Risk Assessment - A tool for decision-making

Lesley Cree
Plant Health Risk Assessment Unit, Science Division, Canadian Food Inspection Agency
Nepean, Ontario, Canada

Pest risk assessments (PRA) are increasingly used by plant quarantine agencies around the world to identify and quantify threats posed to agriculture, forestry, and natural ecosystems by plant pests present elsewhere. Risk assessments can be useful tools in providing a scientific basis on which regulatory programs to reduce risk may be based. In essence, a PRA asks how likely a pest introduction is and how bad the outcome would be if it occurred. International guidelines and terminology provide a common framework on which assessments are based, but formats differ depending on regional differences, the level of detail required, and the extent of information available on the pest in question. In the absence of perfect information, a risk assessment provides a transparent framework for decision-making based on the best available information.

As scientists began to report on a new and serious disease of oak trees in California and identified Phytophthora ramorum as the causal agent of the disease, interested parties in other regions became concerned with the question of whether or not outbreaks of P. ramorum could happen elsewhere and what measures, if any, should be taken to reduce the threat. Risk assessments of P. ramorum have been completed by a number of agencies worldwide and provide the rationale for regulatory programs that have subsequently been implemented. Among the PRAs that are available are assessments written on behalf of Canada, Oregon, The Netherlands, New Zealand, the United Kingdom, and the United States.

Each of these assessments uses factual information on the organism as a starting point to make predictions about the pest's likelihood to be introduced and its potential impacts in a defined area. Potential host range, distribution, means of spread, and economic or environmental impacts following introduction are considered. Since risk is considered to be a product of likelihood and impact, the factors that contribute to each of these elements are evaluated separately before a final assessment or estimate of risk is derived. Risk assessment is an ongoing process and, as the facts change, so do the conclusions of the assessment. This has presented a significant challenge for risk assessors who must make judgements even when information is incomplete. It has been necessary and valuable to revise PRAs, and subsequently quarantine regulations, as new information on P. ramorum has come to light.

The following table briefly summarizes the findings and conclusions of some risk assessments completed for P. ramorum. Even when the same pest is being considered, risk assessment findings and conclusions can differ. Risk assessments from a diverse range of countries have concluded that P. ramorum is of potential significance and that regulatory measures to prevent its introduction or further dissemination are warranted. However, differences in predictions about the hosts at risk, potential pathways for introduction, or potential impacts are evident. Some of the differences in these risk assessments are due to the timing of the assessment; risk assessments can only draw on the information available at the time of writing with the result that early risk assessments differ from those written later. Other differences stem from differences in climate or host species prevalent in the area of concern in the risk assessment or from differences in trade patterns or economics, particularly for horticultural products.

Table of Risk Assessments for Phytophthora ramorum (pdf)
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The strengths of a risk assessment lie not in its ability to perfectly predict the future, but in its ability to compile and organize information, which is often highly technical, from a broad range of sources and translate it into meaningful terms for managers who must ultimately make decisions regarding appropriate actions. In the case of P. ramorum, the risk assessor's job was made easier by the open sharing of scientific information between individuals and organizations in both the regulatory and scientific community. Despite constantly changing scientific knowledge, each of the resulting risk assessments has provided useful information that has been used to design legally defensible regulatory programs for imports or nursery certification, obtain funding to conduct the program, and ultimately explain that program to a primarily non-scientific audience. Furthermore, these assessments have provided information that has been used in the design and implementation of inspection programs, surveys, and eradication or control programs. Risk assessments have also helped to identify gaps in knowledge and to influence future research efforts that, in turn, contribute to greater accuracy in future revisions of the assessment and better regulatory programs to reduce risk.

Phytophthora ramorum has presented both challenges and opportunities to the regulatory and scientific community worldwide. As scientists have struggled to learn as much as possible about the organism as quickly as possible, regulators have struggled to balance the need to protect plant health with the need to meet international trade obligations. Lessons learned from the P. ramorum experience will no doubt influence future developments in risk assessment and lead to an even more responsive and inclusive process.