Pteridium aquilinum (L.) Kuhn, commonly known as bracken-fern, is a cosmopolitan plant species absent only from polar and desert regions. Worldwide, Pteridium aquilinum has been rapidly spreading due to the neglect of agricultural fields and the growing wave of forest fires. This plant has several toxic components such as thiaminases, brains, active glucosides, beta-glucopyranosides and ptaquloside in its constitution that can be lethal to animals. The animals do not normally choose it as food, except in situations of food shortage, in periods of drought, when the ferns are still green or when the animals graze in places with large amounts of the plant in question. In order to revisit the major syndromes caused be P. aquilinum major toxins, the authors present several cases of acute and chronic ruminant poisoning associated with ingesting P. aquilinum in Portugal in sheep and cattle. The primarily associated syndromes observed in animals that consume this plant were thiamine deficiency, blindness in sheep, acute or subacute poisoning with bone marrow depression, and consequently, leucopenia and bladder tumours in cattle. Given the economic impact of the diseases caused by the toxins of this plant, and possible public health implications, it is imperative to minimise the syndromes in animals and establish measures for their prevention.