Photosensitisation in sheep 

Photosensitisation of sheep is first seen as swelling in the ears and muzzle, then sunburn-like appearance with blistering and scabs, and sometimes seen on other areas of the body if recently shorn.  

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Photosensitisation can result in two forms: 

Primary photosensitisation occurs when plants containing light-sensitive substances are ingested. If enough of these agents accumulate in blood vessels near the skin surface, they react with sunlight creating toxins that cause skin damage.  

Plants that cause primary photosensitisation include St John’s wort and Buckwheat. There are reports that Medics, Lucerne and Biserulla can exacerbate primary photosensitisation by increasing the sensitivity of the skin to sunlight, and by hosting seven-spot ladybird larvae. 

It’s also strongly believed that consuming cowpea aphids while grazing pasture, can cause photosensitisation as they contain a photodynamic pigment that causes phototoxic effects. Cowpea aphids are pests of pasture medics, clover, lucerne, vetch and cowpeas, as well as faba beans, chickpea and lupin crops. 

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Signs of primary photosensitisation due to light-sensitive toxins will appear within 2–3 days of ingestion of toxic compounds, and improve within 4-5 days after removal. 

Secondary photosensitisation occurs when the liver is damaged by toxins.  Usually, the green plant pigment chlorophyll is metabolised into a light-sensitive substance phylloerythrin, which the liver excretes in bile to the intestine. When the liver is damaged, phylloerythrin can’t be excreted and builds up in the bloodstream. If enough phylloerythrin builds in blood vessels near the skin surface, sunlight converts it into a toxin which causes skin damage. 

Plants with toxins that cause liver damage and secondary photosensitisation include Hairy panic, Sweet grass and Caltrop. 

Other plants and toxins that cause liver damage and possibly secondary photosensitisation include Heliotrope, Paterson’s curse, Lantana, Fungus of facial eczema, Fungus of lupinosis and Blue-green algae. 

In cases of liver damage and associated secondary photosensitisation, signs can occur some weeks after the animals have been removed from the toxic plants. 

 

Young lambs and sheep are more prone to sensitisation, whereas sheep with dark pigmented skin or wool coverage are less susceptible. 

Treating animals requires removing them from grazing directly toxic and green plants which cause liver damage (or move to a paddock with less toxic plants), or from ingesting cowpea aphids. Place stock in shaded areas if possible, and provide feed with no green colour such as cereal hay or lower quality hay. For severely affected or valuable animals, seek veterinary advise.  

Many outbreaks are mild, with only a few sheep affected and recovery within a few days. In more severe cases, some sheep may die and recovery can be more prolonged when there has been liver damage. Badly affected stock may die from shock or infection of damaged skin, and euthanasia may be required. Early recognition of the condition with appropriate action will minimise losses. 

References 

https://www.lls.nsw.gov.au/help-and-advice/livestock-health-and-production/diagnosing-livestock-diseases/photosensitisation 

https://www.flockandherd.net.au/sheep/ireader/photosensitisation.html