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.)

ragwort plant | Mad Barn Canada

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 plant | Mad Barn Canada's tongue plant

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.