I had the same kinda thing going on with my Havoc for several months last year. He'd get the respiratory thing, I'd take him to the vet, they'd give him some antibiotics, and he'd get better. Within a week of being off of his meds he'd start sneezing again and the whole process would start over. This went on for most of the summer. 2 vets said the sneezing was a result of a respiratory infection, but a 3rd finally said the opposite: allergies caused irritation, irritation caused mucus, mucus led to infection. I finally put him on a slightly-longer than usual round of antibiotics to knock the infection out, then switched to allergy meds (as needed). I also got a tabletop air purifier since it seemed that he was allergic to something in the house. He started getting sick off and on in May or June and it finally went away for good around last November. I don't currently have him on any meds but the air purifier is still going and I haven't had any sinus issues at all so far this year.

to the first 2 vets it looked like an ordinary (but very persistant) upper respiratory infection. I used to work at a vet and I know that URIs are really contagious and I kept explaining that Toby never so much as sneezed once, but they insisted that was Havoc's problem and gave antibiotics, because they did seem to help a bit. Thank goodness I got tired of the same old routine and saw vet #3. Months of misery got resolved in a couple of weeks. They're closer to me and have better hours anyway!