Pleural effusion is an abnormal buildup of fluid up in the pleural cavity, the thin fluid-filled space that lies between the lungs and the chest wall. It is a symptom, with an underlying disorder causing this fluid to build up. The buildup of excess fluid leads to difficulty breathing, due to the inability of the lungs to fully expand. Causes include congestive heart failure, liver failure, fluid overload, blood clotting disorders (usually due to ingestion of rat poison), lung lobe torsion, pulmonary edema, and diaphragmatic hernia.
Without examining Luna it is difficult to weigh in on her conditions, the rapid breathing is concerning and is possibly due to anaemia if her gums are pale; blood counts would be a logical test to run if the gums are pale so that we would know if there really is a decrease in red blood cells as well as whether or not there is an increase in white blood cells or other tell tale signs. I understand that you’ve spent a lot already, but without examining Luna I cannot say for certain what the cause is, I would recommend visiting another clinic or a charity clinic for a better look at Luna’s symptoms. Regards Dr Callum Turner DVM
Rapid breathing (tachypnea) is a respiratory disorder characterised by abnormal breathing that is rapid and shallow. It is caused by a reduced level of oxygen, mechanical disorders (where the lungs aren’t able to expand as they should, usually due to a build-up of fluid in or around the lungs), and physiological disorders in which the cat’s respiratory centre in the brain is over stimulated.