I remember years ago, just outside the village of Croxton Kerrial in Leicestershire, there was (possibly still is) a fresh water spring beside the main road. Water cress use to be in abundance, and it was delicious, always worth a break stop.
Watercress is a common source of the liver fluke parasite for humans. The fluke has a complex life cycle that involves water snails, and commercial growers take steps to ensure there's no snail contamination of their crop. The same can't always be said for watercress that grows in the wild, or that which is cultivated by amateur growers.
It's wild watercress that is the danger, as the water in which it grows may come from streams that are contaminated with sheep faeces. Commercial growers work to a strict code of practice which lays down strict rules about water sources. It's therefore safe to eat watercress that has been grown commercially.
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