cause cancers in digestive system among animals that eat these regularly. These ferns contains ptalquiloside, which is a well known carcinogen ( a substance that causes cancer).
However, ptalquiloside can be easily broken down by 1) soaking in water(change water frequently) because it's water soluble, and 2) high temperature. So wash them thoroughly and cook in boiling temp, they should be safe to eat. Don't eat too much. :))