Bracken and horsetail poisoning

Abstract

Bracken (Pteridium spp.) and horsetail (Equisetum spp.) are ancient plants that contain thiaminase. Chronic grazing by monogastric animals, such as horses, results in an induced deficiency of thiamine (vitamin B1). This manifests as ataxia, lack of co-ordination (‘bracken staggers’), loss of condition and stumbling. As the disease progresses, there is weakness, inability to stand, muscle tremors, coma and convulsions. In addition, haemolytic anaemia and haemoglobinuria may occur. Diagnosis of bracken or horsetail toxicosis is based on clinical signs and history of exposure. Post-mortem findings are generally non-specific. Treatment is supportive with prompt removal of the plants, and provision of good quality forage and nursing care. Supplementation with thiamine can be effective. Prevention of poisoning involves ensuring adequate forage is available, and preventing access to the plants. Clearing the bracken or horsetail can be difficult and in the case of bracken, specialist advice should be sought.