Evidence that high potency foot-and-mouth disease vaccine inhibits local virus replication and prevents the "carrier" state in sheep.

The ability of a single administration of a high, medium and low potency foot-and-mouth disease (FMD) vaccine to decrease or inhibit local virus replication and excretion in the oropharynx of sheep following aerosol challenge with homologous live virus 14 days later was examined. Unvaccinated sheep showed signs of clinical FMD, whereas all of the vaccinated sheep, regardless of antigen payload, were protected against clinical disease and development of viraemia. Virological and serological results confirmed that there had been no local virus replication in the oropharynx of sheep from the high potency vaccine group in contrast to moderate or substantial virus replication in the oropharynx of the low potency vaccinated or unvaccinated sheep respectively. The vaccines showed no evidence of promoting a local mucosal antibody response at the time of virus challenge, but were capable of stimulating a systemic gamma interferon response, the level of which was related to the antigen payload. This suggests that the systemic gamma interferon response could be a useful indicator of the ability of a FMD vaccine to elicit a sterile immunity and indicates that further work is warranted to investigate the role of systemic gamma interferon in this immunity. This is the first experiment to clearly show that high potency, high payload, FMD vaccines are capable of inhibiting local virus replication and consequently persistence and the carrier state in this target species.