How ragwort causes photosensitisation

Your Horse

Ragwort is a major cause of secondary photosensitisation, so keeping your horse’s paddocks clear of it is vital in preventing the condition. But how is ragwort involved? Vet Leona Bramall explains.

While intact, ragwort is generally quite unpalatable and horses don’t tend to eat it unless no alternative forage is available. However, it becomes much more palatable when it has been treated using a herbicide but hasn’t yet fully decomposed, or when it is cut and subsequently dries out. So one of the main sources of exposure for horses is when ragwort is inadvertently incorporated into hay or haylage.

Pyrrolizidine alkaloid, the toxin in ragwort, is generally a cumulative toxin. While a toxic dose may be eaten on one occasion, it’s much more common for a horse to consume a toxic dose over a longer period, even years.

The toxin causes irreparable damage to the liver, which can lead to fatal liver failure. Clinical signs are often only apparent when more than 75% of the liver is affected. Signs can include depression/ abnormal demeanour, reduced appetite, weight loss, jaundice, diarrhoea and photosensitisation.

Diagnosis is based on seeing compatible clinical signs, whether or not the horse has a history of grazing ragwort-infested pasture, plus blood tests and ideally, a liver biopsy.

Treatment is generally of a palliative nature to keep the horse comfortable before eventual death.

Meet the expert: Leona Bramall MVB CertAVP(EM) MRCVS graduated from University College Dublin before completing an internal medicine fellowship at Hagyard, Kentucky, and a hospital internship at Leahurt. She is an RCVS Advanced Practitioner in equine internal medicine. Her primary areas of interest lie in dermatology and gastrointestinal disease. 

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