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Hi - I'm Dr. Tim and I'm a licensed veterinarian with over 25 years of experience with dogs and cats. I'd be happy to help you out. It certainly sounds like the signs some dogs get when their heartworm disease progresses. Especially as the worms get larger or start to die off, it can cause damage to the lungs which can then produce coughing, gagging, difficulty breathing.
Yes, if there is enough lung damage to produce fluid in her lungs, it could be coming out through her mouth. Is she coughing?
That is certainly possible. I would consider asking about changing to a different antibiotics (or adding a second antibiotic) and possibly increasing the dose on the prednisone. Sometimes dogs go through a bad period when one or two of the worms die off and if she can be made comfortable to survive through that, she still has a chance.
That is really hard for me to tell without seeing her bloodwork and her chest x-rays -- those are the things that are needed to give a prognosis. At least your vet has held out some hope so it is worth asking about what to do to make her feel better in the short term while you are thinking over your options, I would say.
The best I can offer is that some dogs aren't strong enough to survive the treatment and the best we can do is to keep them comfortable as I described above. Ultimately if the antibiotics and prednisone can't keep the signs to a minimum, you might be left making that final decision to put her to sleep, unfortunately.
I hope this helps and is a satisfactory answer. Please remember to rate my response since that is the only way I am compensated for my answers. Good luck with your little one and please let me know if you have any other questions and I'll do my best for you. Take care.