Appendix 37 Quantification of experimental transmission of FMDV O Taiwan in pigs P.L. Eblé1*, A.A. de Koeijer2, A. Dekker1 : Central Institute of Animal Disease Control (CIDC-Lelystad), P.O. Box 2004, 8203 AA Lelystad, The Netherlands 2 : Department Quantitative Veterinary Epidemiology, Division of Infectious Diseases, Animal Sciences Group Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands 1
Introduction To quantify reduction of virus transmission, transmission experiments in which an SIR model is used to estimate reproduction ratio R with a final size method, have shown to be very suitable [1,2]. R is the average number of secondary cases per infectious individual during its entire infectious period [3]. This implies that an infection will fade out in a population when R < 1, but can spread on a large scale when R > 1. Besides R also the transmission rate β is an important parameter, because it can be used in models to optimise control strategies. A GLM method [4] in which information on the time span of the infectious process is used, allows for an accurate calculation of β. Moreover, point estimates of R can be made, using the results of β and the infectious period T [5]. In this study, we quantified the reproduction ratio R and transmission rate β of FMDV after several vaccination strategies. Materials and Methods We performed ten transmission experiments with groups of ten pigs. In each group, five randomly selected pigs were challenged by intra-dermal inoculation in the bulb of the heel of the left hind-foot with 0.1 ml of challenge virus containing 105 TCID50/ml FMDV O Taiwan. The remaining 5 pigs of a group were contact exposed. Four of the groups were non-vaccinated, two groups were vaccinated at 7 days before inoculation (7dpi), one group was vaccinated with a 4-fold vaccine dose (4-FD) at –7dpi, two groups were vaccinated at –14dpi and one group was vaccinated with an (intra-type) heterologous vaccine at – 14dpi. For vaccination we used a commercially available double-oil-in-water emulsion [DOE] containing 3µg of FMDV O Taiwan or O Manisa (heterologous vaccine) 146S antigen per 2 ml dose. Contact infections were determined by inspection of clinical signs, by virus isolation of oropharyngeal fluid (OPF) and plasma samples, (boost of) neutralising antibody titre (VN-titre) and response in an NSP-ELISA. OPF was collected daily after challenge, heparinised blood samples and serum samples were collected at days -14, -11, -7, -4, 0, 3, 7, 10 and 14 dpi. OPF samples and plasma samples were assayed for the presence of virus by plaque titration on monolayers of secondary pig-kidney cells [6]. VN-titres to FMDV O Taiwan and VN-titres to FMDV O Manisa in serum samples were measured using a neutralisation assay [6]. Serum samples were also tested in a commercially available ELISA (UBI® FMDV NS EIA Swine) for detection of antibodies against non-structural proteins of FMDV (NSP-ELISA). Contact pigs were considered infected if FMDV was isolated in OPF or plasma samples or a statistically significant rise in VN-titre was detected or a response in the NSP-ELISA was detected. In the final size method, the estimation of R was based on the final size of the outbreak observed in the experiments. For calculation of R, a stochastic susceptible-infectious-removed (SIR) model was used [7] in which R was calculated using a maximum likelihood estimator [1,8]. For the calculation of R, all inoculated pigs were considered infectious. To determine whether transmission differed significantly between groups, we tested the null hypothesis that there was no difference in transmission between the non-vaccinated and the vaccinated group. In the GLM method, the information of the VI of OPF was used. Data for similar experiments were pooled and the results per day were entered in a GLM (Generalized Linear Model) analysis to estimate the transmission rate using the methodology of Velthuis et al. [4]. We assumed that animals were infectious only at he days that the VI was positive. If euthanasia was applied, the calculation was adjusted by reducing the total group size with one. In cases where the first contact animal became VI-positive simultaneously with the inoculated animals, we assumed that the inoculated group had become infectious half a day earlier. To quantify the infectious period T we used a survival analyses for censored data, using a twoparameter Weibull distribution. Subsequently, R was calculated with the results of β and T as in Klinkenberg et al. [5]. For all three parameters (β, T and R) differences between the vaccinated and non-vaccinated groups were analysed with a T-test. Results In the four non-vaccinated groups, all contact pigs became infected. In the –7dpi vaccinated groups, also all contact pigs became infected. In the group that was vaccinated with a 4-fold vaccine dose at –7dpi, 3 of the 5 contact pigs became infected. In the –14dpi vaccinated groups, both homologous 232
