Respiratory Infection or Something Else??!!

[mommachix]

We started our small backyard flock last year with two red sex links who have been wonderfully healthy and laying consistently until two weeks ago. We recently acquired five new pullets (four months old) that we have kept properly quarantined this entire time. Two weeks ago, one of my sex links started gasping for air, stretching her neck, and laying down quite a bit. She was still laying, eating, and drinking on the first day we noticed this...no discharge from her nostrils, only other symptom was a darker wattle and Comb than usual. We took her to an avian vet who diagnosed her with a lower respiratory infection (I asked about gapeworm and her crop was fine). She was out on doxycycline and ivermectin (just in case it happened to be gapeworm). We also quarantined her in a quiet cage away from her sister immediately. She ended up passing the next day (two days of symptoms). So now, two and half weeks later, her sister has similar symptoms. She as been kept separate from the other birds because of the illness. I already wormed her with the ivermectin and now I have given her a dose of the antibiotics...I'm quite worried because I wouldn't think this thing would linger without contact with other birds after over two weeks unless it's being spread through some other method. We do have other animals that wander our yard at night...definitely possum and raccoons maybe rats, so I am worried it is being transmitted by something else. And now I am concerned if we lose her that our new chickens will be in danger if they are put in the yard (if it's infected with whatever this is). Thoughts?