Vaccine 25 (2007) 6381–6391 Foot and mouth disease virus transmission among vaccinated pigs after exposure to virus shedding pigs K. Orsel a,∗ , M.C.M. de Jong b , A. Bouma a , J.A. Stegeman a , A. Dekker c a Faculty of Veterinary Medicine, Department of Farm Animal Health, Utrecht University, P.O. Box 80.151, 3508 TD Utrecht, The Netherlands b Animal Sciences Group, Wageningen University and Research Centre, Wageningen, The Netherlands c Central Institute for Animal Disease Control Lelystad (CIDC, Lelystad), Wageningen UR, The Netherlands Received 19 December 2006; received in revised form 17 May 2007; accepted 6 June 2007 Available online 28 June 2007 Abstract The aim of this study was to design a transmission experiment that enabled quantification of the effectiveness of vaccination against foot and mouth disease (FMD) virus in groups of pigs. Previous experiments showed that intradermal injection of pigs with FMD virus 14 days after vaccination was not suitable to start an infection chain, as inoculated vaccinated pigs resisted challenge. Therefore, we carried out two experiments in which we used direct contact to a non-vaccinated pig as route of infection. In the first experiment only the vaccine effect on susceptibility was quantified by exposing pigs, either vaccinated 14 days before or not vaccinated, each to a non-vaccinated seeder pig inoculated with FMD virus O/NET/2001. Since no significant differences were observed between contact infections in vaccinated or non- vaccinated pigs, we performed a second experiment in which both susceptibility and infectivity were subject to vaccination. We quantified virus transmission in homogenous groups of vaccinated or non-vaccinated pigs in which the infection chain was started by exposure to a third group of non-vaccinated infected pigs. Transmission occurred to all contact-exposed pigs in the non-vaccinated groups and to 9 out of 10 contact-exposed pigs in the vaccinated groups. The rate of transmission (β) was significantly reduced in the vaccine group. Yet, the estimated reproduction ratio in both groups was still above 1. In conclusion, by adjusting our transmission study design and challenge method, we were able to quantify transmission of FMDV among vaccinated pigs. According to this study a single vaccination was not sufficient to stop pig to pig virus transmission. With these results major outbreaks may still be expected, even in groups of vaccinated pigs. © 2007 Elsevier Ltd. All rights reserved. Keywords: Foot and mouth disease; Pigs; Study design; Transmission; Vaccination; Virus excretion 1. Introduction In strategies on control and eradication of foot and mouth disease (FMD), vaccination can be considered as an impor- tant additional tool in conventional strategies as described in the terrestrial code of the OIE (World organization for Ani- mal Health). Depending on the control policy, vaccination should reduce clinical signs after infection and preferably also prevent transmission of virus in a population [1,2]. ∗ Corresponding author. Tel.: +31 30 2531248; fax: +31 30 2521887. E-mail address: k.orsel@vet.uu.nl (K. Orsel). The capacity of a vaccine to reduce virus spread can be experimentally studied in transmission experiments, in which virus transmission and the effect of intervention measures thereon can be quantified [3]. An important aspect in these studies is the start of the infection chain in a group of animals [4]. Preferably, inoculation should mimic natural infection. Since the primary site of infection for FMD virus (FMDV) is assumed to be the pharyngeal area [5], intranasal inoculation seems to be appropriate to induce infectious animals. In previous experiments with cattle [6,7], this route of infection was successfully applied in both non-vaccinated and vaccinated animals, and enabled us to quantify virus transmission both in non-vaccinated and vaccinated cattle. In 0264-410X/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2007.06.010