Publication Abstract

Making the case for disease freedom - what can epidemiology contribute?

E.J. Peeler

A central tenet of EU aquatic animal health legislation and international standards is that susceptible species can only be traded live between areas of equal health status (or from a higher to lower status) for listed diseases. Thus demonstrating disease freedom is crucial to maintaining a high health status. Epidemiology provides the scientific underpinning to the design and analysis of surveillance to demonstrate freedom. Methods, implemented in free software (e.g. FreeCalc), to calculate sample sizes for structured surveys now take account of the minimum detectable prevalence and test characteristics, leading to improved design and interpretation of surveillance to demonstrate freedom. Scenario tree modelling (STM) goes further by allowing non-structured data sources to be used and for sampling to focus on high risk regions or farms, i.e. risk based surveillance. These methods enabled a move from input (e.g. 30 samples from every farm) to output based standards (e.g. demonstrate freedom with 95% confidence that the pathogen is not present above 2%).

Recent decisions in Europe on disease freedom illustrate epidemiology’s wide role, over and above the technical challenges of surveillance. In the UK a decision not to attempt eradication of koi herpes virus was in part based on mathematical modelling of the spread of disease under different scenarios that quantified the likelihood of success, and time-scale, of an eradication programme. No EU Member State has made a case to demonstrate freedom from white spot syndrome virus, despite an absence of outbreaks. This is discussed alongside consideration of how import risk analysis (IRA) supports decisions about mitigation measures to maintain freedom. The EU ceased listing epizootic ulcerative necrosis as an exotic disease in part due to the likely costs of necessary trade restrictions (the decision was also informed by risk assessments of the likelihood and consequences of introduction).

Epidemiology provides many of the solutions to the technical questions that arise when making the case for disease freedom. However, governments must balance the potentially competing interests of all stakeholders, including consumers. To this end socio-economic analysis, informed by epidemiological data, of the costs and benefits of disease freedom is also crucial.