St. John's Wort
Hypericum perforatum
Horse
Moderate
Get veterinary advice the same day and remove access. Regular consumption (1% of body weight cumulatively) combined with strong sun exposure.
Equio informs, it does not diagnose. When in doubt, call your vet.
What to do now
- Antidote
- No specific antidote. Removal from sun and supportive care.
- Time to signs
- A few hours to 1 day after sun exposure.
- Prognosis
- Good, but skin lesions are very painful and slow to heal.
- Immediately remove the horse from sunlight (bring into stable).
- Apply soothing local treatments to burns.
- Call a veterinarian if lesions are extensive.
Its ID card
- Height
- 30 cm – 1.2 m
- Flowering
- June to September.
- Toxic parts
- Entire plant, especially the flowers.
- Toxin
- Hypericin (a naphthodianthrone)
- Habitat
- Dry meadows, dry soils, embankments, wastelands, path edges.
- Soil indicator
- Light, dry, poor, very sunny soils.
- Regions
- Entire Europe
- In hay
- Yes — stays toxic once dried
- Palatability
- Not very palatable, but accidentally consumed in hay or if pasture is too dry and lacks grass.
When the risk rises
June to September (strong sun + flowering).
Spotting it in the field
Ragwort (also yellow but very different shape), Buttercup. — smell : slight aromatic smell when crushed, but not very pronounced.
- Erect stem.
- Opposite leaves appearing perforated with tiny holes when held to light.
- Bright yellow flowers with 5 petals.
- Flowers stain fingers red/purple when crushed.
Signs to watch for
Acute toxicity (photosensitization) and potentially chronic if consumption is repeated.
Dose : The exact dose is not clearly established, but regular consumption of significant amounts (several hundred grams) by a light-colored horse exposed to the sun is dangerous.
- Severe sunburns on unpigmented skin (snip, socks)
- Red, swollen, cracked, oozing skin
- Intense itching (pruritus)
- Agitation, horse seeking shade desperately
- Possible long-term liver disorders
Who is affected
Other species : Light-colored sheep and cattle are very sensitive.
In pregnant mares : no major direct documented risk.
Getting rid of it
Method : Pull the plant before flowering, ideally wearing gloves to avoid skin staining. Burn waste to prevent dissemination.
- Avoid overgrazing pastures where St. John's Wort is present.
- Mow infested areas before flowering.
- Pay particular attention to horses with light coats or depigmented areas.
- Ensure access to shade or a stable during periods of strong sunlight.
Frequently asked questions
Can Equio identify St. John's Wort from a photo?
It can help organize visual clues, but plant identification must remain cautious when horse health is at stake.
Is St. John's Wort dangerous in hay?
This entry flags hay vigilance. Isolate suspicious forage and ask for professional advice if exposure is possible.
This page is informational. If ingestion is likely, if signs appear, or if the horse is fragile, contact a veterinarian immediately.