Vaccination against foot-and-mouth disease II: Regaining FMD-free status.

An epidemic of foot-and-mouth disease (FMD) can have devastating effects on animal welfare, economic revenues, the export position and society as a whole. The preferred control strategy in the Netherlands has recently changed to vaccination-to-live, but - not have been applied before - this poses unprecedented challenges for effectively controlling an epidemic, regaining FMD-free status and minimizing economic losses. These three topics are addressed in an interdisciplinary model analysis. In this second part we evaluate whether vaccination-to-live poses a higher risk for regaining FMD-free status than non-vaccination strategies and whether the final screening can be improved to reduce this risk. The FMD transmission model that was developed in the first part, predicted the prevalence of infected animals in undetected herds for 1000 hypothetical epidemics per control strategy. These results serve as input for the final screening model that was developed in this part. It calculates the expected number of undetected infected herds and animals per epidemic after final screening, as well as the number of herds and animals to be tested. Our results show that vaccination strategies yield a larger number of undetected infected animals in the whole country per epidemic before final screening than preemptive culling (median values and 5-95% interval): 8 (0-42) animals for 1 km preemptive culling, 50 (7-148) for 2 km vaccination and 35 (6-99) for 5 km vaccination. But the final screening reduced these to comparably low numbers: 1.0 (0-9.1) for 1 km preemptive culling, 3.5 (0.3-15) for 2 km vaccination and 2.1 (0.3-9.4) for 5 km vaccination. Undetected infected animals were mainly found in non-vaccinated sheep herds and vaccinated cattle and sheep herds. As a consequence, testing more non-vaccinated cattle and pig herds will not reduce the expected number of undetected infected animals after the final screening by much, while the required testing resources drastically increase. However, testing only a sample instead of all animals in vaccinated pig herds will not increase the expected number of undetected infected animals by much, while the required testing resources reduce by half. In conclusion, vaccination and preemptive culling strategies yield comparable numbers of undetected infected animals after final screening and the final screening costs can be reduced by testing a sample instead of all vaccinated pigs.