Plants causing photosensitisation

calendar icon 3 December 2018
clock icon 3 minute read

The sap of at least 30wilds and cultivated plants including parsnip tops, parsley, celery tops and giant hogweed contains substances (furocoumarins) which , in contact with bare skin, enhance the skin's sensitivity to the ultra violet rays in direct sunlight. The affected skin becomes red and inflamed and then develops blisters and extensive angry looking skin erosions. Similar signs may occur following the ingestion of plants such as buckwheat, white and alsike clover and Saint Johns Wort. The toxic principle may either be absorbed unchanged into the blood stream and be deposited in the skin or it may damage the liver with subsequent photosensitisation. These types of photosensitisation tend to occur in herbivores and people but not in pigs although there are reports of photosensitisation in pigs in central Europe from eating buckwheat. Celery tops sometimes have the same effect but this is due to fungi growing on them.

Sows and boars may be most obviously affected on their snouts and ears. The relatively hairless udders of lactating sows, which have most contact, may also be badly affected and become too painful for them to allow their piglets to suck. Boars sometimes become affected on their undersides which makes them reluctant to mate.


This is based on the appearance of typical skin blisters and erosions and the availability of plants containing the toxic substances.

Similar diseases

The early blisters and erosion on the nose could be mistaken for foot and mouth disease, swine vesicular or vesicular stomatitis but the history of exposure to plants containing furocoumarins and the progress of the disease over a day or two would allow ready differentiation.


  • There is no antidote to these photosensitising plant substances.
  • Affected pigs should be removed from the plants and from direct sunlight or at least provided with shade, until the skins heal.

Management and prevention

  • Give antibiotic cover to prevent secondary bacterial infections.
  • Remove offending plants.
  • Move pigs to different pastures.

Emily Houghton

Editor, The Pig Site

Emily Houghton is a Zoology graduate from Cardiff University and was the editor of The Pig Site from October 2017 to May 2020. Emily has worked in livestock husbandry, and has written, conducted and assisted with research projects regarding the synthesis of welfare and productivity of free-range food species.

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