The term photosensitivity refers to an increased sensitivity to ultraviolet light and resembles a bad case of sunburn.

When grazing lush green feed, cattle and sheep take in large amounts of the plant pigment, chlorophyl.

After the pigment is digested and absorbed, it passes to the blood from where it is normally filtered by the liver, broken down and passed out of the body. If an excessive amount of chlorophyl is consumed its end products may not be fully removed and they can build up to high levels in the blood.

This may also happen if the liver function is impaired in some way, for example, by liver fluke damage or damage from certain toxic plants such as heliotrope. There are other plants eaten by stock which have high levels of preformed photosensitising pigments. An example in the north-east is St John's wort.

The pigment can accumulate in the surface layers of the skin, and the interaction between the pigment and ultraviolet light results in a burn-like effect in the skin. This happens on uncoloured, less hairy areas of the skin such as the unpigmented skin and teats of cattle and the ears and nose of sheep.

In the early stages of photosensitivity, affected animals become agitated due to the burning sensation in their skin.

Affected animals will shake their heads, rub against trees and kick at their bellies in an attempt to relieve the pain. Their ears may become swollen and droopy and will seek shade during the day.

Photosensitivity can be severe enough to put an animal into shock which can be fatal.

On many occasions farmers only see the end results of photosensitivity where the affected skin begins to lift off as a result of the damage the burn has caused.

If detected in the early stages, antihistamines may help. There are ointments available for darkening teats which can be particularly useful for dairy cattle.

Affected animals should be moved into shaded areas and taken off green feed.