There, but for the grace of God, go any of us Genghis. Had I been in your situation at the time, I would undoubtedly have acted the same way. I'd have trusted the bloke to do the right thing, especially as he'd agreed. No reason at the time to suspect that he wouldn't keep good faith.
You did as much as you could do in the circumstances, so don't flog yourself over the outcome. After all, as you say, it's within the realm of possibility that the FMD outbreak could have occured there anyway, by other means. I hear what you say about that scenario not being very likely but, still, there really is nothing more that you could've done, given the circumstances - and the element of trust.
Hopefully, we all live long enough to learn from our mistakes.