How SAD! All I can think of, having only two cats, is to isolate the kitties...but you have more than two, so don't know how you would do that.

HUGS and prayers going out, I hope someone here will have ideas!

ETA: The only thing I can think of is to borrow (if you don't have one) an ionizing air cleaner. Also, a small room steamer (like you use when you have a cold) with some tea tree oil in it will really help - the TT oil really cleans up the air.