The radiographic lesions coupled with Max’s age make you most suspicious of a neoplastic process. The diffuse nature of the lesions makes a transthoracic aspirate a viable diagnostic step. Infectious causes of pneumonia are still a possibility as well, and pulmonary fine-needle aspiration is a relatively safe procedure that may provide a quick diagnosis. Parasitic infections may be a consideration, but you think this is less likely given Max’s history and environment.
You discuss obtaining an aspirate with the owners, noting the risk of hemorrhage or pneumothorax. But with a 1.5-in, 25-ga needle, the risk should be minimal. The owners agree to the procedure as well as baseline laboratory tests to look for any other systemic problems (you draw a little extra blood in case you decide to add other tests later). With Max lightly sedated and by using the radiographs as a guide, you aseptically prepare an area likely to provide the best access to lung lesions and perform the aspirate.
Max does well and is hospitalized overnight to be monitored closely for worsening respiratory signs. In case Max has a protozoal infection such as toxoplasmosis, you start clindamycin at 25 mg/kg given orally divided b.i.d.
The following day you receive the results of his laboratory tests:
Serum chemistry profile