Pteridium aquilinum (king fern, eagle fern); Pteris aquilina. Pteridium ferns belongs to the family POLYPODIACEA. Large, dark green, pinnate leaves grow up from a rhizome. Fixed spore cases form a brown line underside of the pinnules of the leaves, ripening to release thousands of small round spores. Common in woods and on heaths and moorlands mainly on light acid soils and often the dominant flora in these areas.

Animals most affected
Cattle, (sheep, horses).

Etiology
Ingestion of the green plant, during periods of food shortage, or when dried plant material is mixed in with hay or straw. Ingestion over a prolonged period (15 - 30 days) causes toxicity. Poisoning is frequent in areas where the fern grows naturally. Bracken poisoning is termed pteridism.

Toxicity
Contains:

• a thiaminase which hydrolyses vitamin B1 (depletion of vitamin B1 causes a clinical syndrome of thiamin deficiency which occurs in non-ruminants, particularly in horses);
• unidentified substances which depress the medulla, and are responsible for specific problems in cattle (e.g. Bright blindness).

Drying the plant does not reduce its toxicity. Toxic doses not fully known, however large quantities of the plant need to be eaten continuously to cause toxicity. (The effects upon the bone marrow appear to be cumulative, though they have been reported after ingestion of bracken by ruminants over a short period.)

Clinical features
In cattle
Important note: The latent period between ingestion of bracken and the appearance of symptoms is generally long, from several weeks to several months:

• thus the clinical signs may appear after buying and transporting animals to regions where ferns are uncommon;
• the acute stage is preceded by a state of general denbilitation, occasionally with salivation and nasal discharge.

The clinical signs of bracken poisoning have a characteristic pattern of presentation:

• depression, weakness, anorexia;
• haemorrhagic syndrome (variable);
• with haemorrhagic enteritis, haematuria, cutaneous bleeding ('sweating blood').

These latter symptoms are often absent or minimal in particular breeds of cattle (e.g. Charolais).

• hyperthermia (41o - 42o C) may be concurrent with the above syndrome or occur at the end-stage of poisoning;
• death within 4 - 10 days.

Haematological effects:

• severe leucopaenia with agranulocytosis and thrombopaenia, pancytopaenia;
• depression of bone marrow function exhibited by granulocytosis and thrombocytosis.

In horses
Disturbances of nerve function associated with thiamin (vitamin B1) deficiency:

• muscular weakness and tremor, motor incoordination, irritability, asthenia, weakness and collapse;
• death preceded by tonic-clonic spasms and opisthotonos.

Lesions

In cattle
Lesions indicative of a general haemorrhagic syndrome:

• haemorrhages in muscles and organs; petechiae evident in the mucous and serous membranes, on the surfaces of the heart, kidneys, etc.;
• ulcers in the abomasum and intestine.

In horses
Non-specific.

Treatment

In cattle
No treatment found to be fully effective:

• blood transfusion (the most efficacious)

• iv toluidine blue (1 g/l serum), at a rate of 1-2 1/24 hours. Note: to be used with caution;
• batyl alcohol (2 g), plus Tween 80 (5 g) and 1% NaCl (100 ml) given by slow iv infusion, 20-50 ml/day.
  

In addition:

• antibiotics;
• antihistamines.

In horses
Antidote: thiamin (vitamin B1), 100 mg/day im.