Photosensitisation is the clinical condition in which skin is damaged by photodynamic agents of dietary origin that react to sunlight.
Phylloerythrin is the most important of the light-sensitive pigments causing skin disease in grazing ruminants in Australia, but there are other agents that may also cause the same disease. Phylloerythrin is produced by fermentation in the rumen of chlorophyll that is present in all green plants. Phylloerythrin is absorbed and carried in blood and normally excreted in bile by the liver. Sunlight penetrating unprotected skin can cause fluorescence of pigment within superficial blood vessels and result in damage to tissues of the skin including the small blood vessels.
Phylloerythrin may be retained in the circulation by simple overload of the excretory capacity of the liver when young sheep and goats are exposed to succulent, immature forage such as early growth lucerne, perrenail rye grass, clover or rape. Outbreaks also occur secondary to liver damage from some other cause that impedes the excretion of even normal levels of the pigment.
Acute clinical cases of photosensitization at assembly points would be unusual and dependant on exposure of animals in the period prior to assembly, but significant residual lesions on the skin can affect large numbers of animals. The highest risk is generally in late winter and spring when pastures are rapidly growing and there may be less cloud cover.
Non-pigmented skin areas with minimal wool covering and exposed to direct sunlight are more likely to be affected (face, ears and feet). Animals with short wool, especially with open fleeces, may be affected along the dorsal midline. Head shaking and shifting of feet results from damage to head and feet. Face and ears are swollen, ears droop, tears are excessive and a bright band of redness is present on the coronet. Affected areas are itchy, blisters develop and scabs form. Affected skin may become necrotic and ulcerate and covering fleece may be ragged and lost along the back.
Affected cattle may show skin damage along the back and on the sides of the udder depending on where non-pigmented skin is located.
Clinical signs are usually adequate for diagnosis but must be differentiated from sunburn. Jaundice if present, will indicate liver disease.
Protect from direct sunlight. Deny access to green feed. Secondary skin infections may require antibiotics. Prevent flystrike.
Screen out animals with skin lesions during selection. Avoid grazing animals on hepatotoxic and phototoxic plants that may have emerged in holding paddocks at assembly points.