There are a couple of common names for this worm. One is the stomach worm. Another is the abomasal worm. Either way, they are the same worm and they need to be watched carefully. It’s a kind of parasitic worm that is often found in, surprise, the stomachs of horses and ruminant species - such as cattle and deer. The worm is small, thin, and hasn’t really been studied a lot.
However, what is known about these worms isn’t uplifting. They are known to leave small lesions on the animal’s skin, especially when they first appear in the animal. Often, the animal will also become uncomfortable and have some kind of diarrhea. The good news for the horses, however, is that this parasitic worm is only able to travel if they share pastures with some kind of ruminant animal.