Photo Sensitivity…(Sunburn / Mud Fever) 08/09/2015

Many horses with white faces and/ or white socks, suffer from ‘sunburnt’ noses, and/or chronic mud fever. Some get ulcers in their mouths.

The first line of thought with mud fever is that it is caused by ‘mud’. The first line of thought with scabs on the nose is ‘sunburn’. However, the truth is that it can be a very complex issue that doesn’t respond to external treatments. This is because the real cause of ‘mud-fever’ and ‘sunburn’ is Photosensitisation.

This is caused by eating plants which contain certain photodynamic (or fluorescing) pigments. These pigments enter the bloodstream and eventually reach the unpigmented skin of white faces and white socks, where they are exposed to UV rays, they oxidise, and thereby create the oozy sores in the surrounding skin.

Affected skin rapidly becomes reddened, painful, and raised above areas of adjacent pigmented skin. Serum often oozes through the affected skin to form crusts in the hair. Soon, the dead skin becomes dry and parchment-like, and the hair and white skin slough leaving ulcerated areas that may develop secondary bacterial infections, especially in muddy conditions. Hence the name ‘mud fever’. Yet the bacterial infection could be secondary to the real cause which is photosensitization.

When this occurs on the muzzle, it resembles, but is not, sunburn. It is a reaction caused by eating these plant pigments, which are exposed to UV rays in the vulnerable unpigmented skin areas. Most commonly affected areas are the muzzles of horses with white faces and white socks as in mud-fever.

This explains why some horses that have ‘heaps of white’ never sunburn or get mud-fever, while others do so, chronically and exasperatingly!! Plants known to cause this kind of photosensitization include Perennial rye-grass, (you might have guessed!) clovers, especially white clover, alfalfa, lucerne, St John’s Wort and buttercup. Many horses are grazing pastures that comprise these species. Buttercups also contain a chemical that causes dermatitis from direct contact with it.

Protection from UV rays is a huge help in prevention, however, this is tricky on the legs. There are vast numbers of topical applications for treating mud fever, which ‘work’, but often on some horses but not others. Quite often, just when you think you’ve got it beat, hey presto, it’s back!!

Preventing the horse eating the offending plants is obviously the best option but not always easy. Keep in mind that rye grass, clover and lucerne hay may also cause skin photosensitivity. It is yet another really good reason to work out ways to change your pastures to encourage other species than those listed above.

If your horse is prone to sunburn from sun exposure we suggest applying a natural sunblock which is easy to apply, non greasy and stays on in rain Click here to view our favourite

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