‘Mud Fever & Sunburn’

It is so easy to misunderstand these two conditions.

The names give the impression that they are caused by external factors such as ‘mud’ or ‘sun’. In actual fact the real name for both of these conditions is ‘Photosensitisation’ caused by the horse consuming plants high in ‘photodynamic’ (or fluorescing) pigments.

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There are two types: primary and secondary.

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Primary occurs due to the ingestion of lush, dark green plants which are high in photodynamic pigments, such as Clovers, Lucerne (alfalfa) rye-grass, St John's Wort or even blue-green algae like spirulina. Via digestion, these pigments enter the bloodstream and eventually reach the un-pigmented skin of white faces and white socks where they are exposed to UV rays which damage the skin.

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Serum often oozes through the sores to form the crusty edges. Then, the damaged skin sloughs off leaving ulcerated areas that are susceptible to secondary bacterial infections, especially in muddy conditions, hence the name 'mud fever'. This is how horses can develop ‘mud fever’ even when there is no mud in sight.

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Secondary occurs when the normal breakdown of chlorophyll is disrupted and liver dysfunction leads to its accumulation in the blood and body tissues. In the skin this allows excessive absorption of UV light, resulting in the inflammation and sun-burn-like lesions.

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The offending ‘photodynamic’ pigments are found in all sorts of dark green plants especially legumes such as clover (of any variety) and Lucerne (Alfalfa). Dark green grasses like Rye Grass are also high in fluorescing pigments as is blue green algae (eg Spirulina), St John’s Wort and Buttercup.

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One summer we learned the hard way. Before we understood about these things we bought and fed to our horses, some freshly made hay that was full of clover. Big mistake! Within 24 hours our horses white legs flared up, swelling to the point of almost splitting the skin, followed closely by the appearance of the oozy sores.

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The more ‘bluey green’ the plant, the higher in the ‘photodynamic pigments’ they are.

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Paddocks of brand new grass/clover mixes are particularly dangerous to all livestock especially those with large areas of unpigmented skin like coloured horses.

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While it is necessary to treat existing ‘mud-fever’ topically (there are many topical treatments that work well) what you really need to do is remove the cause which means being meticulous about pasture management and being aware of what plants are in your horse’s hay. Particularly avoid clover and lucerne if your horse has trouble with either ‘mud-fever’ or ‘sun-burn.

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Then you will never have to worry about your white-faced, white socked horses again.

In the case of Albinos, Cremellos, Paints and Appaloosas – some individuals have very sensitive skin and little protection from hair around their eyes and muzzles and are consequently susceptible to real sunburn and therefore do need protection from the UV rays. There are excellent masks available which are designed for the purpose.

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Prevention involves:

- taking steps to ensure plants like clovers, plantain, cat’s ear, capeweed and ragwort don’t become predominant as part of pasture management and be very mindful of what is in your hay

- making sure you feed a high spec multi vitamin and mineral which contains the organic forms of copper and zinc along with all the other minerals necessary for optimal health of the whole horse  - Premium NZ Horse Minerals, Premium MVA or Supreme Vit & Min (Australia).

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