Tiamat
I've seen the future...
The idea of infecting birds with disease to spread around is as old as biological warfare. Studies and theories were made on it back in WW1 and Interwar not to mention WW2 and the Japanese Gamer House in Manchuria known as Unit 731. But no one has ever put it in action because it's not only really hard to control it also invites complete fucking hell onto you as you are pretty much giving anyone who gets a single citizen sick a casus beli to push your shit in. Not to mention the treaties and whatnot.
Oh, it is indeed an old idea that's been well researched for millenia, and with an obvious list of problems as you mentioned.
Personally, I would use infected furries on the local populace. Both cuddly, and terrifying all at the same time.