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ES Insulin dysregulation in horses: a new epidemic or a new perspective?
June 6, 2025
Depending on the horse's “instinct” is no safety net against ragwort poisoning. Research, autopsy reports, and field studies show that both fresh and dried weed is indeed ingested and that the liver is irreversibly damaged.
Seven persistent claims under scientific scrutiny.
Fable. Under normal circumstances, most animals avoid the bitter, fresh plant. Still, plenty of exceptions have been documented—in times of hunger, overgrazing, young or recently imported animals. A recent Czech case report describes a mare that is fresh Senecio ovatus (Hoary ragwort, related to tansy ragwort) ate it and required euthanasia due to liver failure.
Fable. Pyrrolizidine alkaloids (PAs) remain chemically stable in hay. As a result, horses are less likely to recognize them and ingest them unnoticed. Royal GD emphasizes that both acute (with a large dose) and chronic (with a small, repeated dose) liver failure occurs.
Fable. PAs are converted in the liver into reactive pyrroles which cumulative destroy tissue; recovery is not possible once scar tissue (fibrosis) has formed. The European review of Cortinovis & Caloni shows that repeated intake in the order of milligrams per kg of body weight can already cause fatal damage.
Fact. In a BMC study Of the six warmblood horses, only one-third were able to consistently sort out the JKK from hay containing 5–10 % contaminants; four animals continued to eat the hay and were removed from the experiment early for safety reasons.
Fable. Bitterness helps – but only if there is sufficient alternative roughage. As soon as the ration becomes scarce, the aversion compellingly weakens and the horse can learn to ignore the taste. Royal GD explicitly warns that instinct is no guarantee.
Fact. Clinical symptoms (weight loss, lethargy, ataxia, jaundice) do not appear until 70 % or more of the liver tissue has been irreversibly damaged. Autopsies at universities in Belgium, Germany, and France typically show fibrosis and bile duct proliferation in horses that without previous clear symptoms died.
Fable. Flowering does help the plant, though more visible, but withered stems with seed heads remain standing all winter long. Furthermore, hay from earlier cuts can supply PAs well into the housing season. Case Reports show poisonings in all seasons.
The ruling “Horses do NOT eat fresh ragwort (JKK)” sounds reassuring, but is scientifically untenable. What research and field reports show:
| What does the literature say? | Finding |
|---|---|
| Outbreak in pasture (UK, 1983) | 5 of 20 horses died from acute liver failure; autopsy pointed to ragwort in the fat, not in hay. |
| Case report mare, Czech Republic 2024 | Toward Senecio ovatus in the meadow; PA metabolites in blood + liver confirmed. |
| BEVA position paper 2024 | “Most horses avoid fresh tansy ragwort, but with limited pasture or high stocking density, the risk of consumption increases.” |
| European toxicological review (Cortinovis & Caloni 2015) | Classifies Senecio-species as a risk for horses in both hay and pasture; emphasizes cumulative liver toxicity. |
Bitterness is relative. Hunger, overgrazing, young/inexperienced animals, or competing herd pressure can break the aversion.
Gradual habituation. A horse that accidentally eats a few tops without getting acutely ill and‘reads’therefore the taste is not immediately harmful. The horse will therefore simply swallow a bite of ragwort the next time as well. .
Detection errors of the meadow manager. JKK grass quickly disappears among other greenery; livestock farmers rarely see it happen, but liver biopsies and autopsies reveal the damage afterwards.
Absolute “never” is not correctThere are documented cases of fresh JKK consumption with a fatal outcome.
Usually horses avoid it, provided there is enough palatable grass.
Practical message: Do not rely solely on instinct. Keep pastures free of tansy ragwort, especially during dry periods or when grazing is tight.
| Action | Why? |
|---|---|
| Pasture management before flowering (May–July) | Young rosettes are easy to dig out; prevents seed bank |
| Visually and analytically inspect hay | Dried ragwort is unappealing, but less recognizable and equally toxic |
| Periodic liver enzyme check | Damage is already detectable early in blood, clinically not |
| Education of stable mates | Sharing this ‘Fact or Fiction’ prevents casual assumptions |
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