Be on the lookout for photosensitisation in sheep grazing lucerne

02 Aug 2023

Local Land Services is encouraging producers to be on the lookout for signs of photosensitisation in sheep, particularly those grazing lucerne.

Photosensitisation is where stock become abnormally sensitive to sunlight due to the presence of photodynamic agents (PDA) in their skin – sheep get sunburnt on exposed, unpigmented parts of the body.

Central West Local Land Services District Veterinarian, Dr Nik Cronin said that a number of cases of photosensitisation in sheep have been seen across the region in the last few months.

“I have seen cases with slightly unusual presentation with producers reporting behaviour they had not seen in sheep before,” said Dr Cronin.

“Three cases were in weaned lambs, and another in a mob of ewes with lambs at foot. The weaned lambs were short woolled, and affected animals were described to be biting, or chewing at themselves along their body, to the point where wool had been pulled out, and in a proportion of animals there was extensive damage to the skin along the back.

“In the ewes and lambs, the presentation was more classic with skin damage of the face and swollen puffy ears.”

In all cases the sheep were grazing lucerne, and on paddock inspection, cowpea aphids were identified on the plants.

Dr Cronin said that while photosensitisation can be seen in sheep grazing lucerne independent of aphids, there has been suggestions that cowpea aphids may contain a pigment or other substance that is photodynamic itself. However, such a substance is yet to be identified.

“So while the jury is still out on the exact cause of photo in these cases, we are encouraging producers to check lucerne paddocks for aphid infestation and monitor grazing stock.

“We are very interested in gathering more information and potentially work towards a clear determination about the role of the cowpea aphid in photosensitisation of livestock.

“We are also interested to know if you have sheep grazing lucerne where cowpea aphids are present with no notable disease.”

Dr Cronin advises that if signs of photosensitisation are noticed, once stock are moved, they seem to settle within a short period of time.

However, if left unchecked affected stock can suffer a set-back, and particularly in the case of young lambs at foot, there may be permanent damage and even mortalities.

If you have any questions or would like to report unusual symptoms or behaviour in your sheep, please get in touch with your local Central West Local Land Services district vet on 1300 795 299.