Foot and mouth disease virus in different host species; the effect of vaccination on transmission

Abstract

Foot and mouth disease (FMD) is a contagious disease, affecting important livestock species like cattle, sheep and pigs. Therefore, FMD is listed as a notifiable disease to the Office International des Epizooties. The outbreaks of FMD in Europe in 2001 triggered the discussion about the use of vaccination as an additional control measure. The aim of vaccination during an epidemic of FMD is to stop virus transmission among animals or between herds. The main purpose of the work presented in this thesis was to quantify the efficacy of vaccination against FMDV in cattle, sheep and pigs, and to compare virus excretion and virus spread before clinical signs become apparent in vaccinated and non-vaccinated populations of the three host species. Transmission was quantified with reproduction ratio R, i.e. the expected number of secondary cases produced by a typical infected individual during its entire period of infectiousness in a completely susceptible population. With a reproduction ratio significantly below 1, no major outbreaks are likely to occur. Also, the size of an outbreak on a herd to be expected at the moment of first detection of clinical signs was estimated. Differences were observed between vaccine efficacy in the various species. In vaccinated calves R was significantly below 1. In vaccinated dairy cows no transmission was observed. In sheep, R was estimated below 1, although not significantly. Limited vaccine effectiveness in pigs was found; R was estimated above 1. When the vaccination capacity is limited, these findings indicate that a differentiated vaccination programme for various species might be applied. At the moment of clinical detection, most infected individuals can be expected in a pig herd. Combined with the limited vaccine efficacy priority might best be given to culling of pig herds.