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Assessing Risk of Introduction via Import

EFSA Journal 2012;10(4):2657 [20 pp.]. doi:10.2903/j.efsa.2012.2657
European Food Safety Authority Acknowledgment EFSA wishes to thank the members of the AHAW Working Group: Giovanni Alborali, Labib Bakkali Kassimi, Jef Hammond, Frank Koenen, Mo Salman, Arjan Stegeman and Hans-Hermann Thulke and EFSA staff: José Cortiñas Abrahantes, Katriina Willgert, Andrea Gervelmeyer and Tilemachos Goumperis for the support provided to this scientific output and Mo Salman for peer reviewing the publication. Contact sas@efsa.europa.eu
Type: Scientific Report of EFSA On request from: EFSA Question number: EFSA-Q-2011-01182 Approved: 17 April 2012 Published: 19 April 2012 Affiliation: European Food Safety Authority (EFSA) Parma Italy
Abstract

A quantitative approach to estimate the likelihood of introduction of an infectious disease agent into a disease-free country through the movement of animals is essential to assess the risk of introduction of such disease agent. This approach was used to establish the likelihood of introduction of vesicular stomatitis virus (VSV) in Europe based on probability theory and a set of assumptions in relation to the shipment size of importation (values ranging from 1 up to 4,000 animals), prevalence scenarios from the exporting country for each of the susceptible species and the characteristics of the testing system in place to detect VSV. Several scenarios were modelled. Considering animals imported to Europe with a testing system sensitivity of 0.98 and a prevalence of 1 out of 4,000 in the population of concern (i.e. belonging to a susceptible species and destined to export) the number of single shipments that are needed in order to have VSV introduced in Europe are 199,951. Taking into account that each year approximately 4,000 animals are imported to Europe, at least 50 years are needed for the introduction of an infected animal. When the prevalence is around 9 out of 10,000 and keeping the sensitivity at 0.98, it takes 54,193 single shipments (at least 14 years) to introduce an infected animal. Similarly, it can also be stated that if the prevalence is 1 out of 4,000 and the sensitivity is 0.99, then around 400,000 animals are to be imported (100 years) to introduce an infected animal in Europe. When the prevalence is around 9/10,000 and the sensitivity is 0.99, then the number of imported animals needed to introduce an infected animal in Europe is 108,385 taking at least 27 years.

© European Food Safety Authority, 2012

Summary

A quantitative approach to estimate the likelihood of introduction of an infectious disease agent into a disease-free country through the movement of animals is essential to reduce the risk of introduction of such disease agent. Movement of animals has been considered to be the main risk factor for the introduction of several infectious diseases, into disease-free areas (Horst et al., 1999; Gilbert et al., 2005; Bigras-Poulin et al., 2006; Green et al., 2006; Kiss et al., 2006). The approach proposed to establish the likelihood of introduction of vesicular stomatitis virus (VSV) in Europe is based on probability theory specifically to respond the following risk question:

  • Which approach could be proposed to estimate the likelihood of introduction of vesicular stomatitis virus (VSV) through importation of live animals due to imperfection of available testing procedures? Such an approach should be based on animal species that could be a source of introduction in Europe and on the available disease background information from exporting countries.

The proposed approach to estimate the likelihood of introduction was illustrated for the specific case of VSV introduce to Europe. Given that information needed to estimate the likelihood of VSV introduction is not available, solutions have been proposed to deal with this lack of information. In particular, it was proposed to set up assumptions in relation to the shipment size, the species-specific prevalence estimates in the exporting country to Europe, and the characteristics of the testing system in place to detect the disease. In detail:

  • Shipment size (NH): the values ranged from 1 up to 4,000; the upper bound depending on the disease under study and on how many animals are imported in a year;
  • Prevalence estimates: it was proposed to pool all susceptible species and exporting countries. In this case, prevalence ranged
    from  1/NH, up to
    1/ F2,2NH;0.975
    NH+F2,2NH;0.975
    , where NH ≅ 4000.
    These figures were retrieved from historical information about imports in Europe from those countries which could potentially trade infected animals. Note that the number of potentially infected animals is disease-specific
  • Testing system characteristics: for the specific case of VSV, it was assumed to have sensitivity ranging from 0.98 to 1 and specificity = 1, since the only animals that pose a risk for introduction are those which test negative but are truly infected (false negative).

In summary, for each of the input information needed to estimate the likelihood of introduction of VSV in Europe, a range of values was used, aiming to account for variation among countries and species-specific testing system characteristics, as well as different shipment sizes entering in Europe.

It can be said that for the particular case of VSV if we consider animals imported to Europe, with a testing system sensitivity of 0.98 and a prevalence of 1 out of 4,000 in the population of concern (i.e. belonging to a susceptible species and destined to export), then the number of single shipments that are needed in order to have VSV introduced in Europe is 199,951. Considering that each year approximately 4,000 animals are imported to Europe at least 50 years are needed for the introduction of an infected animal.

When the prevalence is around 9 out of 10,000 and keeping the sensitivity at 0.98, it takes 54,193 single shipments (at least 14 years) to introduce an infected animalSimilarly, it can also be stated that if the prevalence is 1 out of 4000 and the sensitivity is 0.99 around 400,000 animals are to be imported (100 years) to introduce an infected animal in Europe. When the prevalence is around 9 out of 10,000 and the sensitivity is 0.99 the number of imported animals needed to introduce an infected animal in Europe is108,385 taking at least 27 years.

Keywords

Likelihood of introduction, negative predictive values, sensitivity, specificity, binomial distribution