Predicting the level of herd infection for outbreaks of foot-and-mouth disease in vaccinated herds

Foot-and-mouth disease (FMD) is a highly contagious virus infection of shee p, goats, cattle, pigs and other, non-domesticated species of artiodactyls, and causes both clinical and subclinical infection according to the natura l or acquired immunity of the host. Within vaccinated dairy herds FMD may a ppear as an acute, mild or subclinical infection, dependent upon the immune status of the herd, the level of challenge and the efficacy of the vaccine used. In the large dairy herds of Saudi Arabia, sub-clinical FMD was on a number of occasions, found to have spread amongst the cattle before signs o f disease were seen. Such undetected transmission resulted in a large incid ence on the first day of diagnosis and curtailed the impact of post-outbrea k vaccination (PoV). First day incidence (FDI) for these herds was found to correlate with the final cumulative incidence of clinical disease. Since F DI is available at the start of an outbreak it can be used as a predictive tool for the eventual outcome of an FMD outbreak. During the past 11 years 47% of dairy herds examined in Saudi Arabia have experienced FMD initially as sub-clinical disease. For the remaining 53%, waning vaccinal protection did not suppress clinical disease in the initially infected animals, and th ese showed severe rather than mild signs. Hence, in such herds there was a very low initial level of subclinical infection, so PoV was more effective, and the timing of PoV was found to give a good correlation with cumulative herd incidence: an early PoV resulted in low prevalence of clinically infe cted animals whilst late PoV permitted high prevalence. PoV timing can ther eby be used in tandem with FDI as a predictive tool for future outbreaks, e stimating the final cumulative incidence (or prevalence) of clinical FMD ca ses.