Around 700 PAs and their derivatives are known to be present in over 6,000 flowering plant species. Thirteen plant families have been reported to include PA-producing species, specifically Asteraceae, Fabaceae and Boraginaceae. Common examples are Senecio, Heliotropium, and Symphytum, but also animal products, such as honey and eggs, from animals that forage on these species may end up containing traces of PAs. Pyrrolizidine alkaloids are toxic to foreign organisms, but not all are created equal. Some PAs are hepatotoxic, others are carcinogenic, mutagenic, pneumotoxic, genotoxic, fetotoxic and teratogenic to varying degrees. However, PAs without the 1,2-double bond do not show these toxicities. About 50% of all PAs fall into the toxic, the other half into the harmless category (Brown, 2015; Fu, et al., 2004; Chojkier, 2003; Stewart & Steenkamp, 2001; Huxtable, 1980; Mattocks, 1968).

Contamination of herbal products with PAs is not a new phenomenon, however, nowadays highly sensitive analytical methods will detect even minute amounts of PAs in food and medicines. Many episodes of human and animal intoxication were shown to be caused by PAs. First scientific evidence for ragwort (Senecio jacobaea, Asteraceae) causing chronic liver disease in livestock was provided at the beginning of the 20th century. Seeds and dust from plants producing PAs growing as weeds can contaminate cereal grains. Liver damage in humans caused by cereals contaminated by seeds of Senecio spp. and seeds and dust from heliotrope or turnsole (Heliotropium lasiocarpum, Boraginaceae) has been reported in the 1920s and 30s from South Africa and the former Soviet Union, and from Afghanistan and Tajikistan in the 1970s and 90s, respectively. But there is also potential for PAs to occur in other foods. For instance, honey derived from the flowers of PA-containing plants has been shown to contain PAs, and also milk, meat, teas, salads, as well as eggs may contain PAs as carry-over from contaminated feed (Aniszewski, 2007; Wiedenfeld, et al., 2008; HMPC, 2014). Furthermore, medicinal plants containing PAs have been found to potentially cause significant liver damage, especially in children. Recently, PA contamination has been found in various plant materials of various origins at an increasing rate (Bodi, et al., 2014). It has also been demonstrated that PAs can reside in soil (from decayed plant material or mulch) and taken up and stored by crops which otherwise do not themselves contain PAs (Selmar, 2015).

On the other hand, PA-containing plants play an important role in their ecosystem. For instance, PA-containing species are visited by a range of generalist pollinators like bumblebees and honeybees (Irwin, et al., 2014). However, feeding only on the pollen of one species is not typical of bee behavior or natural honey production. Additionally, nectar and pollen contain considerably lower levels of alkaloids compared to other plant parts. Thus, the accumulation of alkaloids in honey is low in natural environmental conditions (FSANZ, 2016). Danaine (e.g., Monarch) and Ithomiine butterflies gather PAs and utilize them for biosynthesis of their major pheromone components and other purposes. Courtship behavior and successful mating depends on their release of these pheromones (Bropée, 1986; Irwin, et al., 2014; Harborne, 2001). Certain standards and certifications (e.g., organic wild, and FairWild) require maintaining or improving the biodiversity of the harvesting areas. If PA-containing plants are removed from the ecosystem, are we then working against biodiversity conservation and long-term survival of pollinators? In the US, for instance, the eastern, migratory population of monarch has declined by about 80% in the ten years since 2005. It has been estimated that ‘the population has a substantial probability of quasi-extinction, from 11–57% over 20 years’ (Semmens, et al., 2016).