Let me preface this by saying I am no veterinarian, just a guy that reads a lot. I defer to the judgement of professionals with direct experience evaluating and treating animals with conditions such as you describe, with which I have no experience whatsoever.
That said, it sounds like what you are describing could be consistent with Large Lung Worm. This occurs most frequently in fawns, usually fawns that are malnourished and may be host to other parasitic diseases as well. It usually gets picked up from vegetation that the deer consumes, so it could be that after weaning the fawn picked this up off of contaminated browse. The frothy mucus in airways is typical of this condition. If the fawn is particularly lethargic or appears to be having difficulty breathing, then this would also point to the condition. If you can look down the animal's throat, then you may be able to see some of the worms (they would be thin and roughly an inch long). From what I have read, this can spread among deer in close proximity, but healthy deer are usually asymptomatic (not sick).
My instinct would be to quarantine this animal and keep it from contact with the rest of the stock pending professional evaluation. If this is lung worm, then it is entirely possible that other members of the herd are already carrying this, but not ill, so I would consider getting a vet on this sooner than later to make sure that it isn't a bigger problem concerning more animals. Also, if you have other livestock, be careful. Most ruminants (sheep, goats, cattle, etc.) are potential carriers for this that could also be affected.
Also, sounds like if you can confirm its worms Ivomec is a pretty common solution. When you dewormed did you repeat ~10-12 days later?
