With the warmer weather and green feed that comes with spring, livestock farmers are likely to see an increased incidence of photosensitisation in cattle and sheep during the coming weeks.
The term photosensitivity means an increased sensitivity to ultraviolet light and resembles a severe case of sunburn.
When grazing lush, green feed, cattle and sheep take in large amounts of the plant pigment, chlorophyll.
After the pigment is digested and absorbed from the digestive tract, it passes to the bloodstream from where it is normally filtered by the liver, broken down and passed out of the body.
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If an excessive amount of chlorophyll is ingested, its end products may not be fully removed and may 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 by damage from certain toxic plants such as Heliotrope and Paterson's curse.
The pigment then accumulates in the surface layers of the skin.
The interaction between the pigment accumulated in the skin and ultraviolet light results in a burn like effect in the skin.
There are other plants grazed by stock which have high levels of these pigments preformed in the plant.
An example in the north east is the weed St. John's Wort.
The ultraviolet light must be able to penetrate the surface skin layers.
This happens on uncoloured, less hairy areas of the skin such as the unpigmented skin, teats of cattle and the ears and nose of sheep.
In the early stages of photosensitivity, an affected animal becomes agitated due to the burning sensation in their skin.
They 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 they seek shade during the heat of the day.
Photosensitivity can be severe enough to put an animal into shock and can sometimes be fatal.
Many times, 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 be helpful. There are ointments available for darkening teats which are particularly useful for dairy cattle. All affected animals should be moved immediately into shaded areas and taken off the lush green feed.
For further advice, please contact your local veterinarian or Agriculture Victoria veterinary or animal health officer, or in NSW your Local Land Services.
Photosensitisation of sheep can resemble the exotic diseases foot-and-mouth disease, bluetongue and sheep pox.
If you have any suspicion of an exotic disease, call the emergency animal disease (EAD) watch hotline on 1800 675 888.
- Dr Jeff Cave is a senior veterinary officer with Agriculture Victoria
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