Phytophotodermatitis is a phototoxic inflammatory reaction of the skin resulting from contact with a light-sensitizing botanical agent (such as lime juice) followed by exposure to ultraviolet A (UV-A) light (from the sun, for instance). Symptoms include erythema, edema, blisters (vesicles and/or bullae), and delayed hyperpigmentation. Heat and moisture tend to exacerbate the reaction. A reaction may be elicited in any person who has been exposed to adequate amounts of both a photosensitizer and UV-A light. Phytophotodermatitis is not an immunologic response; no prior exposure to the photosensitizing agent is required. The photosensitizing substances found in phototoxic plants belong to a class of chemical compounds called the furanocoumarins, which are activated by long-wavelength ultraviolet (UV-A) light. The most toxic of these organic compounds are the linear furanocoumarins, so called since they exhibit a linear chemical structure. Bergapten and xanthotoxin (also known as methoxsalen), two linear furanocoumarins derived from psoralen, are invariably found in plants associated with phytophotodermatitis.
Signs and symptoms
A reaction typically begins within 24 hours of exposure and peaks at 48–72 hours after exposure. Initially, the skin turns red and starts to itch and burn. Large blisters (or bullae) form within 48 hours. The blisters may leave black, brown, or purplish scars that can last for several years. This hyperpigmentation of the skin is caused by the production of melanin triggered by the furanocoumarins. Although media reports have suggested that eye exposure to the agent can lead to temporary or permanent blindness, the risk of permanent blindness is not supported by existing research. Phytophotodermatitis can affect people of any age. In children, it has sometimes been mistaken for child abuse.
Causes Plants associated with phytophotodermatitis mainly come from four plant families: the carrot family (Apiaceae), the citrus family (Rutaceae), the mulberry family (Moraceae), and the legume family (Fabaceae).
Apiaceae The carrot family Apiaceae (or Umbelliferae) is the main family of plants associated with phytophotodermatitis. Of all the plant species that have been reported to induce phytophotodermatitis, approximately half belong to the family Apiaceae. False bishop's weed (Ammi majus), the world's major source of the linear furanocoumarin xanthotoxin, has been used since antiquity to treat vitiligo but accidental or inappropriate use of this plant can lead to phytophotodermatitis. Despite this danger, A. majus continues to be cultivated for its furanocoumarins, which are still used for the treatment of skin disease. Numerous species in the family Apiaceae are cultivated as food products, some of which exhibit phototoxic effects. In particular, celery, parsnip, and parsley have been reported to cause phytophotodermatitis among agricultural workers, grocery workers, and other occupational food handlers.
A number of phototoxic plant species in the carrot family have become invasive species, including wild parsnip (Pastinaca sativa) and the tall hogweeds of the genus Heracleum, namely, Persian hogweed (Heracleum persicum), Sosnowsky's hogweed (Heracleum sosnowskyi), and giant hogweed (Heracleum mantegazzianum). In particular, the public health risks of giant hogweed are well known. Other plant species in the family Apiaceae that are associated with phytophotodermatitis include blister bush (Notobubon galbanum), cow parsley (Anthriscus sylvestris), wild carrot (Daucus carota), various species of the genus Angelica (e.g., Korean angelica Angelica gigas), and most (if not all) species of the genus Heracleum (esp. the tall invasive hogweeds and the cow parsnips, Heracleum sphondylium and Heracleum maximum).
Rutaceae The citrus family Rutaceae is the second most widely distributed family of plants associated with phytophotodermatitis.
Numerous citrus fruits in the family Rutaceae exhibit phototoxic effects. Of these, perhaps the best known is lime. Phytophotodermatitis associated with limes is sometimes colloquially referred to as "lime disease," not to be confused with Lyme disease. In the family Rutaceae, the most severe reactions are caused by the essential oil of the bergamot orange (Citrus bergamia). Bergamot essential oil has a higher concentration of bergapten (3000–3600 mg/kg) than any other citrus-based essential oil, even lime oil, which contains 1700–3300 mg/kg of bergapten. Other plant species in the family Rutaceae that are associated with phytophotodermatitis include burning bush (Dictamnus albus), common rue (Ruta graveolens), and other plants in the genus Ruta.
Moraceae The mulberry family Moraceae is often associated with phytophotodermatitis. Multiple species in the genus Ficus are known to exhibit phototoxic effects. Of these, the common fig (Ficus carica) is well known and thoroughly documented. Like Ammi majus in the family Apiaceae, the common fig has been used since antiquity to treat vitiligo but the milky sap of fig leaves can cause phytophotodermatitis if used accidentally or inappropriately. A literature search revealed 19 cases of fig leaf-induced phytophotodermatitis reported between 1984 and 2012. In Brazil, several hospitals reported more than 50 cases of fig leaf-induced burn in one summer. In most cases, patients reportedly used the leaves of the fig plant for folk remedies, tanning, or gardening. Other plant species in the family Moraceae that are associated with phytophotodermatitis include Ficus pumila and Brosimum gaudichaudii. Like Ficus carica, the South American species Brosimum gaudichaudii has been shown to contain both psoralen and bergapten.
Prevention The first and best line of defense against phytophotodermatitis is to avoid contact with phototoxic substances in the first place:
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