Transmission dynamics and emergency control in an FMDV-free country with pasture-based livestock production systems

Foot-and-mouth disease (FMD) remains one of the most economically important livestock diseases worldwide, despite decades of control efforts. This thesis advances understanding of foot-and-mouth disease virus (FMDV) transmission in a previously FMDV-free country, focusing on between-farm spread in pasture-based livestock production systems. Using Uruguay’s 2001 epidemic as a case study, the research quantified key drivers of transmission, assessed the role of livestock movements, estimated between-farm transmission parameters, evaluated the effectiveness of emergency vaccination, and compared alternative control strategies.

Analyses showed that by the time the first outbreak was detected, infection had likely already spread extensively, reaching more than 200 farms. Reconstruction of infection timelines suggested a detection delay of at least two weeks and indicated that livestock movements during the high-risk period played a major role in long-distance dissemination. Estimates of the movement-related reproduction number (Rₕ) indicated that livestock movements alone were sufficient to sustain epidemic spread, emphasizing the critical role of highly connected premises such as livestock markets.

Between-farm transmission was quantified using spatially explicit kernel models. Transmission patterns differed from those reported in more intensive production systems, with cattle density emerging as a key determinant of spread. Farm heterogeneity strongly influenced transmission dynamics: larger cattle farms were both more likely to become infected and more likely to generate onward transmission. In contrast, sheep and pig farms contributed relatively little to epidemic spread. These findings identify large cattle farms and highly connected holdings as priorities for risk-based surveillance and early detection.

A major contribution of this thesis is the estimation of vaccine effectiveness under field conditions during an FMDV epidemic. Using farm-level data, vaccination with commercial bivalent vaccines was associated with substantial reductions in between-farm transmission within the first week after administration. These findings provide rare field-based estimates of vaccine effectiveness against transmission and demonstrate that good-quality commercially available vaccines can play a critical role in epidemic control when combined with movement restrictions and biosecurity measures.

Finally, the estimated transmission kernels were incorporated into a stochastic simulation model to evaluate alternative control strategies. Preventive targeted vaccination, particularly when directed toward cattle farms with the highest transmission potential (Rₕ≥0.8), consistently achieved the greatest reductions in epidemic size while requiring substantially less operational effort than reactive culling or small-radius ring vaccination.

Overall, this thesis demonstrates how quantitative epidemiology and field data can be used to generate evidence directly relevant to outbreak preparedness and response. The findings show that delayed detection and livestock movements during the high-risk period can drive extensive dissemination before control measures are implemented, that transmission in pasture-based production systems is strongly shaped by cattle density and farm-size heterogeneity, and that emergency vaccination can rapidly reduce between-farm transmission when integrated with movement restrictions and biosecurity measures. By providing empirical estimates of movement-mediated spread, transmission parameters, and vaccine effectiveness, this research strengthens the scientific basis for risk-based surveillance, epidemic modelling, and contingency planning for future FMDV incursions in pasture-based livestock production systems, including Uruguay and other regions where extensive livestock production predominates.