In addition to Mycobacterium tuberculosis and Treponema pallidum, other organisms such as Staphylococcus aureus, streptococcal species, Corynebacterium diptheriae, Candida species, and Aspergillus species have been implicated as causes of mycotic PAA.1
Mycotic PAAs usually form as a result of bloodstream infection and endovascular seeding. This mechanism is supported by the fact that mycotic PAA have occurred in conjunction with IV drug abuse and endocarditis in which endovascular seeding is the only source of vessel infection. In cases of pneumonia, some authors have documented angiocentric infiltrates without evidence for airway inflammation, suggesting that endovascular seeding is again the primary event. In cases in which an aneurysm develops within the pneumonic lung, it is impossible to rule out a pathogenesis similar to that associated with Rasmussen aneurysm in which mycobacteria invade the outer vessel wall and spread medially, leaving it vulnerable to rupture. In our case, the patient had no history of syphilis or tuberculosis and, although there was no history of IV drug abuse or evidence for endocarditis, the PAA was not within an area of pneumonia, suggesting that it stemmed from vascular seeding.