They ending up mostly in the mesenteric artery as it branches from the aorta. The worm's irritating presence makes the arterial wall thicken. The higher blood pressure may cause the wall to balloon out to form an aneurysm.

Adult large strongyles attach to the mucosa of the cecum and large colon and suck blood, causing inflammation and bleeding ulcers in the intestinal lining.

The large strongyle, 1/ S. vulgaris, causes severe damage to the anterior mesenteric artery and its branches, resulting in aneurysms, emboli and thrombi.