As spring brings warmer weather and abundant green pasture, livestock producers should be alert to an increased risk of photosensitisation in cattle and sheep.

The condition, which can resemble a severe case of sunburn, can cause significant animal welfare issues.

Photosensitisation is an abnormal sensitivity of the skin to ultraviolet light.

It occurs when light-sensitive pigments accumulate in an animal's bloodstream and skin.

When these pigments are exposed to sunlight, they trigger a painful burn-like reaction.

In grazing animals, the process is caused by an excessive intake of chlorophyll, the green pigment found in pasture plants.

As chlorophyll is digested, by-products are normally processed and removed by the liver.

However, when animals consume large amounts of lush green feed, or when liver function is compromised, these pigments can build up in the body.

Conditions that cause liver damage, such as liver fluke infestation or poisoning from certain plants, including heliotrope and Patterson's curse, can reduce the liver's ability to remove these pigments from the bloodstream.

Some plants also contain photosensitising compounds directly.

One well-known example is St John's Wort.

Damage occurs most readily in areas where sunlight can easily penetrate the skin.

In cattle, this typically affects unpigmented skin and teats.

In sheep, the ears and nose are particularly vulnerable.

Early detection is important.

Animals suffering from photosensitisation may become restless and agitated due to a painful burning sensation.

Common signs include head shaking, rubbing against trees or fences, kicking at the belly, swollen or drooping ears and seeking shade during daylight hours.

As the condition progresses, the affected skin may begin to peel away or slough off.

In severe cases, animals can go into shock, which may prove fatal.

Prompt action can reduce the severity of the condition.

Antihistamines may be useful in the early stages, and special teat-darkening ointments are available for dairy cattle to help protect sensitive skin.

Affected animals should be moved into shaded areas immediately and removed from lush green feed until recovery occurs.

Producers should also avoid activities that may worsen the condition, including transport, until the skin has healed.

Photosensitisation can develop quickly and may have serious consequences if left untreated.

For further advice, contact your local veterinarian, an Agriculture Victoria Veterinary or Animal Health Officer.

– Dr Jeff Cave

Agriculture Victoria senior veterinary officer

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Friday, 21st July 2028

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