Modelling the maintenance and transmission of foot-and-mouth disease virus in sheep

The aim of this study was to investigate the characteristics of an outbreak of footand-mouth disease (FMD) in Greece during 1994. This epidemic suggested that under certain circumstances transmission of foot-and-mouth disease virus (FMDV) may not be as efficient as is classically thought. Here, analysis of field data from the epidemic has shown that conditions at the source of the epidemic may have provided an initial amplification of the force of infection, enabling the virus to cause a limited number of outbreaks elsewhere. These favourable conditions included a high density and large number of sheep flocks. Later in the epidemic, the distribution of sheep flocks was more dispersed and led to an epidemic that was termed "self-limiting". Seemingly, without the influence of control measures, the infection rate of FMDV within sheep flocks fell dramatically until no cases of clinical FMD occurred. The virus was not sheep-adapted.Serial passage experiments described here have been designed to ensure a consistent exposure period for sequential groups of sheep, such that each group was exposed to the same proportion of the previous group's total excretion. Two identical experiments were performed with four groups of sheep. Serial passage of this isolate showed a significant reduction in the viral load of infected animals after passage of virus through two groups of contact infected sheep. This reduction corresponds with a decrease in the estimated force of infection. Transmission of this isolate through contact infected sheep failed to amplify or maintain the level and force of infection.Virological factors contributing to "self-limitation" have also been investigated. Experiments described here show that dose-dependent transmission of this isolate is a complicated process. Immunologically, inoculated sheep responded well to this isolate, such that with higher doses immune mechanisms reduced the infectiousness of inoculated animals. Sheep inoculated with the highest dose of virus did not transmit infection to in-contact animals. Above the threshold dose for infection there was a decline in the infectiousness of inoculated animals and a corresponding reduction in the level of infection for in-contact animals. Principal component analysis of infection parameters for inoculated animals shows that characteristics of infection group well with dose.The results of the experiments described here suggest that isolates of FMD that are not adapted to sheep may be unable to maintain themselves in sheep populations. FMDV in cattle and pigs transmits rapidly and has the potential to create concentrated foci of infection. The potentially different behavior of FMDV in sheep should be carefully considered when attempting to control FMD epidemics