I stand corrected

Liver fluke larvae can penetrate the skin of a human host.

Wikipedia

I had  been ill advised to think that they entered  through mouth, ear and nose membrane, including anal and sexual organs and open cuts.
read about the leaf shaped flatworm Trematode

The larvae attaches to fish skin and plants such as watercress.
Snails and slugs are the main carriers of liver fluke and transfer them to vegetation.
Liver flukes are also common in a exposed wild and domestic animals.
Nong Han (Lake) is known to be infested with the parasite.  
It stands to reason that surrounding waterways are similarly invested.