I have never heard of denegard until I read this post. But I did have several birds that I lost last year to some kind of respiratory illness. Same symptoms as you listed. I assumed mine to be IB. I gave mine Tylan 50 for a week. I got it from my local co-op. I used a syringe and squeezed a .5cc in their beak until they swallowed it. Many people will probably tell me that giving antibiotics isn't good unless u have ur birds tested for chronic respiratory infection, or IB, or New Castles, etc...but I am unable to do that where I live. It is cost prohibitive for us. I have 4 birds with symptoms now--and I vaccinated them for NC/IB last week. I'm wondering if they have symptoms because they are building up the anti-bodies to fight the infection...you know, similar to like getting the flu shot. Every year I get that shot and within a few days I have a sore throat, runny nose and generally feel yucky...
The Tylan was around $12 at my co-op, not sure how expensive denegard is.