Strongylus vulgaris (large strongyles), commonly known as the blood worm, is a common horse parasite in the phylum Nematoda. It looks like a long worm with a large biting mouth, like the picture shown to the left. They are usually reddish in color because of all the blood they take from the equine host. 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 environments.