IMPLICATIONS DERIVED FROM A MATHEMATICAL-MODEL FOR ERADICATION OF PSEUDORABIES VIRUS

Simple mathematical models based on experimental and observational dat a were applied to evaluate the feasibility of eradicating pseudorabies virus (PRV) regionally by vaccination and to determine which factors can jeopardise eradication. As much as possible, the models were uncom plicated and our conclusions were based on mathematical analysis. For complicated situations, Monte-Carlo simulation was used to support the conclusions. For eradication, it is sufficient that the reproduction ratio R (the number of units infected by one infectious unit) is <1. H owever, R can be determined at different scales: at one end the region with the herds as units and at the other end compartments with the pi gs as units. Results from modelling within herds showed that contacts between groups within a herd is important whenever R between individua ls (R-ind) is >1 in one or more groups. This is the case within finish ing herds. In addition, if the R-ind is more than 1 within a herd, the size of the herd determines whether PRV can persist in the herd and d etermines the duration of persistence. Moreover, when reactivation of PRV in well-vaccinated sows is taken into account, R-ind in sow herds is still less than 1. In sow herds with group-housing systems, it is p ossible that in those groups R-ind is > 1. Results from modelling betw een herds showed that whether or not R-herd is <1 in a particular regi on is determined by two factors: (1) the transmission of infection bet ween nucleus herds and rearing herds through transfer of animals and ( 2) contacts among finishing herds and among rearing herds. The transmi ssion between herds can be reduced by reduction of the contact rate be tween herds, reduction of the herd size, and reduction of the transmis sion within herds. (C) 1998 Elsevier Science B.V.