|
Plant toxins and photosensitivity
Photosensitivity reactions in cattle occur when light reactive plant components (called phototoxins) enter the skin and are exposed to ultraviolet light. The phototoxin reacts to UV in light coloured or thin-skinned or hairless areas such as around the eyes, nose or teats. The wavelengths of UV that cause photosensitivity reactions are a wider spectrum than those that result in sunburn.
What causes photosensitivity?
Cattle may become photosensitised directly by eating plants containing phototoxins, for example brassica crops (especially if fertilised with high levels of nitrogen) or weeds (e.g. St Johns Wort). A few plants (e.g. parsnips; celery and parsley) are able to cause blisters and photosensitivity on contact with skin.
When cattle and sheep eat plants, the chlorophyll in the plants breaks down to release a phototoxin called phylloerythrin. Usually the animal processes phylloerythrin safely through the liver. Sometimes this is not possible if the liver is damaged by toxic plants such as cape weed or rough dog’s tail. In this situation cattle become photosensitised as a result of un-processed phylloerythrin entering the blood stream.
|
