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:
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.