Photosensitisation

Description

Photosensitisation is a condition of the dermis caused by a reaction induced by photodynamic agents (PDAs) exposed to UV light in the skin of animals that have eaten certain plants. Photosenstisation is primarily a condition of sheep, cattle and horses but other species may also be susceptible. Photosensitisation can be divided into two categories; primary and secondary photosensitisation). Primary photosensitisation occurs due to the normal ingestion of growing lush green plants containing PDAs. Secondary (hepatogenous) photosensitisation occurs as a result of liver or bile duct damagem, leading to accumulation of phylloerythrin in the blood, body tissues and skin due to insufficient conjugation and excretion by the liver.

Clinical signs

The condition is characterised by 'sunburn-like' lesions that are confined to unpigmented areas of the skin

• • 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

• 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