Pteridium
a fern in the family Dennstadiaceae.
The fern is classified by some authorities as more than one species including: P. aquilinum, P. esculentum, P. revolutum, P. yarrabense. Called also bracken. It causes poisoning in several unique ways: (1) Ingestion by cattle over a short period causes depression of bone marrow activity, leading to pancytopenia evidenced principally as ecchymotic hemorrhages in mucosae and terminal septicemia. Severe diarrhea and dysentery may be terminal events. Ingestion over a long period causes proliferative lesions in and bleeding from the urinary bladder mucosa. See enzootic hematuria. Bright blindness of sheep also occurs when intake of bracken is prolonged. There is also a relationship between access to bracken and a higher than normal occurrence of intestinal carcinoma in ruminants. (2) A thiaminase in bracken causes a clinical syndrome of thiamin deficiency in horses. Signs are muscle tremor, incoordination, frequent falling and bradycardia and cardiac irregularity. 
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Pteridium aquilinum.By permission from Knottenbelt DC, Pascoe RR, Diseases and Disorders of the Horse, Saunders, 2003
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