NADIS is a network of 40 veterinary practices and six veterinary colleges monitoring diseases of cattle, sheep and pigs in the UK.

NADIS data can highlight potential livestock disease and parasite incidences before they peak, providing a valuable early warning for the month ahead.


June 2004

By Neil Sargison BA VetMB DSHP FRCVS

NADIS Sheep Disease Focus

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.

 

FACE RUBBING IN A ROMNEY RAM

 

CURLING OF THE EAR TIP IN A BLACKFACE LAMB

 

LOSS OF THE EAR OF A SHETLAND EWE
 

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.


• While every effort is made to ensure that the content of this forecast is accurate at the time of publication, NADIS cannot accept responsibility for errors or omissions. All information is general and will need to be adapted in the light of individual farm circumstances in consultation with your veterinary surgeon.

Copyright © NADIS 2002



 

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