Hi Don
I don`t think there is much to worry about but Im no expert, There has been a bit in the news lately about rats on farms and the health risks to farm workers but rats are always there anyway. I suppose the sheer numbers increase the chance of contracting something nasty. As bluebell says they are very tricky to deal with, we catch a few in Fenn traps but I get the feeling its just the tip of the iceberg.
Cheers Pete