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Dermatitis from plants of the new world

European Journal of Dermatology. Volume 19, Number 5, 423-30, September-October 2009, Review article

DOI : 10.1684/ejd.2009.0714

Summary

Author(s) : Denis Sasseville , Division of Dermatology, McGill University Health Centre, Royal Victoria Hospital, Room A 4.17, 687 Pine Avenue West, Montréal, QC H3A 1A1, Canada.

Summary : This review focuses on the different types of phytodermatitis from a pathophysiological and clinical point of view, followed by a more in-depth review of some dermatotoxic plants from the American continent.

Keywords : dermatitis, phytodermatitis, dermatotxic plants

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ARTICLE

Auteur(s) : Denis Sasseville

Division of Dermatology, McGill University Health Centre, Royal Victoria Hospital, Room A 4.17, 687 Pine Avenue West, Montréal, QC H3A 1A1, Canada

accepté le 23 Février 2009

Plants are called “the lungs of the planet” because they have developed photosynthesis to capture atmospheric carbon dioxide and release vital oxygen. Without plants, life as we know it would not exist. Over millennia, man has learned to exploit various plants in his quest for fire, food, shelter, clothing, tools, paper, dyes, perfumes, drugs and poisons.

Most plants are harmless, but some exert noxious effects by ingestion or skin contact. Botanists, gardeners and dermatologists are familiar with the irritant or allergenic potential of numerous plants. The layman, however, is often unable to recognize toxic plants that leave painful memories after too close encounters.

This review focuses on the different types of phytodermatitis from a pathophysiological and clinical point of view, followed by a more in-depth review of some dermatotoxic plants from the American continent.

Irritant phytodermatitis

Many plants are equipped with thorns, spines, barbs and cutting leaves that cause lacerations and penetrating wounds. Foreign body granulomas secondary to the dermal implantation of broken thorns present as papules or nodules that often suppurate and are notoriously slow to heal. Their infectious complications are not limited to simple staphylococcal or streptococcal abscesses and cellulitis. Direct inoculation of pathogenic microorganisms will give rise to the indolent or ulcerated granulomas of atypical mycobacteria, to the chains of abscesses along lymphatic vessels of sporotrichosis or, in tropical areas, the vegetating plaques of chromomycosis [1].

Other plants contain cytotoxic chemicals that can induce true chemical burns. These noxious substances are either excreted at the plant surface, or become accessible only if the plant is bruised or broken. Skin contact with oxalates, saponins, thiocyanates, phorbols and proteolytic enzymes will provoke erythematous, bullous or necrotic plaques. Accidental contact with the oral mucosa will be followed by severe pain, considerable edema, sialorrhea and loss of speech [2].

Allergic phytodermatitis

Plant allergens are produced in amounts that vary with the climatic conditions, species and growth cycle of the plants that synthesize them. They are more often complex mixtures of terpenes, esters, aromatic and aliphatic alcohols, resorcinols or hydroquinones. The end result of cutaneous exposure is a delayed hypersensitivity reaction triggered by the release of pro-inflammatory cytokines by previously sensitized T lymphocytes.

Catechols produced by plants of the Anacardiaceae family are among the most potent allergens. They are composed of a benzene ring that bears a hydroxyl radical at positions 1 and 2, and a 15- or 17-carbon aliphatic side chain at position 3. Allergenicity increases in proportion with the length of the side chain and the number of double bonds that it contains. These penta- or heptadecylcatechols are collectively known as urushiol, a name derived from the Japanese word “kiurushi” that designates the sap of the lacquer tree [3]. In sensitized individuals, the first lesions may appear as quickly as 8 to 12 hours after contact, increase in number over many days and persist for a few weeks. Intensely pruritic, they initially present as irregular or linear urticarial plaques that rapidly become studded with vesicles and bullae, at times hemorrhagic. They favor uncovered areas but can appear on the genitals or the trunk through hand transfer of the plant sap. A papulo-vesicular dermatitis without characteristic streaks will involve the anterior aspect of the body when exposure occurs from plant particles forcefully projected by a weed strimmer. Smoke and ashes from burning plants can cause airborne contact dermatitis. In all types of exposure, the morphology of the lesions is eczematous, but a clinical pattern of erythema multiforme is sometimes seen.

Among Asteraceae (formerly known as Compositae), a family that harbors more than 25,000 species all over the world, are found weeds as well as ornamental and edible plants. They all synthesize sesquiterpene lactones (SLs), of which more than 800 are potentially allergenic. These 15-carbon cyclical molecules become even more allergenic if they bear an α-methylene radical on their lactone ring. Sesquiterpene lactone dermatitis mostly affects adult outdoor workers. Pruritic and excoriated, it is located on exposed areas and mimics photodermatitis, with the difference that upper eyelids, the submental area and retroauricular regions are involved. Initially seasonal, it gradually becomes perennial. In its chronic stage, massive lichenification confers a leonine aspect on the face. True persistent photosensitivity at times supervenes, but its etiology remains mysterious because SLs are not photosensitizing [4].

Alstroemeriaceae share with tulips (family Liliaceae) a common allergen, tulipalin, generated by hydrolysis of the glucoside tuliposide A. Tulipalin, or α-methylene-γ-butyrolactone, is another SL. The dermatitis that it induces is mostly occupational and affects tulip bulb sorters and florists who cut the stems and leaves of Alstroemeria, also known as Peruvian lily. Hyperkeratotic and fissured, the dermatitis is localized on the pulp of the first three fingers and has been called “tulip fingers” [5].

The allergens of exotic woods are quinones or phenols [6]. The same molecule can be present in different species, and cross-reactions among various quinones are not exceptional. Skin lesions tend to be localized when they are secondary to contact with solid objects such as necklaces, bracelets, musical instruments and tool handles [7]. Most often, however, the dermatitis affects cabinetmakers, carpenters, sawyers and sanders whose activities generate large amounts of wood dust. In those cases, the airborne dermatitis involves the eyelids, the face, the ears, the neck and other exposed areas. Very fine dust remains trapped under clothes and gives rise to lesions in intertriginous and genital areas. Quinones are potent allergens that can at times cause generalized erythema multiforme-like dermatitis [8].

Phytophotodermatitis

Some plants manufacture psoralens and furocoumarins that, when applied on the skin, enter cell nuclei and intercalate into DNA strands. Upon activation by ultraviolet light, they form interstrand cross-links, leading to cell death. This reaction is phototoxic and can therefore affect all exposed individuals. Plants responsible for most of these reactions belong to 4 families: Apiaceae, also known as Umbelliferae (carrot, parsnip, parsley, fennel, celery, etc.), Rutaceae (citrus fruit trees, gas plant, rue), Moraceae (fig tree), and Fabaceae or Leguminosae (essentially Psoralea coryfolia, which gave its name to psoralens). The most common presentation is Oppenheim’s meadow dermatitis, which appears within 24 hours of contact with the plant, followed by sun exposure. The lesions, distributed in linear or irregular plaques, look like burns and are often bullous. They heal with dark residual hyperpigmentation that disappears slowly over many weeks. The term “berloque dermatitis” was used initially to describe the pendant-like hyperpigmentation on the sides of the neck that would occur after sun exposure in women who had applied oil of bergamot-containing perfumes. These accidents are rare nowadays because most perfumers, such as Guerlain with his famous Shalimar, no longer use bergamot in their products. These days, clinicians most often see the pigmented spots and blotches caused by citrus fruit juices that splashed or dribbled over uncovered skin on the beach or besides the swimming pool.

Urticarial phytodermatitis

Some plants bear tiny urticating hairs called trichomes along their stems or under their leaves. The calcified tips of these structures break after penetrating the skin and inject a mixture of vasoactive compounds that include histamine, serotonin and acetylcholine [9]. The reaction is immediate and non-immunological, characterized by initial pruritus and wheal formation at sites of contact with the plant. Visible lesions regress within minutes while itch is replaced by stinging dysesthesias worsened by touch, heat and water, that persist 1 to 2 days. Urticaceae grow on all continents and are most often responsible for those annoying but benign symptoms. Some tropical plants can, however, cause more severe reactions.

Plants that cause mechanical irritation: cactaceae

This family of succulent plants comprises about 2000 species, mostly originating from the American continent and well adapted to desert conditions. Formidably equipped against predators, these plants possess long and rigid spines borne on areolae, pincushions covered with tufts of fine, pointed hairs called glochids. Opuntia ficus indica, imported in Israel and grown for its edible fruits, is well endowed with glochids that break easily in the skin, causing a pruritic dermatitis in harvesters of prickly pears that mimics scabies, and is known as “sabra dermatitis”.

Cacti can be small and grown as indoor plants, or gigantic such as the saguaro (Carnegiea gigantea) that grows in the Sonoran desert and is the state plant of Arizona (table 1).
Table 1 Examples of American plants that cause mechanical or chemical irritant contact dermatitis

Plant family	Genus and species	Vernacular name
Cactaceae	Opuntia ficus indica Carnegiea gigantean	Indian fig tree Saguaro cactus
Agavaceae	Agave Americana	Century plant
Apocynaceae	Plumeria rubra, etc.	Frangipani
Araceae	Dieffenbachia picta, etc. Philodendron scandens, etc.	Dumb cane
Euphorbiaceae	Euphorbia pulcherrima	Poinsettia, Christmas flower
Hippomane mancinella	Manchineel tree
Hura crepitans	Sandbox tree
Jatropha curcas	Physic nut tree, piñon
Sapium laurocerasus, etc.	Milk tree, palo de flecha

Plants that cause chemical irritation

Araceae and Agavaceae

Popular indoor plants, Dieffenbachia ssp. and Philodendron ssp. originate from tropical South America. Fanning at the tips of fleshy stalks, the leaves of Dieffenbachias are wide, oval and spotted. Most Philodendrons (philos = to like, dendron = tree) are epiphytic climbers. Their thick and glossy leaves are entire (smooth edged) or profoundly dissected. The blue agave, Agave americana or tequilana, grows mostly in Mexico or Central America. Its thick, rubbery, sword-like leaves, that bear spiny edges, can grow to 2 m in length. They arise close to the ground in a wide rosette pattern (figure 1). Sisal is extracted from the harvested leaves, while the core of the plant is fermented to yield pulque that, after distillation, becomes mescal and tequila. The cells of all plants of these families are loaded with bundles of calcium oxalate crystals called raphides and are cutaneous irritants [10]. Dieffenbachias have been named “dumb canes” because chewing their leaves causes severe oral burns accompanied by considerable edema and aphonia. Oxalate crystals are responsible for “el mal de agaveros”, a dermatitis involving uncovered areas in agave harvesters who chop the plant with machetes (table 1).

Euphorbiaceae

This family comprises more than 10,000 species and 300 genera of plants that grow as herbs, shrubs, trees or thorny specimens and are often mistaken for cacti (table 1). Members of this family include croton (Codiaeum variegatum), castor bean plant (Ricinus communis) and the rubber tree (Hevea braziliensis). All of these plants secrete a white, viscous latex rich in irritant phorbol esters.

Among plants native to the American continent, the showy Christmas flower, or poinsettia (Euphorbium pulcherrima) is dazzlingly spectacular with its scarlet bracts surrounding a cluster of tiny yellow flowers. The sap of this common ornamental plant has caused a few cases of irritant contact dermatitis [11]. In addition, it also contains proteins, similar to hevein, that have provoked an anaphylactoid reaction in a latex-allergic individual [12].

The manchineel tree, Hippomane mancinella, is believed to be the world’s most toxic tree! It can grow up to 6 m in height, has a wide span and dense foliage. Its range includes all areas of tropical America, including the Caribbean archipelago and the Florida Everglades, but is limited to a narrow band along the shoreline and beaches. Its bark, sap, leaves and apple-like fruits are all corrosive and cause severe burns with considerable edema, blisters and necrosis. Ingestion of the fruit can be lethal, and blindness can follow ocular contact. Severe dermatitis rewards those careless enough to seek shelter from showers under its foliage, as rainwater dripping from the leaves becomes laden with toxic alkaloids [13].

The sandbox tree, Hura crepitans, also known as monkey pistol and monkey no climb, is another remarkable tree, native from Guyana and widely distributed in Central and South America. Easily reaching 20 m in height, its grayish trunk is covered with sharp spines that make climbing impossible. Its caustic and vesicant sap contains sterols and huratoxin, a daphnane diterpene. The toxin is present in all parts of the plant, including the commercially harvested wood and the fruit, a pumpkin-like capsule divided in crescent-shaped pods. When ripe, the fruit explodes with a loud bang, projecting seeds for a distance up to 60 m.

The genus Sapium includes many species that inhabit a wide area that extends from Southern California down to Patagonia. Some of these plants, such as Sapium laurocerasus, glandulosum, biloculare (milk tree, palo de flecha) etc., secrete a caustic latex with which native Amerindians poisoned the tips of their arrows. They also used to throw pieces of bark in the water to kill fish.

Jatropha curcas, the physic nut tree, or piñon in Spanish, grows between 1 and 8 m in height. Graced with broad leaves, it bears a large number of fruits. From its seeds is extracted an excellent biofuel. The active ingredient of its corrosive latex is 12-deoxy-16-hydroxy-phorbol ester.

Apocynaceae

The seven species of frangipani, Plumeria rubra, etc, originate from Mexico, Central America, and Venezuela (table 1). Related to the oleander, this shrub with lanceolate leaves can sometimes grow as a tree and reach 12 m in height. It displays clusters of numerous, fragrant white or pink flowers that are used to make leis in the Hawaiian archipelago. The toxic alkaloids of its caustic latex, similar to that of Euphorbiaceae, have not been identified.

Plants that cause allergic sensitization

Anacardiaceae

The genus Toxicodendron is well distributed across North America (table 2). It comprises five species, of which one, Toxicodendron radicans, is subdivided in nine subspecies [14]. Plants of this genus are responsible for 90% of all cases of phytodermatitis in North America. It is estimated that 50% of North Americans are sensitized to their usushiol.

Poison ivy is represented by two species, Toxicodendron rydbergii and Toxicodendron radicans. The former is a shrub that can reach a height of 1 m while the latter is a climbing vine that can reach a height of 30 to 40 m by climbing trees. Their leaves, initially small and coppery, enlarge to about 10 to 30 cm as they ripen to a glossy green shade. Each leaf is divided in three leaflets linked by a long central and two short lateral stalks (figure 2). This constant botanical characteristic renders identification of the plant rather easy. The edges of the leaflets may be smooth, finely serrated or slightly notched. Small, white flowers are borne on panicles in the axilla of the twigs.

Poison oak is found in the United States. The species Toxicodendron diversilobum grows on the West coast while Toxicodendron toxicarium occurs on the East coast and the Mississippi valley. Both species possess deeply notched leaflets that resemble oak leaves.

Poison sumac, Toxicodendron vernix, is a tree that grows to 5 to 10 m in height. Its distribution is limited to wet and boggy areas around lakes. Its ovoid or lanceolate leaflets, 5 to 10 cm in length, are grouped on each side of a long stalk in odd numbers of 7 to 13.

The cashew nut tree, Anacardium occidentale, is a 10 to 15 m tree with thick ovate leaves. Its fruit, the cashew apple, is reddish when ripe and contains a kidney-shaped nut within a three-layered pericarp. The middle layer is filled with a brown oil rich in cardol and anacardic acid, chemically related to urushiol and equally allergenic. This oil is commercially used in the manufacture of brake and clutch linings. The nut itself is hypoallergenic but, when contaminated by cashew oil, its ingestion can provoke systemic contact dermatitis in previously sensitized individuals. The clinical manifestations of this condition include a recall of prior Toxicodendron dermatitis, or the baboon syndrome [15, 16].

The mango tree, Mangifera indica, is found not only in India, but also in most of tropical and sub-tropical America. This tall tree (6 to 30 m) gives the world’s most consumed tropical fruit. Its numerous flowers are borne on terminal panicles at the tip of its branches [17]. The allergens, urushiol and cardol, are present in all parts of the tree and in the skin of the fruit. The dermatitis affects harvesters, and also the unwary, who bite directly into unpeeled mangoes and subsequently develop a severe perioral eczema with marked edema, vesiculation and oozing.

Schinus terebinthifolius, the Brazilian pepper tree, is also known as Florida holly because it was imported from Brazil to Florida as an ornamental plant. It rapidly became invasive in its new habitat and is now the most common cause of phytodermatitis in that State. A tall shrub or a small tree measuring 2 to 10 m in height, it displays compound leaves separated in 3 to 13 leaflets. The small fruits, grouped in dense clusters, take on a shining red color when ripe.

The three species of Lithraea (litre in Spanish, aroeira in Portuguese), Lithraea molleoides, braziliensis and caustica, all originate from the Amazon basin. Their wood is commercially exploited even though it can cause severe phytodermatitis. Cross-reactions among the three species and with other Anacardiaceae is likely, given that they all produce penta- and heptadecylcatechols [18].

Among the twenty species of Comocladia that populate Central America and the Caribbean region, seventeen, including Comocladia glabra, dentata, engleriana, mollifolia, etc., can sensitize and cause dermatitis after contact with their wood or foliage. The same is true of Metopium toxiferum, or poisonwood, that grows in Florida and South America, and secretes an urushiol-rich sap. Smoke that arises from burning its wood can cause airborne dermatitis.
Table 2 Examples of American plants and trees that cause allergic phytodermatitis

Plant family	Genus and species	Vernacular name
Anacardiaceae	Anacardium occidentale	Cashew nut tree
Comocladia glabra, etc.	Carrasco, maidenplum
Lithraea molleoides, etc.	Litre, aroeira
Mangifera indica	Mango tree
Metopium toxiferum	Poisonwood
Schinus terebinthifolius	Brazilian pepper tree, Florida holly
Toxicodendron radicans, etc.	Poison ivy
Toxicodendron toxicarium, etc.	Poison oak
Toxicodendron vernix	Poison sumac
Alstroemeriaceae	Alstroemeria aurantiaca	Peruvian lily
Ambrosia trifida, etc.	Tall ragweed
Asteraceae	Echinacea purpurea	Purple coneflower
Erigeron Canadensis	Fleabane
Parthenium hysterophorus	Congress grass, Santa Maria feverfew
Bignoniaceae	Paratecoma peroba.	Peroba do campos, ipe peroba
Tabebuia avellanedae	Lapacho, ipe
Cupressaceae	Thuja plicata	Western red cedar
Fabaceae	Bowdichia nitida	Sucupira
Dalbergia cearensis	Kingwood
Dalbergia nigra	Brazilian rosewood, jacaranda
Dalbergia retusa	Nicaraguan rosewood, cocobolo
Dalbergia stevensonii	Honduras rosewood
Machaerium scleroxylon	Pao ferro, jacaranda
Meliaceae	Swietenia macrophylla	American mahogany
Rutaceae	Zanthoxylum flavum	Yellowheart, West Indian satinwood

Asteraceae

The family Asteraceae is the largest on Earth after Orchidaceae. Most plants bear numerous tubular florets grouped on a floral disk supported by an involucre, a whorl of specialized bracts. The florets of the outer row often display fused petals called rays. The majority of these plants are ubiquitous, but some are indigenous to the American continent. Foremost among them are the common and tall ragweeds, Ambrosia artemisiifolia and Ambrosia trifida. These plants are doubly noxious because they contain phytodermatitis-producing SLs, but also produce a troublesome pollen. The plants pollinate by wind dispersion instead of through insects. The very fine pollen particles become airborne between mid-August and the first frosts. Their protein fractions provoke IgE-mediated rhinitis and asthma, illnesses that affect millions of North Americans every year.

The coneflower, Rudbeckia purpurea, is a robust and erect plant that can reach 1.3 m in height. Its brownish flowers are clustered on a rough, highly arched floral disk. The peripheral purple rays are slanted downwards, giving the whole inflorescence the aspect of a badminton cock. A decoction of the plant is said to possess antidepressant and immunostimulant effects that controlled studies have failed to demonstrate.

Erigeron Canadensis, or fleabane, originates from Canada and was introduced in the botanical garden of the city of Blois in 1655. Its seeds are easily propagated by wind and the plant now grows all over Europe. An erect plant with straight stems bearing narrow lanceolate leaves, it produces numerous yellow tubular florets. In hybrid ornamental varieties, delicate lilac-colored rays border the yellow floral disk (figure 3).

Parthenium hysterophorus is an invasive annual plant from the North American Southwest, where it thrives in arid or semi-arid soils. It is known under various vernacular names such as feverfew, wild wormwood, dog-flea weed, white top, etc. It is an erect plant, up to 80 cm tall, and its hairy stems become woody with aging. Its drooping lanceolate leaves are profoundly dissected [3]. The small and numerous tubular florets are creamy white and grouped by 4 or 5 on panicles at the tips of the stems. Allegenic SLs, concentrated in pollen, flowers and leaves, predominantly include parthenin and ambrosin, and, to a lesser extent, hysterin, hymenin and tetraneurin A. Sensitization is preferentially airborne, either by pollen, or by trichomes and dried particles carried by the wind over great distances. In 1956, the plant was accidentally introduced in the Indian city of Poona through a shipment of American wheat [19]. It rapidly propagated through the country, displacing indigenous species, and continues to affect a dense, mostly rural population. It is believed to cause 40% of all cases of allergic contact dermatitis in this country, and is thus called “the scourge of India” [20]. Its other moniker, “congress grass”, is of more uncertain origin: it may describe the similarity of its white flowers with the traditional cap of members of the Indian Congress, or refer to the American bill of Congress that authorized the 1956 wheat shipment [4, 21].

Exotic woods

Exotic woods from the American continent have been responsible for numerous cases of allergic contact dermatitis and respiratory tract hypersensitivity. Most inhabit tropical regions and are subdivided in many families (table 2). Their allergenic quinones often share similar chemical structures and cross-reactions are common among species, even if they belong to different families. In addition, primin being a quinone (2-methoxy-6-pentylbenzoquinone), cross-reactions between Primula obconica and exotic woods are not rare [3].

Among Fabaceae (Leguminosae), the genus Dalbergia numbers more than 300 species. Their wood is valued by cabinetmakers because of its hardness and its beautiful reddish color laced with dark streaks. American species include Nicaraguan rosewood, or cocobolo (Dalbergia retusa Hemsl.), Brazilian rosewood or jacaranda (Dalbergia nigra Allem.) and Honduras rosewood (Dalbergia stevensonii). These trees are 15 to 25 m tall and were named “rosewoods” because of their characteristic odor when cut. Dalbergia cearensis, or kingwood, is called “bois de violette” in French, not because of its smell but because of its peculiar color. The allergenic quinones of the Dalbergia genus are called dalbergiones. R-4-methoxydalbergione and S-4’-hydroxy-4-methoxydalbergione are common to all species, while obtusaquinone occurs only in cocobolo.

Two other Fabaceae, pao ferro, also called jacaranda (Machaerium scleroxylon Tul.), and sucupira (Bowdichia nitida Spruce) grow in South America and produce quinones that cross-react with those of Dalbergia ssp. Their allergens are R-3,4-dimethoxydalbergione and 2,6-dimethoxybenzoquinone, respectively [3]. The latter is equally present in an African species, the African dry zone mahogany (Khaya senegalensis) that belongs to the family Meliaceae, which also includes the American mahogany, Swietenia macrophylla. The wood of the latter species, used in cabinetmaking and shipbuilding, has provoked many cases of allergic contact dermatitis, but its allergens have not been identified.

Bignoniaceae are represented on the American continent by the genera Tabebuia and Paratecoma. Among the 20 or so species of the former, the better known is undoubtedly the Brazilian Tabebuia avellanedae (Griseb.) Lor., colloquially named lapacho or ipe. Brazilian altogether, the genus Paratecoma has only one species, Paratecoma peroba Kulm., peroba do campos or ipe peroba. Species of the two genera secrete an oil rich in allergenic quinones. The allergens are probably deoxylapachol, lapachenole and, to a lesser extent, lapachol [3]. Cross-reactions may occur with teak (Tectona grandis) of the Verbenaceae family, which contains the same allergenic molecules.

Zanthoxylum flavum, also called Fagara flava, yellowheart or West Indian satinwood, hails from Florida and the Caribbean islands. The scent of its flowers is very odorant and its wood is used to make small objects. Cases of contact dermatitis have been reported, but the allergens remain unknown. The plant belongs to the Rutaceae family, therefore reported reactions could have been phototoxic.

In more Nordic climates, growing on the American and Canadian West coast, Thuja plicata D. Don., or Western red cedar, has caused cutaneous sensitization and professional asthma among sawmill workers. The allergens found in the rot-resistant wood of this giant member of the Cupressaceae family are once again quinones: thymoquinone, methylbenzoquinone and β-thujaplicin.

Phototoxic plants

The range of Cneoridium dumosum is limited to Southern California and the Mexican Baja California (table 3). A member of the Rutaceae family, this plant therefore produces psoralens. Called bull rue or bush rue, this low-lying shrub is well adapted to semi-arid conditions. Its white flowers are delicately scented and its red berries are edible. Multiple cases of phytophotodermatitis have been recognized, and two have been published: both victims had hiked during a sunny day in an area where the plant grew profusely [1, 22].
Table 3 Examples of American plants that cause phytophotodermatitis and contact urticaria

Plant family	Genus and species	Vernacular name
Rutaceae	Cneoridium dumosum	Bull rue, bush rue
Euphorbiaceae	Acidoton urens Cnidosculus stimulosus	Mountain cowitch Spurge nettle, bull nettle, finger rot
Hydrophyllaceae	Phacelia crenulata, etc. Wigandia caracasana	Cleftleaf wild heliotrope, scorpion weed Mala mujer
Loasaceae	Blumenbachia hieronymi, etc. Caiophora coronata, etc.	Sting lily Ortiguilla
Malpighiaceae	Malpighia fucata	Olaya, palo bronco
Urticaceae	Laportea canadensis	Wood nettle

Urticarial plants

Among true nettles, or Urticaceae, wood nettle (Laportea Canadensis) is indigenous to Eastern Canada and United States (table 3). It reaches a height of 1.5 m and, contrary to other nettles, its leaves are not lanceolate but ovate or cordate (figure 4). Urticating hairs are easily visible along its stems.

Some Euphorbiaceae are not only irritant but can also induce non-immunological contact urticaria. The mountain cowitch, Acidoton urens, is widely distributed in the Caribbean area as well as in Central and South America. Another plant, Cnidosculus stimulosus, favors the Southeastern regions of the United States, bears small white flowers and trilobed leaves, and grows to 30 to 60 cm. It is also known as spurge nettle, bull nettle or finger rot, this last name probably describing what happens to the fingers of the unwary who touches it: he will retain for a long time the burning memory of the unwanted serotonin injection from the plant trichomes.

The genera Blumenbachia (12 species) and Caiophora (56 species) belong to the family Loasaceae. These plants are native to Central and South America. They grow as weeds, shrubs or small trees, and bear strange flowers with curious designs. All harbor strongly urticating hairs.

Growing in Puerto Rico where it is called olaya or palo bronco, Malpighia fucata blongs, as its name implies, to the Malpighiaceae family. This shrub or small tree (6 m) bears its urticating hairs on the undersurface of its ovate leaves.

Most plants of the Hydrophyllaceae family come from Central America or the Southwestern United States. Wigandia caracasana Kunth, or “mala mujer”, is said to be one of the most virulent nettles in Panama. Usually growing as a small shrub, it can sometimes reach the proportions of a tree bearing large ovate leaves. Its purple or blue, bell-shaped flowers are very pretty. Its urticating hairs are loaded with quinones and prenylated phenols [23]. Belonging to the same family, the genus Phacelia is widely distributed throughout the Western United States. Many species are urticating, and some cause a vesicular allergic dermatitis. In these cases, the allergen is thought to be a geranylhydroquinone.

Conclusion

This brief overview covers only the essential aspects of the cutaneous toxicity induced by plants of the New World. Its flora is so diverse, especially in its tropical regions, that it is impossible to be all-inclusive in a few pages. Man and plants travel, however, and the victims of accidental contact with noxious plants from the Americas may be seen in European clinics [24, 25]. I hope that this short essay will help the clinician to handle plant contact dermatitis that may be a major concern for patients, especially in professional settings [26].

Acknowledgements

Financial support: none. Conflict of interest: none.

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