Negative stool test? Very common. Give up on them that's my advice.
I'm not a big fan of herbs. Some ppl report here that herbs might help while others blame them for 'systemic escape' but by no means they can get rid of parasites on their own.
Harmful? No. You can never be sure if it's the one or not. Also anti- parasite drugs in general attack many classes of parasites.
Well... Unless you get it out alive (like I did by accident LOL! And even than... It comes down to risk assessment/matching your symptoms to their cycle with support of some medical evidence such as elevated specific white blood cells, etc. So to know for sure is very hard.
First get rid of them... It's not as easy as many ppl think. Reg reinfection... It's impossible to know what was and is the source of infection. Your body can still be infected by them again at any given time. Scary as hell.