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| Brackenfern (Pteridium aquilinum) |
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ALSO KNOWN AS:
DESCRIPTION:
DANGEROUS PARTS OF PLANT:
SIGNS: Horses: The toxin in brackenfern is thiaminase, an enzyme that destroys thiamine (vitamin B1). The horse then essentially suffers from a vitamin deficiency of thiamine, which causes myelin degeneration of peripheral nerves ( a loss of the fatty insulation layer to nerves that primarily control muscles). Poisoning can occur at any time of year, but is more likely in the late summer when other forages are scarce and the level of thiaminase is at its peak. Bracken is not considered palatable, but horses will eat it if no other forage is available, or they will consume it in hay or bedding, where it remains toxic. Some horses are believed to acquire a taste for it, and these horses will consume it even if other forages are available. Horses need to consume bracken for one to two months prior to manifesting clinical signs. After this time horses may then be fed bracken-free forage and yet still develop clinical signs within 2 to 3 weeks. The first signs in horses is weight loss after a few days on bracken. Later, weakness and gait abnormalities are present, which progress to staggering, hence "bracken staggers". Affected horses may stand with their legs widely placed and their back arched. Muscle tremors and weakness is apparent when the horses are forced to move. Early in the course of the syndrome, a slow heart rate and abnormalities of the heart rhythm may be noted. Near the end of the clinical course, the heart rate and temperature rise, and the animals cannot get up and may have spasms and an upward arching of the head and neck. The syndrome runs its course, with death occurring within 2 to 10 days of the onset of signs, but it can be treated. Swine would show signs similar to those in horses. Ruminants, especially cattle: Thiaminase does not adversely affect ruminants since the ruminal bacteria degrade the enzyme. However, other toxins in bracken affect ruminants, most notably ptaquiloside, a lactone toxin that affects the bone marrow. The toxin is present in all parts of the plant, but is concentrated in the rhizomes, and is toxic in fresh as well as dried plants. Consumption of bracken results in the depression of bone marrow (and thus red and white blood cell and platelet production), and the plant has a direct or indirect anti-coagulant property. Cattle show signs after grazing bracken for 1 to 2 months, although death may occur within this time frame as well. Affected cattle have an increased temperature, weight loss, and exhibit increased bruising and bleeding. From the excessive bleeding, cattle are anemic, and can die within a week of showing signs. Young cattle may develop swelling in the larynx and have difficulty breathing. Sheep may be poisoned in a similar manner, but are apparently more reluctant to consume bracken. The plant is also reported to contain carcinogenic substances, but instances of cancer in animals resulting from bracken fern ingestion is not well reported.
TOXICITY RATING:
ANIMALS AFFECTED:
CLASS OF SIGNS:
FIRST AID: Horses: If horses are observed eating bracken, immediately remove them from the pasture, or in some way prevent access to the plant. Hay with bracken in it should never be fed. If large amounts were consumed, and especially if clinical signs are present, call a veterinarian immediately. The antidote is daily injections of thiamine for up to two weeks. Do not wait until the animal cannot rise, by then it may be too late. Provide similar first aid to swine.
SAFETY IN PREPARED FEEDS:
PREVENTION: |
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