Third stage larvae of Strongylus vulgaris migrate along the intima from intestinal vessels up through the mesentery to reside in the root of the cranial mesenteric artery.
There they develop into fourth stage larvae, which eventually go back to the gut.
While they are hanging out in the intima at the root of the mesentery, they are very irritating, and the vessel responds with an inflammatory reaction. The end result is a mesenteric arteritis with significant thrombosis.
Migration of strongyle larvae in the anterior mesenteric arteries of horses damages endothelial cells resulting in thrombosis.
It is common for pieces to break off and cause an infarct in the intestines.