NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare.

Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

Photosensitisation

NADIS receives reports every summer of swollen heads in white-faced sheep, caused by photosensitisation. Photosensitisation occurs in cattle, sheep, goats and horses following the accumulation of photosensitive metabolites under the skin and their reaction with sunlight to cause necrotic damage.

SWOLLEN HEAD IN A BLUEFACE LEICESTER EWE DUE TO PHOTOSENSITISATION

Clinical signs

The first sign of photosensitisation is usually swelling of wool-free unpigmented areas of the face, ears and limbs. In thin sheep, or certain sheep breeds, such as the Scottish Blackface, the skin of the midline of the back at the parting of the fleece is also involved. Affected animals are restless, rub or scratch affected areas and seek shade. Severely affected animals don’t eat and rapidly lose body condition. These signs are followed by seepage of serous fluid through the skin, which dries to form yellow crusts. Skin necrosis, scab formation, (in particular around the muzzle, eyes and on the ears), sloughing and regeneration follow over a period of several months. Ears are sometimes lost or shrivel and curl at the tips.

Cases of photosensitisation occur either as a primary condition or secondary to liver damage.

Primary photosensitisation

Most cases of photosensitisation seen in the UK follow the movement of sheep from poor to lush green pasture such as silage aftermaths, and are associated with failure to adapt to increased amounts of chlorophyll.

PHOTOSENSITISATION IN A GREYFACE GIMMER, 5 DAYS AFTER TURNING ONTO LUSH GREEN PASTURE

Certain toxic plants contain photodynamic agents that are absorbed and carried systemically to the skin. For example, clinical signs of photosensitisation are seen within 2 – 3 days of ingestion of St. John's wort (Hypericum perforatum), which is common in hedgerows and scrub ground in the UK, and contains the photodynamic agent hypericin.

Hepatogenous (liver) photosensitisation

Photosensitisation can occur when the excretion of phylloerythrin, a normal degradation product of chlorophyll, which is normally excreted in bile, is obstructed due to various liver diseases. High levels of phylloerythrin make the skin sensitive to sunlight.

RAISED WOOL OVER AN AREA OF SKIN NECROSIS ON THE BACK OF A BLACKFACE RAM LAMB

Hepatogenous photosensitisation is associated with the ingestion of a number of plant or fungal toxins, or may be due to an inherited defect in Southdown and Corriedale sheep. In many Scottish hill areas, annual outbreaks occur, with up to 10% of lambs affected during early summer. These cases are probably associated with the ingestion of bog-asphodel (Narthrecium ossifragum). While ragwort is much less toxic to sheep than to cattle, long term ingestion can result in photosensitisation.

BOG ASPHODEL

Management

There is no satisfactory treatment for photosensitisation. However, affected animals should be immediately housed in darkness for up to 3 weeks to avoid further reaction with sunlight until the lesions heal.

Neil Sargison BA VetMB DSHP FRCVS