk assessments. In probabilistic risk assessment, inputs are described by probability distributions resulting in a probability distribution for the output. The output of a probabilistic risk assessment might be, for example, that we are 95% certain there will be between 10 and 35 cases of infection in a given country annually. In qualitative risk assessment, the probability of an event’s happening is described using qualitative terms such as negligible, low, medium or high. The qualitative estimates of risk for each step in the risk pathway may be combined multiplicatively to give an overall estimate of risk.

How do we know if a model is right? How can risk assessments be validated?

Models do not have to accurately model every single biological process in question, but need to provide a useful answer. According to Prof. George E. P. Box 1987 “Essentially, all models are wrong, but some are useful”. Determining if the model is useful requires an understanding of the underlying scientific assumptions, and the uncertainty in the model parameters and framework, so that the final estimates of the uncertainty in the predicted risk can be clearly understood. Importantly, risk assessment has to provide plausible estimates and be fit for purpose. It may be possible to validate a risk assessment model by comparing its predictions with observed values from, for example, epidemiological data or surveys. However, a different data set to that used to parameterise the model is required as a comparator. In comparing the model outputs, account would need to be taken of systematic differences between the data sets used in validation and model parameterisation.

How are risk assessments focusing on an emerging agent conducted?

Pathogenic agents may emerge through mutations, or through taking advantage of changes in their environment or both. Assessing the risk to human or animal health from these agents is challenging as historical epidemiological data may be limited or absent. The first step in developing a risk assessment for an emerging agent is to review the current state of scientific knowledge associated with the agent and environment. Secondly, theories should be formulated on the nature of the changes, perhaps using comparisons with similar pathogens where data are available. Where knowledge is limited, worst case assumptions may be used, and a greater reliance placed on the use of multi-disciplinary teams of experts to inform the risk assessment process. It is important for these pathogens to bear in mind that risk assessment is an iterative process, and as more information and data are obtained, the risk assessment can be revised and updated.

What is a dose-response curve and how do you incorporate it in a risk assessment?

A dose-response curve is a graph which depicts the risk of a specific response occurring given exposure to a defined dose of a pathogen. The response used is often infection or disease although morbidity and even death may be used. Dose-response curves are obtained from experiments in which groups of subjects are given a specific dose of pathogen and the proportion in each group showing the specified response is recorded. The dose-response curve is fitted mathematically to the experimental data. Experimental data to which dose-response curves have been fitted include studies in which human volunteers were given oral challenges of doses of Cryptosporidium oocysts, and cattle feeding studies with masses of brain from BSE-infected cattle (Chappell et al. 1999; Lasmézas et al. 2005). Two parameters are of particular interest in dose-response curves for microbial agents. The first is the risk from a dose of just one pathogen. The second is the dose which when given to each and every member of a group infects half of that group and is called the infectious dose 50% (ID50). Most dose-response curves for pathogens assume there is no threshold dose and that a dose of one pathogen can initiate infection, albeit with low risk. The risk then increases linearly with dose. In a risk assessment, a given exposure (either an integer dose or a fraction of a pathogen) is translated directly into a risk using the dose-response curve, which can be extrapolated to doses of less than one pathogen.

What data are used in risk assessments? Where do the assumptions come from?

The actual data required depends on the structure of the risk assessment and, importantly, the risk pathway. The pathway clearly sets out the model inputs for which data are required. General data sources include the scientific literature, unpublished data, and expert opinion. For an animal import risk assessment in which release, exposure and consequence assessments are conducted, the following types of data are required:- Release assessment: • Prevalence of the disease of interest in the country of origin • Sensitivity of pre and post-export tests • Number of animals entering the country • Selection of animals in the country of origin. Exposure assessment: • Destination and use of animals upon arrival in country • Processes and interventions that might impact on exposure • Exposure routes through, for example, direct contact, fomites, vectors and wildlife. Consequence assessment: • Amount of pathogen required to infect an animal Where data are missing, or inadequate, the risk analyst may use assumptions to fill the gaps. Assumptions often have to be made about the operational efficiency of processes. The assumptions are derived from an understanding of the relevant processes and need to be transparent.

What is an expert opinion? How do you obtain it and how is it used?

An expert is defined as a skilful person who has training and knowledge in some special field. The expert is the provider of an opinion in the process of expert-opinion elicitation. Expert opinion can be defined as the formal judgement of an expert on a matter on which his or her advice is sought. It is a subjective assessment, evaluation, impression, or estimation of the quality or quantity of something of interest that seems true, valid, or probable to the expert’s own mind (Vose, 2000). Expert opinion is elicited in such a way as to enable the risk analyst to extract the specific expertise of the expert into a form that can be used in risk assessment. This may involve, for example, asking the expert to qualify the magnitude of a risk of a particular event’s occurring. Quantitative data may also be gathered, and experts may be asked to give an indication of the uncertainty in their answer. The modified Delphi technique is one approach for eliciting expert opinion and has been used, for example, to estimate the risk of importation of foot-and-mouth disease into Europe (Gallagher et al. 2002). The basic steps include:- • Selection of questions and development of questionnaires • Selection of experts • Provision of sufficient details (definitions and relevant information) on the issues outlined in the questionnaires • Elicitation of experts’ opinions through completion of questionnaires • Aggregation and presentation of results • Presentation of the results to the experts – this may involve a workshop. • Elicitation of experts’ opinion through completion of questionnaires for a second time • Preparation of a summary of results from the second questionnaire.

What risk assessments have been done within EPIZONE?

There are four risk assessment work packages within Theme 7 of EPIZONE. These are:- WP 7.1: Standardisation of import risk assessment WP 7.2: European online database on epizootic diseases as an early warning system WP 7.3: Decision support system for CSF WP 7.4: Impact of environmental effects on the risk of introduction of epizootic diseases in Europe: Identification and Prioritisation.

How does risk assessment complement epidemiology and epidemiology complement risk assessment?

Risk assessment can add to epidemiology by helping to identify data gaps, and by providing an indication of the importance of weak or missing data in terms of their impact on the final risk estimate. Outputs of risk assessment models may also explain epidemiological observations, for example, by giving a prediction of the range of pathogen doses to which a person or animal may be exposed to through a given route, for which monitoring has failed to detect pathogen. Epidemiology complements risk assessment by identifying the pathways from observed data. Risk assessment approaches may be used with epidemiological data to calibrate dose-response models.


  • Chappell, C.L., Okhuysen, P.C., Sterling, C.R., Wang, C., Jakubowski, W. and DuPont, H.L. (1999) Infectivity of Cryptosporidium parvum in healthy adults with pre-existing anti-C. parvum serum immonoglobulin G. American Journal of Tropical Medicine and Hygiene 60(1), 157-164. European Food Safety Authority (EFSA) Good risk assessment practice http://www.efsa.europa.eu/EFSA/AboutEfsa/HowWeWork/efsa_locale-1178620753812_GoodRiskAssessmentPractice.htm Accessed on 11th November 2008 at 11.30 GMT. 
  • Gallagher, E., Ryan, J., Kelly, L., Leforban, Y. & Wooldridge, M (2002). Estimating the risk of importation of foot-and-mouth disease into Europe. Veterinary Record 150, 25, 769-772.  
  • Lasmézas, C.I., Comoy, E., Hawkins, S., Herzog, C., Mouthon, F., Konold, T., Auvré, F., Correia, E., Lescoutra-Etchegaray, N., Salès, N., Wells, G., Brown, P. and Deslys, J.-P. (2005) Risk of oral infection with bovine spongiform encephalopathy agent in primates. Lancet 365, 781-783. Legal texts: the World Trade Organisation (WTO) agreements http://www.wto.org/english/docs_e/legal_e/ursum_e.htm #Agreement Accessed on 19th November 2008 at 11.04 GMT. Terrestrial Animal Health Code 2008 http://www.oie.int/eng/normes/mcode/en_sommaire.htm Accessed on 19th November 2008 at 11.21 GMT.
  • Vose, D (2000). Risk Analysis: A Quantitative Guide. John Wiley and Sons, Chichester, UK 2000.