The early pathogenesis of foot-and-mouth disease in pigs infected by contact: a quantitative time-course study using TaqMan RT-PCR

Foot-and-mouth disease (FMD) is a highly contagious, economically important virus disease of cloven-hoofed animals. The objective of the present study was to examine the early pathogenesis of FMD in pigs by a quantitative tim e-course study. Under experimental conditions, recipient pigs were infected by contact with donor pigs affected by FMD, Every 24 h from day 1 to day 4 after exposure, two recipient pigs were selected randomly, killed and necr opsied. A range of tissues were analysed by a quantitative TaqMan RT-PCR me thod and by titration of FMD virus on primary bovine thyroid cells. The tit res of virus determined by assay in cell culture and calculated from the qu antitative TaqMan data correlated strongly (r > 0.9), thereby establishing the validity of the TaqMan calculations. The data indicated that the replic ation of virus in the lungs contributes only in small part to airborne viru s excretion. Sites in the pharynx, trachea and nasal mucosa are probably mo re important in that regard. The sites of earliest virus infection and poss ibly replication in recipient pigs appeared to be in the pharynx (soft pala te, tonsil and floor of pharynx). The data indicated that FMD virus replica tion in pigs is rapid and that the majority of virus amplification occurs i n the skin. A model for the progression of infection is proposed, indicatin g initial spread from the pharyngeal region, through regional lymph nodes a nd via the blood to epithelial cells, resulting in several cycles of virus amplification and spread.