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Vet. Res.
Volume 31, Number 1, January-February 2000
Page(s) 149 - 150
How to cite this article Vet. Res. (2000) 149-150
Vet. Res. 31 (2000) 149-150

Modelling pseudorabies virus infection supports eradication

A. Van Nesa, M.C.M. De Jongb and J.H.M. Verheijdena

a  Department of Farm Animal Health, Utrecht University, 7 Yalelaan, 3584 CL Utrecht, The Netherlands
b  Department of Immunology, Pathobiology & Epidemiology, Epidemiology, Statistics, and Modelling group, ID-DLO, Lelystad, The Netherlands

Abstract - Mathematical methods benefit the interpretation of results from experiments and observations under field conditions and thus enhance our understanding of the spread of infections within and between herds. Moreover, a mathematical model quantifies the impact of risk factors and thus determines whether or not the risk factor is a jeopardy for eradication. In this paper, we used a stochastic SIR model. The reproduction ratio between pigs $\rm (R_{ind})$ can be calculated from this model. This reproduction ratio is defined as the mean number of pigs that is infected by one typical infectious pig. When $\rm R_{ind} < 1$, infection fades out and eradication will succeed, when $\rm R_{ind}
> 1$ an infection can spread. Hence, eradication is certain if $\rm R_{ind}$ is smaller than 1. However, when $\rm R_{ind}$ is larger than 1, eradication can still succeed when R between herds $\rm (R_{herd})$ is below 1. Of course, when $\rm R_{ind} < 1$, eradication will be more rapid than when $\rm R_{ind}
> 1$ and $\rm R_{herd} < 1$. Measures to eradicate PRV aim to reduce $\rm R_{ind}$ or $\rm R_{herd}$ to below 1. The following conclusions can be drawn from our experiments, observations and modelling with pseudorabies virus (PRV): - Experimentally, it was shown that the number of pigs in a population (with a constant density of pigs per m 2) did not influence the reproduction ratio of PRV. There was, however, a significant difference in transmission between vaccinated not maternally immune SPF pigs and vaccinated maternally immune conventional pigs. Earlier experiments had already shown that transmission in maternally immune SPF pigs, which were vaccinated twice, was below one.

Corresponding author: A. Van Nes Tel.: (31) 30 2531248; fax: (31) 30 2521887;

© INRA, EDP Sciences 2000