Bracken-fern poisoning in horses
Learn how to recognize bracken fern in pastures and the signs of bracken fern poisoning in horses.
ISSN 1198-712X, Published October 2009
Introduction
Bracken fern (Pteridium aquilinum) (Figure 1) is one of the most widespread species of vascular plants, exceeded in geographic range only by a few annual weeds
In horses, thiaminase splits the essential vitamin thiamine (B1) into pyrimidine and thiazole, which individually are inactive in the body
Consumption of bracken fern on pasture is usually limited by horses' preference and the abundance of other palatable plants. The majority of clinical cases are related to the feeding of poor-quality hay that contains high levels of the fern. Bracken fern becomes incorporated in the hay when it invades hayfields from adjacent bush lots and roadsides. Signs of thiamine deficiency occur when hay containing bracken fern at 10%-20%, or more, of the horse's dry-matter intake is fed for approximately 4 weeks
Clinical signs
Thiamine deficiency in monogastric animals and in the horse in particular is marked by loss of condition (a general scruffy physical appearance), weight loss (initially without a loss of appetite) and slightly uncoordinated movements
In ruminants, the acute hemorrhagic syndrome (loss of blood) is linked to the chemical ptaquiloside and chronic ingestion of bracken fern. The concentration of ptaquiloside in bracken fern is dependent on the geographic location of the plant
Poisonings in animals are uncommon since:
- it takes 1-3 months for the toxic effects to accumulate.
- there is variation in susceptibility of animal species.
- large quantities of dry matter must be consumed.
- toxicity depends on geographical location and time of year (since newly emerged fiddleheads are five times as toxic as mature fronds)
footnote 2 .
Acute poisonings are seen after periods of drought when grazing is scarce or when a field is ploughed and animals have access to the rhizomes
Treatment
Early identification of the symptoms is critical in the treatment of bracken-fern poisoning. If caught early, simply removing the source of the fern will lead to a full recovery. Treatment with thiamine is occasionally needed. Treatment regimes or methods vary, depending on the author. One recommendation is to administer thiamine (vitamin B1) at 500 mg-1 gm/day - on day 1 intravenously and then intramuscularly for several days
Identifying Bracken fern
Bracken fern, unlike other fern species, favours dry, well-drained, sandy and gravely soils, such as open woodlots, pastures and roadside environments. They are herbaceous, perennial plants that emerge from the ground as curled fiddleheads. The fiddlehead opens into a compound leaf or frond, which appears to have three major divisions due to the stem branching to expose three leaflets. Each leaflet is about 30–140 cm long, 30–100 cm broad and triangular in shape
Control
Due to the plant's rhizomatous rooting system and its ability to produce mass amounts of spores, control and spread of bracken fern can be difficult. Growth can be slowed over time by regular cutting of the mature plant or by deep ploughing
Ferns commonly found in Ontario
Bracken fern (Pteridium aquilinum)
The leaves are compound and each leaf branches into 3 broadly triangular leaflets
Ostrich fern (Matteuccia struthiopteris)
The leaves are ostrich-plume shaped (oblong and pointed at the tip) and about 1.5 m tall
Sensitive Fern (Onoclea sensibilis)
These ferns are typically found in moist areas. The leaves are about 50–70 cm tall, and the leaflets are much wider, with very shallow lobing along the edges, making a wavy appearance
Consumption of sensitive fern at 10%–20% of the hay content also produces symptoms similar to that of thiaminase-related poisoning seen with bracken fern and horsetail
Footnotes
- footnote[1] Back to paragraph Burrows GE, Tyrl RJ. Toxic Plants of North America. Ames, Iowa: Iowa State Press, 2001:415-422.
- footnote[2] Back to paragraph Knight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Jackson, Wyoming: Teton New Media, 2001:194-197, 222-224.
- footnote[3] Back to paragraph Radostits OM, Gay CC, Blood DC, Hinchcliff KW. Veterinary Medicine, 9th ed. Edinburgh: W.B. Saunders Company Ltd., 2000:1556-1558, 1659.
- footnote[4] Back to paragraph Bracken fern poisoning: Introduction. In: The Merck Veterinary Manual, 9th ed. Whitehouse Station, NJ: Merck & Co., Inc. and Merial Ltd., 2005. Accessed April 8, 2010.
- footnote[5] Back to paragraph Kingsbury JM. Poisonous Plants of the United States and Canada. Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1964:105-108.
- footnote[6] Back to paragraph Alex JF. Ontario Weeds, Publication 505. Toronto, Ontario: 1992:20.
- footnote[7] Back to paragraph Chambers B, Legasy K, Bentley CV. Forest Plants of Central Ontario. Edmonton, Alberta: Lone Pine Publishing, 1996:341, 359, 361.