Let's not over exagerate the risk here. There is exactly a 0% chance of catching West Nile Virus from an infected bird's feathers. The only way to contract West Nile Virus is to be bitten by a mosquito that has recently dined on a blood meal from an infected bird. Even if bitten by such a mosquito, the chances of becoming ill are rare and are typically limited to the very old and those with compromised immune systems.
The only way to contract Avian Influenza (Bird Flu) is to come in contact with the mucous secretions or fecal matter of a live bird that is infected. The influenza virus can only live outside the host for a matter of hours, at best, so handling shed feathers should present almost no risk.
Now getting bird mites, that's a real possibility.... nasty disgusting little critters...
mAngler