There are only certain parasites that invade the body tissue. The ELISA test will knock out some of them. Then if you haven't eaten undercooked seafood it would knock out more and narrow it down. There is Lagochilascaris, but you would be noticing alot of larval migration, probably painful, it sounds like yours are staying in place. If I were you, I would have them biopsied, and looked at by a good lab then they can tell you exactly what it is.
Regarding DEC, it really depends on what you have, because different parasites would require different dosages and length of treatment. For example, Lagochilascaris minor is often dosed DEC 100mg 3x Daily for 6 months. Whereas filarial parasites are usually knocked out it with one dose. Nothing like getting an exact diagnosis.
I would add that a capsule study, is highly effective diagnostic, it would determine if you have any number of parasites that would stay in the intestines, it's much more effective then an upper endoscopy.