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Photosensitivity in Stock

Photosensitivity in Stock

What is Photosensitisation?

Photosensitisation occurs when skin becomes highly susceptible to UV light after eating certain plants and toxins. All livestock species, breeds, and classes can be affected by photosensitisation, although it is most evident in livestock with unpigmented (white/pink) skin, recently shorn sheep and young lambs. Common sites affected are udders, lips, nose, ears, and face.

There are 2 types of photosensitisation:

1. Primary Photosensitisation

Primary Photosensitisation results from animals grazing plants containing light sensitive (photosensitising) compounds. Following their ingestion, these compounds accumulate in the superficial blood vessels at the surface of the skin. Sun exposure activates these compounds into toxins causing local tissue death and swelling. This results in intense irritation and serious skin damage. This form of photosensitisation usually occurs with ingestion of lush, rapid growing plants that are the dominant species in a pasture. Clinical signs are usually noted 3-5 days after access to the toxic plants. This type of photosensitisation is often associated with livestock grazing:

 Plant toxins that cause primary photosensitisation

  • St John’s wort (Hypericum perforatum)
  • Buckwheat (Polygonum fagopyrum)
  • Perennial Ryegrass (Lolium perenne)

2. Secondary Photosensitisation (Liver Form)

We call this secondary photosensitization because the initial damage is done to the liver, by toxic plants. Once the liver is sufficiently damaged then just eating non-toxic plants starts to cause photosensitive skin.

Normally what happens in digestion is the green pigment in plants, chlorophyll, is metabolized into a light-sensitive phylloerythrin. The normal liver then excretes this in bile in the intestines. When the liver is damaged the phylloerythrin build up in the bloodstream instead. Once this photosensitive chemical builds up to high levels the sunlight transforms it into a toxin which severely damages the skin

 Plant toxins that cause liver damage and secondary photosensitisation

  • Hairy panic (Panicum effusum)
  • Sweet grass (Panicum laevifolium)
  • Caltrop (Tribulus terrestris)

Other toxins that cause liver damage and possibly secondary photosensitisation

  • Heliotrope (Heliotropium europaeum)
  • Paterson’s curse (Echium plantagineum)
  • Lantana (Lantana camara)
  • Fungus of facial eczema (Pithomyces chartarum)
  • Fungus of lupinosis (Phomopsis leptostromiformis)
  • Blue-green algae (Anacystis cyanea)

Signs of Photosensitisation

  • Red, weepy skin
  • Swelling causing ears to droop
  • Skin surface may crack, die, turn black or peel off.
  • Animals are intensely irritated and may scratch and rub
  • Appetite loss
  • Milk production drop


First Aid if you suspect Photosensitivity is to:

  • Remove animals from the pasture into deep shade, ideally a darkened shed, sunlight is the enemy. 
  • Provide stock with water and cereal hay with no green colour and contact the vet.

Your Vet will set about determining whether your stock is suffering from primary or secondary photosensitization.  This affects prognosis as stock with severe liver damage may have an illness that lasts for weeks, long term loss of production, and even require euthanasia for liver failure. Up to half affected stock in a photosensitivity outbreak can die.

  • Posted 15 January, 2019

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