Photosensitisation

• '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
• 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

Protosensitising Agents

• Phenothiazine which was used as an anthelmintic
• Tetracycline - if one injects bitches with tetracycline when pregnant, puppies are born with brown teeth, which fluoresce under UV light.
• Haemosiderin may also produce pinky / brown colour to teeth usually due to damage and haemorrhage into tooth.

• Primary
  • Ingestion of photodynamic substance from plants (Hypericum perforatum and Fagopyrum sagittatum - buckwheat) or fungal contaminants
  • Induced by drug administration - phenothiazines, tetracyclines, thiazides, sulphonamides
• Abnormal porphyrin metabolism
  • Bovine congenital porphyria
  • Bovine erythropoeitic protoporphyria
• Hepatogenous
  • Impaired excretion of phylloerythrin (chlorophyl metabolism product) due to hepatic disease
  • Biliary obstruction
  • Inherited defects
  • Hepatic injury due to toxic plants or fungal contaminants, chemicals or some infectious agents
• Grossly:
  • In areas lacking hair or pigment
  • Erythema, oedema -> blisters, exudation, necrosis, sloughing or keratitis
• Microscopically:
  • Subepidermal vesicles
  • Ulceration
  • Secondary infection