The use of vector transition in the modelling of intraherd foot-and-mouth disease

Foot-and-mouth disease (FMD) is a highly contagious viral infection of cattle, sheep, goats and pigs, with complex epidemiological interactions. State-transition simulation models have traditionally catered for complex modelling, yielding detailed representations that are well suited as predictive scenarios. However, results of serological investigations show a variance in antibody levels between segregated age groups on managed farms, and this has further complicated an intraherd model to the extent that a state-transition technique would become cumbersome. Moreover, the distinction between the acute and milder forms of the disease adds three more states to a conventional SIR framework, creating an APRISM model. Consequently a vector-transition technique has been employed. Vector-transition combines daily changes (in both the viral output of infected animals and the antibody titres of susceptibles) with the transition of herd animals between disease states. This means that the probability and herd matrices used in the state-transition approach are no longer required; the model is thus simplified and the processing load reduced. Vector-transition has direct applicability to FMD but could also be used to model similar micropopulation diseases.