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I am not a professional biologist but, as a person interested in wild food, I have had good reason to investigate and consider the possible dangers of wild food gathering. Anyone intending to pick wild food plants, such as watercress, from damp places should be aware of the liver fluke and its life cycle.
Of course the simplest advice is - never pick and eat any wild watercress or other water plant. However with a bit of knowledge you can safely gather and such wild food.
There are many different species of liver fluke world wide. They are members of a large group of parasitic flatworms which all have a very involved and interesting life-cycles. The ones that are potentially dangerous to wild-food gatherers in England have a life-cycle involving sheep or cattle, water snails and forage crops.
The adult flukes are parasitic in the livers or bile ducts of the hosts. The commonest fluke in UK seems to be the sheep liver fluke, Fasciola hepatica. Although commonly called sheep liver fluke it is in practise not fussy about its final host: sheep, cattle or humans will do. Presumably deer, pigs, wild boar and other herbivorous animals will also suffice.
The adult fluke lays its eggs which pass out in the hosts faeces and hatch into a larval form (miracidia) which, if the faeces are deposited in water, swim around and infect water snails (mainly lymnaea truncatula but probably other species as well). Having infected the snail, the larva develops until it is ready for the next stage of its life when it develops into multiple cercaria which leave the snail host and attach themselves to plants growing in the water such as grass or watercress, where they encyst.
The life cycle is completed when the grass (or other plant material) is eaten. The encysted cercariae come to life in the gut, migrate through the gut wall and travel to the liver where the adult fluke develops.
There is a pdf brochure from SAC (Scottish Agricultural College) on Treatment and Control of liver fluke which explains the Liver Fluke life cycle in greater detail.
This looks scary enough to prevent anyone picking wild watercress! But the danger is not at all high, especially if you take a little care. For the larval cercariae cannot climb! They encyst on plants near the water level. Grass is a monocotyledon: it grows from the base and as the grass grows the encysted cercariae will be lifted up to be eaten by the prospective host. But humans don't eat grass, so no problem there!
Watercress on the other hand is a dicotyledon. It grows from the tips, not the base, of the plant. So the encysted cercaria cannot be lifted up the plant but will remain at the water level where they first encysted. Therefore, as long as the water level has not fallen significantly since they encysted and the growth is young and vigorous (as it will be if the watercress is of the quality you will want for consumption), the fresh tips of the watercress should be quite safe to eat as they are well above the water level.
Patient UK has an interesting article on Fasciola hepatica which says "Water-grown vegetables should be washed with 6% vinegar or potassium permanganate for 5-10 minutes which kills the encysted metacercariae.". Or you can of course cook the vegetables. But with careful collecting, such precautions are surely unnecessary!
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