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==Description==

==Description==

Photosensitisation is a condition of the dermis caused by a reaction induced by photodynamic agents exposed to UV light in the skin of animals that have eaten certain plants. Photosenstisation is primarily a condition of sheep, cattle and horses but other species may also be susceptible.  

Photosensitisation is a condition of the dermis caused by a reaction induced by photodynamic agents exposed to UV light in the skin of animals that have eaten certain plants. Photosenstisation is primarily a condition of sheep, cattle and horses but other species may also be susceptible.  

The condition can be divided into two categories; primary and secondary photosensitisation. Primary photosensitisation occurs due to the normal ingestion of lush green plants containing photodynamic agents. Secondary (hepatogenous) photosensitisation occurs as a result of liver or bile duct damage, leading to accumulation of phylloerythrin in the blood, body tissues and skin due to insufficient conjugation and excretion by the liver.

The condition can be divided into two categories; primary and secondary photosensitisation. Primary photosensitisation occurs due to the normal ingestion of lush green plants containing photodynamic agents. Secondary (hepatogenous) photosensitisation is the most commmon type of photosensitivity seen in animals and occurs as a result of liver or bile duct damage. The photosensitising agent, phylloerythrin accumulates in the blood, body tissues and skin due to insufficient conjugation and excretion by the liver. This accumulation in the skin leads to absorption and release of UV light, leading to a phototoxic reaction.

 

==Clinical signs==

==Clinical signs==

The condition is characterised by clearly demarcated 'sunburn-like' lesions that are confined to unpigmented or hairless areas of the skin. The nose, lips, ears, muzzle and udder are commonly affected areas. In the acute stages, the affected area(s) becomes erythematous and oedematous with exudation and crust formation. Animals may be pruritic or painful. other clinical signs may include swollen lips or muzzle, tachypnoea, tachycardia, diarrhoea and salivation.

The condition is characterised by clearly demarcated 'sunburn-like' lesions that are confined to unpigmented or hairless areas of the skin. The nose, lips, ears, muzzle and udder are commonly affected areas. In the acute stages, the affected area(s) becomes erythematous and oedematous with exudation and crust formation. Animals may be pruritic or painful. other clinical signs may include swollen lips or muzzle, tachypnoea, tachycardia, diarrhoea and salivation.

==Diagnosis==

==Diagnosis==

The history and clinical signs are important in order to differentiate whether the condition is primary or secondary.  In the case of primary photosensitisation there is often a history of exposure to plants containing photodynamic agents such as St. John's Wort or clover. Alternatively, there may be a history of administration of drugs such as phenothiazines, sulphonamides or tetracyclines. Signs of liver disease are usually absent.

The history and clinical signs of affected animals are important in order to differentiate whether the condition is primary or secondary.  In the case of primary photosensitisation there is often a history of exposure to plants containing photodynamic agents such as St. John's Wort or clover. Alternatively, there may be a history of administration of drugs such as phenothiazines, sulphonamides or tetracyclines. Signs of liver disease are usually absent.

In animals affected by secondary photosensitisation, the classic skin lesions are accompanied by signs of liver disease. Serum biochemistry often reveals increased liver enzymes (ALP, GGT, SDH and ALT).  

 

In contrast, in animals affected by secondary photosensitisation the classic skin lesions are accompanied by the signs of liver disease listed above. Serum biochemistry often reveals increased liver enzymes (ALP, GGT, SDH and ALT) but bilirubin and bile acid concentrations are usually normal. Definitive diagnosis may be obtained by liver biopsy