Over the past five years, vet Stephen Ashdown has written many articles, spoken on equine matters and contributed widely to horse magazines.

Here he reproduces in the form of questions and answers about 100 topics, all of them common problems which afflict horses.

To access Topics click on Vet Advice Menu.

Poisoned by St John's wort
Q: My 20-year-old Mare has been diagnosed with St Johns wort poisoning. She is in a pitiable state with a raw nose and terrible puckering like corrugated cardboard on her white parts because of photosensitisation.

My vet has seen her and she has had a course of antibiotics and a steroid and intramuscular injection. She is in a dim stable with open access to a small barn with sheets covering the windows to keep the light out.

I am walking her in the evening and early morning out of the strong light and am covering her with a tight woven cotton sheet when waking her out. How soon should the corrugated skin starts sloughing off and do horses normally recover? I know it is going to be a long process but are there any ideas you can offer us to help?

A: I am sorry that your horse is suffering form photosensitisation. St John's wort (used to treat depression in humans) is one of the plants that cause such conditions, but there are also many other possibilities for such problems. Even plants like rape, alfalfa and clover can cause similar disease when they produce chemicals that sensitise animals to sunlight.

The most common cause of photosensitisation is serious liver damage, such as that caused by ragwort. In mild or moderate cases keeping the affected animal out of the sun for one to two weeks is usually enough. This allows the chemical causing the problem to be eliminated from the body

In you case you need to rely on the advice of your own vet because only he can judge the exact seriousness and progress of the healing. Make sure however that your horse is checked out for liver damage as the healing process will take much longer if the liver is affected.

Normally signs of disease only occur when up to 75% of the liver has been severely damaged. Lastly, make sure that your horse does not have access to St John's wort in the future. There are many different varieties of this plant and some types do not actually cause much of a problem as they contain very little of the key chemical hypercicin. You might be interested to know that the hypericin in St John's wort, which causes the problem of photosensitisation, is not actually the part of the plant that gives most relief for depression in people.


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