COMPUTER-SIMULATION TO SUPPORT POLICY-MAKING IN THE CONTROL OF PSEUDORABIES

A further integration of international markets makes a coordinated pol icy against contagious animal infections increasingly important, In th e future, stricter demands are to be expected concerning the control a nd eradication of such infections. To anticipate these demands, a comp uter simulation model is created in which scenarios can be evaluated w ith respect to epidemiological and economic effects of the infections and control strategies. In this paper, the simulation model is describ ed for Pseudorabies in swine. In the model, the population of herds is subdivided into two main herd types: breeding and finishing. Each her d is in one of 24 states per herd type, The states are based on (1) th e reproduction ratio R which is the number of secondary cases caused b y one infectious herd, (2) the prevalence for each value of R and (3) the expected number of infectious animals in an infectious herd within each prevalence range and for each R. The different values of R are b ased on experiments and field data in which different vaccination stra tegies were used. The transition matrix with the probabilities of ever y transition from one state to another is calculated on a weekly base. With this matrix the distribution of herds over states from week to w eek is derived. To include a dynamic element in the transition probabi lities, the number of newly infectious herds per week is a function of animal and other contacts, including aerial, material and personal co ntacts. Calculations show that the infection in the Dutch swine popula tion will not disappear without vaccination. With a vaccination scheme in which sows are vaccinated 3 times per year and fattening pigs 1 ti me per cycle the infection will ultimately be eradicated, but 2 vaccin ations per cycle for fattening pigs are needed to eradicate the infect ion within an acceptable timespan (i.e. 2 to 3 years). The latter stra tegy will become compulsory in the Netherlands from October 1st 1995. (C) 1997 Elsevier Science B.V.