As a practicing veterinarian, I would have a couple of questions. One, how did they diagnose lungworm? Was it clinical suspicion because of the xray, did they find it with a fecal sample, or did they get a sample from her lungs by sedating her and infusing saline? If they got it with an xray, their diagnosis may be inaccurate, as there are other things that can cause that type of pattern in the lungs, or it may have started as worms, and has turned into a chronic inflammation. If they got it with a fecal, she may have 2 types of lungworm, onc that is evident in the stool, but she may have another in the lungs that is not responsive to the medication they've used. If they got it from a tracheal wash, then either they may have identified the wrong worm, she could have 2 types of worms, or there is another underlying problem preventing her from getting rid of the worms, such as an immune problem. The other possibility, is that she may have had lungworm, it was treated, but it set up for this chronic pulmonary problem, causing the appearance of the xrays to be worse. In that case, steroids may be recommended. If you're fairly sure they've been very thorough, and you've had the transtracheal wash (with sedation) done at least twice, then I would think about taking her to an internal medicine specialist. If they still have a sample of the lung wash, then maybe they could sent it to Cornell or UPenn for a definitive identification. There can be different lungworms and different meds to treat them.
While I absolutely have no idea what is really going on with her since I haven't seen her or treated her, I would be concerned that this there may be another underlying problem going on with her respiratory tract, whether it is what made her more susceptible to lungworm or whether a chronic problem has been set up by the lungworm. At this point, you've been to 3 vets, I would definitely take her to a specialist at one of the larger referral places in the state. Good luck!
