With the development of modern methods of clinical diagnosis and pathologic study, it has become universally recognized that arterial aneurysms, in the great majority of cases, particularly aneurysms involving the arch of the aorta, are due to syphilis. Other etiologic factors of clinical importance, although ranking far below syphilis, are bacterial or so-called mycotic infections of arteries, trauma (most frequently in connection with war wounds) and arteriosclerosis. Extremely rare causes are congenital defects of vessels, adhesions pulling the artery wall outward (traction aneurysms), vascular tumors and chemical erosion, such as may occur in the walls of gastric ulcer or carcinomas. In horses, a parasitic worm, the Strongylus armatus, has caused aneurysms involving most commonly the mesenteric artery.
Bacterial infections may invade the vessel wall either from the outside or from within. Those coming from without are usually due to the extension of infection to the vessel walls from neighboring or