Thistles are flowering plants, often recognized by their sharp spines on leaves and stems. They belong mostly to the Asteraceae family, which includes daisies and ragweed. While often viewed as common weeds, the term “thistle” encompasses many species whose impact can differ significantly. This article clarifies common questions surrounding thistle toxicity to humans and animals.
Thistle Toxicity to Humans
Most common thistle species are generally not considered chemically toxic to humans if ingested. The primary concern is physical irritation from their sharp spines. Contact can lead to skin irritation, punctures, or discomfort. If parts of the plant are ingested, the spiny nature can cause mild digestive upset or irritation in the mouth and throat.
Serious chemical poisoning from common thistle ingestion is rare. Some sources suggest that thistles within the Cirsium and Carduus genera are edible, though processing is required due to their spiny nature. While certain species like Acanthus gummifera have been linked to severe toxicity, these instances are not typical for thistles commonly encountered. It is important to distinguish between mechanical irritation from spines and actual chemical toxicity.
Thistle Toxicity to Animals
Thistles can affect various animals, including pets and livestock, primarily due to their physical structure rather than widespread chemical toxicity. The sharp spines can cause physical harm, leading to mouth sores, digestive tract irritation, or even impaction, especially if large quantities are consumed. For most thistle species, like Globe Thistle (Echinops spp.), the plant is considered non-toxic to dogs, cats, and horses, though large quantities might still cause gastrointestinal upset.
However, specific thistle species are chemically toxic to certain animals. Yellow Star Thistle (Centaurea solstitialis), also known as Barnaby’s Thistle, is particularly problematic for horses. Ingestion of significant amounts can lead to a neurological condition known as “chewing disease,” which impairs a horse’s ability to eat or swallow, often resulting in severe malnutrition and requiring euthanasia due to irreversible brain damage. This specific toxicity to horses does not extend to cattle or sheep, which can consume the plant without apparent signs of toxicity. Creeping thistle has also been noted to contain nitrates, which can be toxic to dogs, cats, horses, and cattle if ingested in large amounts.
Beyond Toxicity: Culinary and Medicinal Uses
Beyond their reputation for prickliness and potential for physical irritation, some thistle species offer culinary and medicinal benefits. Various thistles are edible, with parts like young stems, leaves, and roots being consumed after proper preparation to remove spines. For example, the edible thistle (Cirsium edule) has edible roots and young shoots, and its stems can be eaten when peeled.
Milk Thistle (Silybum marianum) is a notable example with a long history of medicinal use, particularly for liver health. Its active compounds, collectively known as silymarin, are believed to possess antioxidant, antiviral, and anti-inflammatory properties. Milk thistle is often used as a complementary therapy for liver conditions like alcoholic liver disease, non-alcoholic fatty liver disease, and hepatitis, and has been studied for its potential to protect the liver from toxins. While research on its benefits is ongoing, it is generally considered safe when taken orally in appropriate doses, with common side effects being mild digestive issues.
What to Do in Case of Ingestion
If a human or animal ingests thistle material, particularly if symptoms develop, prompt action is advisable. For minor irritations, observing the individual for any changes in behavior or discomfort is often sufficient. If severe symptoms arise, such as choking, persistent vomiting, severe pain, or signs of internal bleeding, immediate professional medical or veterinary attention is necessary.
In the United States, the national toll-free Poison Help line, 1-800-222-1222, connects callers to their local poison control center, providing free, expert, and confidential assistance for poisoning emergencies. For animals, contact a local veterinarian or an animal poison control hotline, such as the ASPCA Poison Control Center. It is important to provide as much detail as possible, including the type of thistle ingested, the amount, and any observed symptoms.