THE USE OF VECTOR TRANSITION IN THE MODELING 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 interacti ons. State-transition simulation models have traditionally catered for complex modelling, yielding detailed representations that are well su ited as predictive scenarios. However, results of serological investig ations show a variance in antibody levels between segregated age group s on managed farms, and this has further complicated an intraherd mode l to the extent that a state-transition technique would become cumbers ome. Moreover, the distinction between the acute and milder forms of t he disease adds three more states to a conventional SIR framework, cre ating an APRISM model. Consequently a vector-transition technique has been employed. Vector-transition combines daily changes (in both the v iral output of infected animals and the antibody titres of susceptible s) with the transition of herd animals between disease states. This me ans that the probability and herd matrices used in the state-transitio n approach are no longer required; the model is thus simplified and th e processing load reduced. Vector-transition has direct applicability to FMD but could also be used to model similar micropopulation disease s.