Abstract
The relationship of Foot-and-Mouth Disease virus antigen payload and number of dose of vaccine conferring protection against virus challenge in goats was studied. Goats vaccinated with oil adjuvant Foot-and-Mouth Disease vaccines containing different antigen payloads with or without booster resisted virulent challenge at 21 days post-vaccination or 7 days after booster respectively. However, localized sub-clinical infection was observed in two vaccinated goats on 35 days post-challenge. RNA could be detected from 31.8% of vaccinated goats (10(2.69)-10(4.99) viral RNA copies per cotton swab of nasal secretions) on day 35 post-challenge. Since no live virus could be isolated after 5 days post-challenge, the risk of these animals transmitting the disease was probably very low. The finding showed that oil adjuvant Foot-and-Mouth Disease vaccines containing antigen payload of 1.88 μg may prevent or reduce the local virus replication at the oropharynx and shedding of virus from nasal secretions and thereby reduce the amount of virus released into the environment subsequent to exposure to live virus. This study also showed that goats with poor sero conversion to vaccination can be infected without overt clinical signs and became carriers like sheep.