Ambrus's Leech-Pickers

I've just learned that Ambrus hires people to pick... leeches. There must be some purpose to it.

Ambrus set up a camp full of soldiers and civilians, right in the middle of a swamp, to collect leeches. Why? To what end?

All is clear now - Ambrus and the other vrakhiri need the leeches's spittle, which they use to stop the blood they collect from clotting. If I interrupt the process... My enemies will go hungry.