Introduction to Pyrrolizidine Alkaloids (PAs)
Pyrrolizidine alkaloids (PAs) are a diverse class of secondary plant metabolites found in approximately 3% of the world's flowering plants. These compounds, which number over 660 different types, are produced by plants as a defense mechanism against herbivores. While PAs offer a protective benefit to the plant, they pose a significant health risk to animals and humans when ingested. The primary danger comes from unsaturated PAs, which are metabolized in the liver into highly reactive pyrroles that damage liver cells, potentially leading to veno-occlusive disease (HVOD), cirrhosis, or liver failure.
Contamination of the human food supply often occurs through the co-harvesting of weeds containing PAs alongside food crops, or through the consumption of herbal teas and supplements containing PA-producing plants. PAs can also transfer into animal products like milk, honey, and eggs when livestock forage on contaminated pastures. Recognizing the specific plant families and species that produce these toxic compounds is a crucial step in mitigating this public health risk.
Major Plant Families Containing PAs
While PAs are found across multiple plant families, three families are particularly noteworthy for containing the majority of PA-producing species:
- Asteraceae (Daisy Family): This large family includes several notorious PA-containing genera. The tribe Senecioneae is a major source, with plants like Senecio (ragwort and groundsel) being prominent examples. The genus Eupatorium is also a source of PAs.
- Boraginaceae (Borage Family): Many members of this family are known to contain PAs. Significant genera include Heliotropium (heliotrope), Symphytum (comfrey), Echium (viper's bugloss, Paterson's curse), Cynoglossum (hound's tongue), and Borago (borage).
- Fabaceae (Legume Family): The genus Crotalaria (rattlebox) is the most prominent PA producer within this family. These plants are often found in tropical and subtropical regions and are a common cause of livestock poisoning.
Specific Examples of PA-Containing Plants
Here is a list of some common and medically relevant plants that have pyrrolizidine alkaloids:
- Senecio jacobaea (Tansy Ragwort): A widely known weed that is highly toxic to livestock. It is often responsible for poisoning animals that graze on contaminated pastures or eat contaminated hay.
- Symphytum officinale (Comfrey): Used historically and in some modern herbalism for its medicinal properties, comfrey is a significant source of PAs and has been linked to human poisoning cases. Its internal use is generally not recommended due to liver toxicity concerns.
- Echium plantagineum (Paterson's Curse/Salvation Jane): A common weed in Australia that contributes PAs to honey when bees forage on its flowers. Contamination in grains can also occur during harvesting.
- Heliotropium europaeum (Heliotrope): Another weed species that can contaminate crops, particularly grain crops, and has been involved in human and animal poisoning.
- Borago officinalis (Borage): While its leaves and seeds are sometimes used in food and herbal remedies, borage contains PAs, though in varying concentrations. EU regulations now set maximum levels for PAs in borage leaves.
- Crotalaria species (Rattlebox): This large genus of legumes includes many species that produce monocrotaline and other toxic PAs. They can contaminate animal feed and crops harvested in the same fields.
- Amsinckia intermedia (Yellow Tarweed): Seeds of this plant can contaminate grains and are a known source of pyrrolizidine alkaloid poisoning in livestock.
Comparison of Major PA-Containing Families
| Feature | Asteraceae (e.g., Senecio) | Boraginaceae (e.g., Symphytum, Echium) | Fabaceae (e.g., Crotalaria) |
|---|---|---|---|
| Common Examples | Ragwort, Groundsel, Coltsfoot | Comfrey, Viper's Bugloss, Borage | Rattlebox |
| Primary PA Type | Senecionine-type (macrocyclic diesters) | Lycopsamine-type, Heliotrine-type (monoesters, open diesters) | Monocrotaline-type (macrocyclic diesters) |
| Key Concern | Severe hepatotoxicity from macrocyclic PAs. | Contamination of herbal teas, supplements, and honey. | Livestock poisoning, especially in tropical regions. |
| Toxicity Profile | Potent liver toxins, genotoxic, carcinogenic. | Varies by species and PA type; hepatotoxicity is a key risk. | Hepatotoxic; some species are very potent. |
| Contamination Route | Weeds contaminating fields, herbal medicines. | Herbal supplements, accidental co-harvesting with food crops. | Contamination of grain and feed crops. |
How Contamination Occurs
Pyrrolizidine alkaloids can enter the human and animal food chain through several pathways, highlighting the need for vigilance in food production and personal foraging.
Contamination in Agriculture
- Co-harvesting: During the mechanical harvesting of crops like grains, spices, and leafy vegetables, weeds containing PAs can be collected along with the intended crop. This can lead to contamination of flour, breakfast cereals, or dried herbs.
- Soil Uptake: Research suggests that some PAs can persist in the soil and be absorbed by subsequent food crops planted in the same field. This poses a challenge for organic farming and crop rotation strategies.
- Animal Feed: Weeds with PAs, such as ragwort or rattlebox, growing in pastures can be ingested by livestock, leading to liver damage and the presence of PAs in animal products like milk, eggs, and meat.
Contamination in Herbal and Dietary Products
- Herbal Teas and Infusions: Many popular herbal teas, including those containing borage or comfrey, have been found to contain PAs. Contamination can also occur if PA-producing weeds are mistakenly harvested alongside tea or other herbal plants.
- Dietary Supplements: A significant number of herbal food supplements have tested positive for PAs. The European Food Safety Authority (EFSA) and other bodies have issued guidance and regulations on maximum permissible levels.
- Honey: Bees foraging on PA-containing flowers, like those of Paterson's curse or heliotrope, can collect pollen and nectar containing these toxins, resulting in contaminated honey.
Conclusion
Pyrrolizidine alkaloids are a widespread group of natural toxins present in many plant species, most notably within the Boraginaceae, Asteraceae, and Fabaceae families. From common garden plants like borage and comfrey to notorious agricultural weeds like tansy ragwort and rattlebox, the risk of exposure through herbal products or contaminated food is a serious health concern. As chronic ingestion, even at low levels, can lead to irreversible liver damage, consumer awareness is paramount. Food safety regulations and improved agricultural practices, such as weed management and testing, are essential for minimizing exposure. Anyone using herbal remedies or harvesting wild plants should be fully aware of the potential risks and correctly identify all species involved. For consumers, a varied diet and choosing reputable sources for food products and supplements can help reduce potential long-term exposure.
