Bracken Fern Toxicity: Symptoms, Diagnosis, and Treatment

Bracken is one of the most widespread ferns in the world, and it poisons grazing animals that eat enough of it, usually over weeks or months rather than in a single meal. What the poisoning looks like, and whether it can be cured, depends heavily on which animal ate it. In horses and pigs it causes a fixable vitamin problem. In cattle and sheep it causes damage that no treatment reverses, so for those owners, keeping animals off bracken matters far more than any treatment.

Two Toxins That Make Different Animals Sick

Bracken contains an enzyme that breaks down thiamine, or vitamin B1, so the body can no longer use it.1Proceedings of the Royal Society of Edinburgh, Section B: Biological Sciences. Acute bracken poisoning in homogastric and ruminant animals The enzyme stays active in dried plant material, steadily destroying thiamine wherever it meets it.2PubMed Central. Health and Environmental Hazards of the Toxic Pteridium aquilinum Kuhn Bracken Fern Heat from steam inactivates it.3Proceedings of the Royal Society of Edinburgh, Section B: Biological Sciences. Acute bracken poisoning in homogastric and ruminant animals Animals with one simple stomach, like horses and pigs, end up short of vitamin B1. Cattle and sheep are largely shielded, because the microbes in their stomachs make their own B1.

Bracken also contains ptaquiloside, a compound that turns into a highly reactive form inside the body.4PubMed. Bracken Fern Carcinogen, Ptaquiloside, Forms a Guanine O6-Adduct in DNA That reactive form sticks to DNA and can break it,5Journal of the Chemical Society, Chemical Communications. Modification and cleavage of DNA by ptaquiloside. A new potent carcinogen isolated from bracken fern which can cause the gene changes that lead to cancer.6PubMed. Bracken Fern Carcinogen, Ptaquiloside, Forms a Guanine O6-Adduct in DNA Ptaquiloside is found in every part of the plant, but it is most concentrated in the roots, leaves and young unfolding fronds.7Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology Levels vary a lot by season and location, so no amount of bracken can be called safe.

Hay and young growth are the hidden sources. Bracken baled into hay still carries its toxins, and animals may eat it there when they would avoid it fresh in the field. Young spring fronds are tempting and among the most toxic parts of the plant. Bare or overgrazed pastures push animals to eat bracken they would normally skip.

Bracken Staggers in Horses

In horses the illness is called bracken staggers, after the unsteady walk it causes.8Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals It usually takes a lot of fern over a long time, such as hay that is 20–25% bracken eaten for 3 months or more.9Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals

The signs build in this order:

Later the horse may stumble or sway when turned in a tight circle, or tremble when asked to walk on after standing. Some become reluctant to lie down, or struggle to get up once they have.

Pigs eating bracken roots can develop a similar vitamin B1 shortage. Their signs may centre on weakness and heart problems rather than staggering, and the same vitamin treatment approach applies.

Bone Marrow Poisoning in Cattle

When cattle eat large amounts of bracken over a short period, the toxin wipes out the bone marrow’s blood-making cells.16Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology Platelets, which help blood clot, drop first, and that causes bleeding.17Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology White blood cells that fight infection fall next.18Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology Anemia comes last, because red cells live longer in the blood.19Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology

Signs can appear up to eight weeks after cattle stop eating bracken.25The Cattle Site. Bracken Poisoning In Cattle Write down where and when the animals grazed bracken so you can tell the vet, and keep watching the rest of the herd that shared the same pasture.

Bleeding can be subtle at first. Look for blood in the dung or from the nose, small red spots on the gums or inside the eyelids, and swellings or prolonged bleeding after insect bites or injections.

Long-Term Effects, Bladder Cancer and Bright Blindness

Cattle on bracken-infested land for long periods often develop enzootic hematuria, a chronic condition of bloody urine.26PubMed. Association of bovine papillomavirus type 2 and bracken fern with bladder cancer in cattle Blood in the urine can start as early as about 2 months after regular bracken eating begins.27PubMed. Naturally occurring and braken-fern-induced bovine urinary bladder tumors. Clinical and morphological characteristics. Benign bladder growths can appear after about a year, and invasive cancers after about 2.6 years.28PubMed. Naturally occurring and braken-fern-induced bovine urinary bladder tumors. Clinical and morphological characteristics. In long-term feeding, two in three cows developed bladder tumors within about 5 years.29PubMed. Naturally occurring and braken-fern-induced bovine urinary bladder tumors. Clinical and morphological characteristics. A common cattle wart virus is often found in these tumors and seems to be switched on by bracken’s toxins.30PubMed. Association of bovine papillomavirus type 2 and bracken fern with bladder cancer in cattle

In a herd, chronic bloody urine shows up as red or pink urine, sometimes with clots, in older cattle. Gradual weight loss and pale gums come from steady blood loss. It is usually seen in animals that have been on bracken land for years, and several affected animals in one herd point to the pasture.

Sheep grazing bracken can develop bright blindness, a gradual, permanent breakdown of the retina.31PubMed. Reproduction of progressive retinal degeneration (bright blindness) in sheep by administration of ptaquiloside contained in bracken In lambs fed bracken, the first signs showed after about 4 months.32PubMed. Reproduction of progressive retinal degeneration (bright blindness) in sheep by administration of ptaquiloside contained in bracken Affected sheep often stare with unusually shiny pupils and bump into things.

Affected sheep may walk with heads held high and step carefully. The damage cannot be reversed, so they do best in safe, familiar fields.

How Vets Diagnose Bracken Poisoning

Blood tests in poisoned cattle show low red cells, white cells and platelets.33USDA ARS. Western Bracken Fern Urine tests often find hidden blood and protein.34PubMed. Clinico-pathological findings in cattle exposed to chronic bracken fern toxicity An unusual rise in one type of white cell can appear early, while other results are still normal.35PubMed. Clinico-pathological findings in cattle exposed to chronic bracken fern toxicity So a blood test on herdmates may catch the disease before signs show.

For a vet, the grazing history matters as much as the tests. Bring where the animal grazed and for how long, a sample of the hay or a photo of the plants in the field, when signs began and whether others are affected, and any recent change of feed or pasture.

In horses, other illnesses can look like bracken staggers, so a vet should rule them out.36Mad Barn USA. Bracken Fern Toxicosis (Bracken Staggers) in Horses: Signs, Risks & Prevention They include:

A horse’s response to treatment also helps. Vets may give thiamine and watch for improvement. Rapid recovery points to a vitamin B1 problem such as bracken or horsetail poisoning, while no improvement pushes the vet toward other causes.

Treatment and Chances of Recovery

Giving thiamine is the main treatment for bracken staggers.41Mad Barn USA. Bracken Fern Toxicosis (Bracken Staggers) in Horses: Signs, Risks & Prevention The first step is removing the bracken source entirely. One common approach is 0.5 to 1 gram a day, first into a vein and then into muscle for several days.42ontario.ca. Bracken-fern poisoning in horses Supplements may continue for days or weeks depending on progress.43Mad Barn USA. Bracken Fern Toxicosis (Bracken Staggers) in Horses: Signs, Risks & Prevention

Improvement often begins within a day or two of the injections, and appetite usually returns before full coordination. The hay must be checked or replaced so the problem doesn’t return. Severe cases with seizures have a poorer outlook.

Apart from the vitamin B1 form, bracken poisoning is essentially untreatable.44Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology Antibiotics can help prevent the infections that often kill cattle.45The Cattle Site. Bracken Poisoning In Cattle Blood or platelet transfusions can be tried but need 2–4 litres per cow and often don’t help.46Merck Veterinary Manual. Bracken Fern and Other Fern Poisoning in Animals – Toxicology Bladder tumors and bright blindness don’t reverse. Animals with heavy bleeding rarely recover even with care, so ask the vet about the outlook before paying for transfusions.

Preventing Poisoning and Controlling Bracken

Fence animals out of bracken-heavy areas, especially in spring when young fronds appear. Inspect hay for fern before buying and feeding it. Provide enough good forage that animals aren’t driven to eat bracken.

The best time to treat bracken is summer, just after new fronds fully open, when the roots’ energy stores are lowest.47Zbornik Veleučilišta u Rijeci. Botanical characteristics, toxicity and control of bracken fern (pteridium aquilinum (l.) kuhn) Two or more treatments a year, combining cutting and herbicide, work better than one.48Zbornik Veleučilišta u Rijeci. Botanical characteristics, toxicity and control of bracken fern (pteridium aquilinum (l.) kuhn) Wear a mask when cutting dry bracken to avoid breathing its dust.49PubMed. Land management of bracken needs to account for bracken carcinogens–a case study from Britain Avoid leaving cut bracken piles where runoff can reach wells or streams.50PubMed. Land management of bracken needs to account for bracken carcinogens–a case study from Britain

Expect control to take several years of repeated work. Reseed cleared ground so grass outcompetes returning fern, and keep animals off treated areas until regrowth is dealt with.

Is Bracken Dangerous to People

In Japan and Brazil, where people eat young bracken shoots, eating them has been linked to cancers of the throat and stomach.51PubMed. Human carcinogenesis and bracken fern: a review of the evidence Living long-term near large bracken stands has been tied to higher stomach cancer deaths.52PubMed. Human carcinogenesis and bracken fern: a review of the evidence Soaking in changed water and boiling lowers the toxin.53PubMed Central. Risk Assessment and Risk Reduction of Ptaquiloside in Bracken Fern

Not every coiled fern shoot is bracken. Ostrich fern fiddleheads are the type usually sold for eating. Bracken shoots have fuzzy stems and branch into three parts. When you are unsure of the identity, don’t eat it.

The toxin can also pass into the milk of cows eating large amounts of fresh bracken.54Le Lait. Passage of the bracken fern carcinogen ptaquiloside into bovine milk It can wash from bracken into drinking water as well.55PubMed. Land management of bracken needs to account for bracken carcinogens–a case study from Britain It is best to avoid raw milk from cows grazing heavy bracken.