Protection against direct-contact challenge following emergency FMD vaccination of cattle and the effect on virus excretion from the oropharynx
The ability of emergency foot-and-mouth disease (FMD) vaccine to protect cattle from a heterologous direct-contact challenge and the effect on virus excretion from the oropharynx were examined. An oil adjuvant O1 Manisa FMD vaccine protected 20 cattle from clinical disease following 5 days of direct-contact exposure to five infected cattle at 21 days post vaccination. The donor cattle had been infected by tongue inoculation with a different FMD virus of the same serotype (O UKG 2001). Protection from clinical disease did not prevent localised sub-clinical infection at the oropharynx in most animals, although quantitative reverse transcriptase polymerase chain reaction (RT-PCR) showed that the level of virus replication shortly after direct-contact challenge was greatly reduced in vaccinated animals. Nevertheless, 45% of the vaccinated cattle became persistently infected with 103106 RNA copies per millilitre of oropharyngeal fluid at 28 days post challenge. However, since live virus could not be readily isolated, the risk of these animals transmitting disease was probably very low. The findings show that even after an extremely severe challenge, use of an emergency vaccine will prevent or reduce local virus replication and thereby dramatically reduce the amount of virus released into the environment in the all-important early post-exposure period. These data should help to model the dynamics of virus transmission in future outbreaks of disease where vaccination is considered.