Pyrrolizidine alkaloids (PAs) are naturally occurring compounds that are toxic to horses and found in many species of plants.
Some common plants that contain high concentrations of PAs include Ragwort (Senecio spp.), Rattlepod, Hound’s tongue, Heliotrope, and Fiddlenecks.
Chronic pyrrolizidine alkaloid poisoning, also referred to as ragwort toxicosis, occurs when horses consume large amounts of these plants for a prolonged period.
This severe and often fatal condition affects the horse’s liver and bile system and causes photosensitization (sensitivity to light), which can result in skin damage and pain.
Signs of chronic exposure may not occur until several weeks or months after initial ingestion. Common symptoms observed in horses include lack of appetite, behavioral changes, head-pressing, colic, and exercise intolerance.
Diagnosing ragwort poisoning in horses is challenging as no specific test is currently available, and signs develop months after exposure to the toxic plant.
To date, no antidote is available. Horses that present clinical signs usually have irreversible liver damage and typically require humane euthanasia.
Pyrrolizidine Alkaloids in Plants
Pyrrolizidine alkaloids (PAs) are a group of naturally occurring organic compounds that are toxic to horses. Plants produce these compounds as a defense mechanism against herbivores and microbial pathogens. [1]
Like other toxic plants, these species are generally unpalatable to grazing animals and are not usually eaten in large quantities if other food is available. However, to protect your horse, it is best to remove any known PA-containing plants from horse pastures.
The concentration of toxic alkaloids varies across different plant species and between individuals of the same species. The concentration also varies between different structures within the plant anatomy. [1]
Pyrrolizidine alkaloids are among the leading causes of photodermatitis in horses, which is characterized by extreme skin sensitivity to sunlight. This hypersensitivity is a secondary complication of liver and bile system damage. [1][2]
When a horse’s liver function is compromised by 80% or more, the organ is unable to eliminate phylloerythrin, which accumulates in the blood. When exposed to sunlight, the excess phylloerythrin causes skin damage that, in turn, results in secondary photosensitization and dermatitis. [2]
Relevant Plant Species
Over 6,000 species of plants are known to produce pyrrolizidine alkaloids that are toxic to horses. They include: [2][3]
- Ragwort (Senecio spp.)
- Hound’s tongue (Cynoglossum officinale)
- Fiddlenecks (Amsinckia intermedia)
Ragwort (Senecio spp.)
There are over 1,200 species of Ragwort worldwide. North America has 70 Ragwort species, of which approximately 25 contain toxic amounts of pyrrolizidine alkaloids. [2]
It is prudent for horse owners and caretakers to assume that all Ragwort species are poisonous for horses.
Senecio spp. can be found in a number of different environments. Some species take root in high-altitude, damp, subalpine regions, while others favor dry, rocky terrains.
General characteristics of Senecio spp. include: [2]
- Flower heads: single layer of green bracts (the leaves that support the flower stalk) that touch each other but do not overlap
- Leaves: alternate leaves, typically lanceolate (flat, long, oblong and pointed) to ovate (shorter with rounded tips) in shape, with symmetrical teeth (the jagged edges of leaves)
- Flowers: bright yellow disk florets surrounded by a ray of yellow petals
- Seeds: dense ring of white pappus (the seeds’ wind-dispersal mechanism) at one end
The concentration of pyrrolizidine alkaloids (PAs), the toxic compounds in Senecio, varies significantly. Mature plants pose a greater threat than young plants. Mature Ridell’s ragwort (S. ridellii) contains the highest PA concentration of species in this family (18% of its dry matter). [2]
Hound’s tongue (Cynoglossum officinale)
Hound’s tongue (Cynoglossum officinale) is a widespread weed commonly found in both cultivated areas and wastelands. It can reach up to 3 feet (1 meter) in height. [4]
General characteristics of Hound’s tongue include: [2][4]
- Leaves: The hound’s tongue’s basal leaves are large and tongue-shaped, reaching up to 20 inches (0.5 meters) in length. Higher leaves are lanceolate in shape
- Flowers: small clusters of reddish-purple flowers are found in clusters on the plant’s terminal stalks
- Fruit: large with thick, fleshy walls that, once mature, split into four brown individual seeds (nutlets)
Fiddleneck (Amsinckia intermedia)
Fiddleneck or Tarweed (Amsinckia intermedia) is an annual weed that can be found in fields and wastelands. It reaches 3 feet (1 meter) in height. [2][5]
The fiddleneck plant is sparsely branched and covered in small, lightly colored hairs. As the plant matures and its flowers start to blossom, the coiled stem unfurls, giving the plant its characteristic “fiddleneck” appearance. [2][5]
Other characteristics include: [5]
- Leaves: lanceolate alternate, hairy leaves
- Flowers: small yellow or orange funnel-shaped petaled-flowers characterized by a “fiddleneck” shape, with all the flowers clustering on one side of the stalk
- Fruit: mature fruits split into 2 – 4 black, ridged seeds (nutlets)
Toxicology
After ingesting plants containing pyrrolizidine alkaloids, the toxic compounds are rapidly absorbed by the horse’s gastrointestinal tract. Once absorbed, pyrrolizidine alkaloids are passed into the liver by way of the portal vein (the blood vessel that carries blood from the GI tract to the liver, spleen, and pancreas). [6][7]
In the liver, the alkaloids are broken down into reactive pyrrole compounds, which damage liver cells and can ultimately cause liver tumors and cancers. [6][7]
Pyrrolizidine alkaloid toxicity is cumulative, meaning its effects build in the liver over time. As such, affected horses may not show any symptoms for long periods of time after initial exposure, even if the horse has already ingested a deadly dosage of toxin.
This makes diagnosis, early treatment, and prevention difficult, as the toxic plant responsible for onset of poisoning symptoms may not be present in the pasture by the time medical intervention is needed. [6][7]
It is possible for horses to develop acute PA toxicity after ingesting a large quantity of PA-containing plants in a short period of time. This form of poisoning is very rare compared to chronic toxicity in horses. [3]
Symptoms in Horses
Horses that ingest a toxic amount of ragwort or related plants over time exhibit symptoms related to liver and biliary system dysfunction, including: [6][8]