Photosensitivity is caused by the presence of a photo-activating substance in the skin which increases absorption of UV light resulting in inflammation and lesions very similar to those seen with mud fever. Affected horses develop painful scaling/crusting lesions on all white areas or in some cases just the legs, pigmented areas will not be affected.

There are different forms depending on the origin of the photosensitising agent;

In primary photosensitisation the agent either enters the body directly or is produced within the body. This can be via ingestion of certain plants including clover, St Johns Wort and buckwheat, via injection of some drugs or via direct contact with some plants (eg. buttercups).

Secondary photosensitisation occurs when a horse has a degree of liver compromise which results in elevated blood levels of photo-activating compounds leading to accumulation in the skin. A number of things can cause liver disease but a key one is the ingestion of ragwort.

Diagnosis is made primarily by clinical signs and history and confirmed with a skin biopsy. Blood samples should be taken to identify any underlying liver disease.

Treatment involves identifying and removing the source of the photo-activating agent if possible. Management of the condition involves reducing exposure to UV light by stabling the horse during the day, using boots to cover any white areas/applying sunblock during turnout especially on sunny days. In acute flare ups topical steroid and/or antibiotic creams can be useful and in some cases antibiotics may be needed.


Ragwort poisoning is a cause of liver disease in horses and ponies in the UK, its scientific name is Senecio jacobea and it contains a poison (toxin) that is also found in some other plants. The poison is found in variable concentrations in different parts of the plant and at different stages of its growth. The live plants are not palatable and horses will not usually eat them unless the pasture is heavily contaminated or there is little other food available. However, the poison is very stable and remains toxic even when the dried plant is incorporated into hay which does not affect its palatability. The most common cause of equine ragwort poisoning is therefore from chronic (long-term) eating of hay that includes dried ragwort or on heavily contamined poor pasture.

What are the signs of ragwort poisoning?

Unless very large quantities of fresh plants are eaten, the symptoms of poisoning are usually not seen until 4 weeks to 6 months after the plant is eaten. Small doses of the poison gradually accumulate in the horse’s liver where it causes damage to the liver cells and subsequent fibrosis scarring, eventually causing the liver to shrink in size. Symptoms of liver disease only develop when the organ is sufficiently damaged to the extent that it is no longer able to compensate for the loss of functional tissue. Symptoms usually develop quite suddenly, although in some horses and ponies slight illness can precede more severe symptoms. Early signs include loss of appetite, depression, diarrhoea, weight loss and mild jaundice. More severe symptoms include marked jaundice and collapse or abnormal behaviour, which can range from profound depression to compulsive walking and pressing the head against objects, e.g. walls, apparent blindness, photosensitisation (excessive skin sensitivity to sunlight) and convulsions. These behavioural abnormalities are caused by toxic effects on the horse’s brain (hepatic encephalopathy). Affected horses become dangerous to handle. Most severely affected cases usually those with behavioural abnormalities often die within 10 days.

How is it diagnosed and treated?

The diagnosis of ragwort poisoning is based on clinical signs and laboratory tests. It is often difficult to prove that ragwort has been eaten because of the time lag between ingestion and the development of clinical signs. Laboratory tests, including the measurement of liver enzyme levels in the horse’s blood, confirm a diagnosis of liver disease and bile acid tests assess the liver’s ability to function. To confirm the diagnosis of ragwort poisoning, a liver biopsy is required to demonstrate typical microscopic abnormalities (pathology). If these are not found, the biopsy may help to suggest other possible causes of liver damage. Follow-up blood samples help to monitor progression of the condition in horses receiving treatment for ragwort poisoning.

Mildly affected horses are treated with injections of multivitamins, especially B1, to help support the liver’s function and to allow it’s cells to repair. At the same time the horse’s diet is adjusted to reduce protein and increase carbohydrate intake, also to help the liver’s function. As symptoms often only develop late in the course of the disease, treatment is rarely successful for severely poisoned horses, especially those with behavioural (neurological) abnormalities. Feeding a special diet to try to reduce the severity of nervous symptoms can help in the short term in some cases. The scar tissue which develops in the liver cannot be replaced by normal liver tissue, but less severely poisoned horses can sometimes be helped to compensate for their loss of liver tissue.

Other in-contact horses should be examined for signs of poisoning so that they can receive treatment and extra care. Blood screening tests for liver damage and abnormal liver function are the first steps.

How can I prevent ragwort poisoning?

Ragwort is a two-yearly plant. In the first year a flattish crown of branched leaves is formed. This flat crown is fairly resistant to mowing and is often not noticed. In the second year yellow flowers are produced on stems that are up to approximately 80 cm high. Any plants that are found should be pulled up by their roots and disposed of away from livestock. Do not leave cut or pulled plants in the paddock or they may be eaten when they have dried and are more palatable. Plants on adjacent land should be removed to avoid the spreading of seed back into your paddocks. Always ensure that there is adequate grazing or alternative food sources such as hay, so that your horse or pony is not tempted to eat any ragwort that may have been missed. New weed-killing sprays are now available and manufacturers claim that they are effective in killing ragwort while being safe to use near horses. Individual ragwort plants can be spot sprayed. Do not use pasture that is contaminated with ragwort for hay making because the poison remains active even in dried plants.