The carcinogen, ptaquiloside, would not be destroyed by pasteurisation, according to one expert. It is well known for its ability to cause tumours by damage to DNA, the basic genetic material of all cells. Now a team of scientists in New Zea- land and Venezuela has discovered that almost 10 per cent of the chemical eaten by cows is passed on in milk, a fact the team calls a "cause for concern".
The team suggests that ptaquiloside in milk is "certainly likely" to be responsible for the level of stomach cancer found in farmers in Costa Rica and other Central American countries, where bracken is particularly dense.
Though cows in the UK do not normally eat bracken, they may do if there is a drought, if fields have been overgrazed, or if they wander into thickets. The National Farmers' Union said: "Cows don't like to eat bracken, and farmers don't encourage its growth. Even in winter cows would be fed from winter stocks." But Alan Heyworth of the Bracken Advisory Commission said: "If the animals are in a field which has some fronds, it's obviously going to happen that they'll eat some."
The new research, published today in Nature, investigated the effects of feeding bracken to cows and discovered that ptaquiloside is still excreted in milk almost four days after feeding stops. Pasteurisation would be unlikely to affect the chemical, said Mr Heyworth: "If it survives getting through the cow's digestive system and blood, I don't think heat treatment would have any effect."
In the UK, the plant covers a total area equivalent to the size of Yorkshire. It is poisonous to animals which eat it, and its spores can be carcinogenic when fed to mice and guinea pigs. Although efforts are being made to destroy it using pesticides, European Union rules prevent spraying near watercourses - which can also be polluted by the plant.