I have a flock of 20 hens.6 weeks ago, a bunch started sneezing (2 weeks after introducing new birds). I noticed it was catching, so I put them all on Tet...zine (sorry forget the name). I did it for 5 days, and didn't eat the eggs for 10 days. After the 10 days, there was no more sneezing and everything looked good. A week or two later, I noticed more sneezing. I isolated the birds as I noticed them sneeze, and ended up with 5 in quarantine with the same medication as before. 10 days later I put them back in the flock since they were fine again. However on that day two more from the non-quarantined ones were sneezing. Now it is a week later, and there are probably 7-8 birds sneezing.
One hen has died. Two days ago. Got lethargic and just died 1.5 days later.
Is it possible that a bird can get the same sickness twice? Could the germ have survived the first round of anti-biotics, and is re-infecting the chickens? I was wondering if it could be an allergy, but the only thing I have changed, is adding pine mulch for flooring, and pine shavings for bedding in the nesting boxes. However, the sneezing first started before I added any pine stuff.