Strongylus vulgaris, commonly known as the blood worm, is a common horse parasite in the phylum Nematoda. It generally lives in the large intestine

Large intestine

The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...

 and migrates in the mesenteric arteries

Mesenteric arteries

Mesenteric arteries can refer to:* Superior mesenteric artery* Inferior mesenteric artery...

. This nematode is considered to be one of the "most pathogenic" of the large strongyles subphylum and is distributed worldwide, wherever there are grassland and temperate environment.