Photosensitisation
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- 'sunburn-like' lesions
- cattle and sheep
- sharply confined to the unpigmented areas of the skin
- occurs as a result of the effect of strong sunlight
- due to the effect of UV light on a photodynamic agent (PDA) present in the skin, such as is phylloerythrin derived from chlorophyll
- therefore, the disease occurs mostly in those animals consuming large quantities of grass
- the UV light is changed to a longer wavelength which produces the necrosis of the skin
- many unknown/poorly understood causes
- feeding rape, kale, lucerne, alfalfa
- mouldy hay
- lush pasture
- corticosteroid induced
Primary photosensitisation
Causes
- normal ingestion of growing lush green plants containing PDA
- plants containing such fluorescent pigments
- Hypericum perforatum (St John's Wort) - active principle is hypercin
- Lolium perenne (Perennial Rye Grass) - active principle is perloline
- plants containing such fluorescent pigments
- drugs
- phenothiazine
- thiazides
- tetracyclines
- sulphonamides
- inherited porphyria
Secondary photosensitisation
- photosensitisation secondary to liver damage
Cause
- hepatogenous photosensitisation
- chlorophyll is metabolised to phylloerythrin
- when the liver or bile ducts are malfunctioning (eg hepatitis or bile duct obstruction) due to severe toxic damage, phylloerythrin escapes into the circulation and settles in the tissues including the skin
- the pigment accumulates causing photosensitisation
- hepatotoxic plants
- Senecio jacobea (Ragwort)
- Lantana camara (Lantana)
- Lupinus angustifolius (Lupins)
- Blue-green algae
Clinical Signs
- localised to lightly pigmented skin areas
- clear cut demarcation of affected area
- teats, muzzle, ears
- skin only
- skin and liver signs
Microscopically
- subepidermal vesicles
- ulcerate
- become secondarily infected
Treatment
- remove from sunlight