Idioblasts In Stinging Nettle

Plant-Induced Dermatitis (Phytodermatitis)

Skin-associated complaints are the most common form of plant poisoning reported to Poison Control Centers. Dermatologists often see patients with complaints directly or indirectly related to plant exposures (Table 4). For example, outdoor workers may directly develop dermatitis from the toxin-laden pollen from the various plants in the family Compositae (Asteraceae) (e.g., chrysanthemums, which contain sesquiterpene lactones). Indirectly, the use of perfume or other lotions that contain plant derivatives may produce dermatitis in an unsuspecting user.

Plant-induced dermal disorders are frequently categorized into several groups to generalize their clinical effects and management, but plant-specific mechanisms and therapies may exist:

  1. Mechanical irritants
  2. Chemical irritants
  3. Allergens
  4. Phototoxins
  5. Pseudophytodermatitides

The majority of these complaints are managed by primary care physicians or dermatologists. This section provides a brief overview of the problem and should be supplemented by the use of a reference text or the advice of a dermatologist. Poison Control Centers, physicians, botanists, and toxicologists are frequently confronted by situations in which a plant is implicated in the formation of a rash, suggesting that the ability to recognize plant-induced dermatitis is important for all.

Mechanical Irritants

Mechanical injury, sometimes called toxin-mediated urticaria, is generally induced by plants with obvious physical characteristics that directly injure the skin, such as the barbs of aloe or the trichomes of stinging nettles (Urtica dioica) (Table 5). In the latter case, the stingers are fragile hypodermic syringe-like tubules that contain a mixture of irritant chemicals which are injected into the skin after the trichome breaks the dermal barrier. Following exposure, patients rapidly develop short-lived wheals with intense pruritis. Less-obvious exposures occur when the idioblasts found in common houseplants such as Dieffenbachia

TABLE 4. Representative Plants Causing Contact Dermatitis

Botanical name

Common name

Amaryllidaceae Narcissus species

Anacardiaceae See Table 8


Asimina triloba (L.) Dunal


Allamanda cathartica L. Nerium oleander L. Araceae

See Table 6 Araliaceae

Hedera canariensis Willd. H. helix L.


Aristolochia elegans M.T. Mast A. gigantea Mart. & Zucc. non Hook. A. grandiflora Swartz Asclepiadaceae See Table 7


Campsis radicans (L.) Seem.


Ananas comosus (L.) Merrill


Sarcobatus vermiculatus (Hook.) Torr.


Rhoeo spathacea (Swartz) Stearn (= R. discolor (L'Her.) Hance)

Setcreasea pallida Rose cv. 'Purple Heart' (= S. purpurea Boom)

Daffodil, Narcissus, Jonquil


Allamanda, Canario Oleander

Algerian Ivy English Ivy

Calico Flower Pelican Flower

Trumpet Creeper



Moses-in-a-Boat, Oyster Plant Purple Queen

Compositae (Asteraceae) Ambrosia species Artemisia species Aster species Chrysanthemum species

Erigeron species

Franseria acanthicarpa (Hook.) Coville (= Ambrosia acanthicarpa Hook.) Gaillardia species Helenium autumnale L. H. microcephalum DC. Iva species Lactuca sativa L.

Oxytenia acerosa Nutt. (= Iva acerosa (Nutt.) R.C. Jackson)

Ragweed Mugwort Aster, Daisy Chrysanthemum, Daisy, Feverfew, Marguerite Fleabane Poverty Weed



Marsh Elder Lettuce Copper Weed

TABLE 4. Representative Plants Causing Contact Dermatitis, Continued Botanical name

Common name

Parthenium argentatum Gray P. hysterophorus L.

Rudbeckia hirta L. (=R. serotina Nutt.) Soliva pterosperma (Juss.) Less. Tanacetum vulgare L. Tagetes minuta L. Xanthium species See Table 9 Convolvulaceae

Dichondra repens J.R. & G. Forst.

Cornaceae See Table 5

Euphorbiaceae Hura crepitans L.

Ricinus communis L.

See Table 7


Dicentra spectabilis (L.) Lem. Ginkgoaceae Ginkgo biloba L.

Gramineae (Poaceae) Oryza sativa L. Panicum glutinosum Sw. Secale cereale L.


Phacelia campanularia Gray P. crenulata Torr. ex S. Wats.

P. minor (Harv.) Thell. ex F. Zimm. (= P. whitlavia A. Gray;

Whitlavia grandiflora Harv.) P. parryi Torr.

P. viscida (Benth. ex Lindl.) Torr. See Table 5

Juglandaceae Juglans nigra L.

Leguminosae (Fabaceae)

Prosopis glandulosa Torr. (= P. juliflora (Sw.) DC. var glandulosa

(Torr.) Cockerell) See Table 5


Allium cepa L. A. sativum L. Hyacinthus species Tulipa species

Guayule Parthenium Black-eyed Susan Spurweed Tansy

Mexican Marigold Cocklebur

Monkey Pistol,

Sandbox Tree, Javillo Castor Bean,

Higuereta, Ricino

Bleeding Heart


Maidenhair Tree


Sticky Grass Rye '

California Bluebell Scorpion Flower, Scorpion Weed Whitlavia

Black Walnut Mesquite

Onion, Cebolla Garlic, Ajo Hyacinth Tulip

TABLE 4. Representative Plants Causing Contact Dermatitis, Continued Botanical name

Common name


Phoradendron serotinum (Raf.) M.C. Johnst. (= P. flavescens (Pursh) Nutt.)


Magnolia grandiflora L.


Maclura pomifera (Raf.) C.K. Schneid. See Table 9


Eucalyptus globulus Labill.


Cypripedium species

Palmae (Arecaceae) See Table 6


Abies balsamea (L.) Mill.


Primula farinosa L. P. obconica Hance


Grevillea banksii R. Br. G. robusta A. Cunn.

Ranunculaceae See Table 7


Agrimonia species

Rosa odorata (Andr.) Sweet

See Table 9

Rutaceae See Table 9


Hydrangea species


Lycopersicon esculentum Mill. Solanum carolinense L. S. tuberosum L.

Thymelaeaceae See Table 7


Ulmus glabra Huds. U. procera Salisb.

Umbelliferae (Apiaceae) See Table 9

(American) Mistletoe

Magnolia, Bull Bay Osage Orange

Blue Gum, Eucalyptus Lady's Slipper

Balsam Fir

Birdseye Primrose Primula, German Primrose

Kahili Flower Silk(y) Oak

Agrimony Tea Rose


Tomato Horse Nettle Potato

Wych Elm, Scotch Elm English Elm

Urticaceae See Table 5

TABLE 4. Representative Plants Causing Contact Dermatitis, Continued Botanical name Common name


See Table 6 Zygophyllaceae

Larrea tridentata (Sesse & Moc. ex DC.) Creosote Bush

Coville (= L. glutinosa Englem.)

TABLE 5. Representative Plants with External Irritant, Stinging Hairs, or Detachable Needles Botanical name

Common name


Opuntia species (e.g., O. microdasys (Lehm.) Pfeiff.) Cannabaceae

Humulus lupulus L. Cornaceae

Cornus sanguinea L. Euphorbiaceae Acidoton urens Sw. Cnidoscolus chayamansa McVaugh C. stimulosus (Michx.) Engelm. C. texanus (Muell. Arg.) Small C. urens (L.) Arth. (= Jatropha urens L.) Dalechampia scandens L. Platygyne hexandra (Jacq.) Muell. Arg. Tragia volubilis L. Hydrophyllaceae

Phacelia imbricata Greene P. malvifolia Cham. Wigandia caracasana H.B.K. W urens (Ruiz & Pav.) H.B.K. Leguminosae (Fabaceae) Lupinus hirsutissimus Benth. Mucuna deeringiana (Bort) Merrill (= Stizlobium deeringianum Bort) M. pruriens DC. (= Stizlobium pruriens

(L.) Medik.; Dolichos pruriens L.) M. urens (L.) DC.


Malpighia polytricha A. Juss. M. urens L. Sterculiaceae

Sterculia apetala (Jacq.) Karst. (in exposed fruit) Urticaceae

Laportea aestuans (L.) Chew (= Fleurya aestuans (L.) Gaud.)

L. canadensis (L.) Weddell

Urera baccifera (L.) Weddell

Urtica dioica L.

Bunny Ears, Prickly Pear Hops

Bloodtwig Dogwood

Mountain Cowitch Chaya

Tread Softly, Bull Nettle

Liane Gratte Pringamosa Pringamosa

Stinging Phacelia

Stinging Lupine Velvet Bean

Cowhage, Cowitch, Pica-Pica, Pois Gratté, Vine Gungo Pea

Bejuco Jairey, Ox-Eye Bean, Yeaux Bourrique, Torteza

Touch-Me-Not Cowitch Cherry

Pica-Pica Wood Nettle Ortiga Brava Stinging Nettle Stinging Nettle spp. injure the skin. These idioblasts contain both needle-like calcium oxalate crystals (mechanical irritants) in a soup of irritant chemicals (chemical irritants; Table 6). The crystals are forcibly injected into the skin or mucosa following mechanical stimulation of the idioblasts. Depending on the anatomical location of the crystal deposition, the clinical effects can be minor (e.g., skin), consequential (e.g., eye), or rarely lethal (e.g., airway). Treatment is generally supportive and symptomatic in nature. Skin involvement with a mechanical irritant should be treated with demulcent cremes, ice, and analgesics, and perhaps removal of the offending agent if appropriate. Ocular involvement is similarly managed with symptomatic care that may include ocular irrigation and systemic analgesics, and most exposures should prompt consultation with an ophthalmologist. Oropharyngeal exposures mandate rigorous attention to the airway, and patients may require corticosteroids to limit pharyngeal swelling. Endoscopy is likely indicated in any patient with clinical findings consistent with airway involvement (e.g., dysphonia, dysphagia, stridor).

Chemical Irritants

Chemical irritants differ from the mechanical irritants in that they produce their clinical effects on the basis of a physicochemical quality of the toxin rather than through overt mechanical means (Table 7). Some of these toxins may be introduced along with mechanical irritants, as already noted. Chemical irritants may be directly irritating on the basis of pH or other chemical effects (e.g., solubility), mimicry of an endogenous compound (e.g., acetylcholine or histamine), enzymatic damage, or activation of inflammation. These agents may be protoxins that require metabolic transformation to produce the ultimate toxin [e.g., sinigrin in Brassicaceae (mustard) plants forms allyl isothio-cyanate]. Chili peppers (Capsicum spp.) contain capsaicin, which induces the release of stored neurotransmitter from sensory neurons (substance P) and produces the deep aching pain characteristic of the "Hunan hand" syndrome, for example. Treatment of chemical-induced irritation includes decontamination by thorough washing of the affected area, analgesics, and symptomatic care.


Although any type of allergic response may occur, a type IV, or delayed, hypersensitivity response, also known as allergic contact dermatitis (ACD), is by far the most common caused by plants (Table 8). Many occupations, such as florists, gardeners, and even outdoor workers uninvolved with plant work, are at risk for the development of ACD. Among florists, exposure to Peruvian lily,

TABLE 6. Representative Plants Containing Irritant Raphides

Botanical name

Common name


Alocasia species (e.g., A. macrorrhiza (L.) G. Don)

Anthurium andreanum Linden

Arum italicum Mill.

A. maculatum L.

Caladium bicolor (Ait.) Venten.

Calla palustris L.

Colocasia species (e.g., C. esculenta (L.) Schott

Dieffenbachia species

Epipremnum aureum (Linden & André)

Bunt. (= Pothos aureus Linden & André; Raphidophora aurea (Linden & André) Birdsey; Scindapsus aureus (Linden & André) Engl. & K. Krause)

Philodendron scandens C. Koch & H. Sello ssp. oxycardium

(Schott) Bunt. P. selloum C. Koch Palmae (Arecaceae)

Caryota mitis Lour. Vitaceae

Parthenocissus quinquefolia (L.) Planch. P. triscuspidata (Siebold & Zucc.) Planch.

Elephant's Ear, Taro Flamingo Lily Italian Arum Cuckoopint Caladium

Water Arum, Marsh Marigold

Elephant's Ear



Heartleaf Philodendron

Fishtail Palm

Virginia Creeper Boston Ivy the very common centerpiece flower, results in ACD because of the common sensitizer tulipin A. The general pathogenesis of this reaction involves a primary exposure to a toxin resulting in an immune response (i.e., sensitization) developing in the affected individual. In some cases, particularly with toxins that are too small to elicit an immune response, the binding of the toxin or its metabolite to an endogenous compound (i.e., as a hapten) results in immunological recognition. Upon reexposure to the same, or closely related, toxin the primed immune system recognizes the antigen (or haptenized endogenous compound), and an immunological response is triggered. The result is a slowly developing (over hours to days) rash, consisting typically of pain, itch, redness, swelling, and blisters localized to the affected area. The sensitizing potential of the various plant-borne toxins varies, but urushiol is among the most potent, and "poison ivy" is among the most frequently encountered sensitizers. Nearly everyone is capable of being sensitized to urushiol, accounting for the reason that "Rhus dermatitis" is given its own moniker. Although the reaction resembles irritant dermatitis, it is more slow to develop and requires previous exposure. Many

TABLE 7. Representative Plants Containing an Irritant Sap or Latex Botanical name

Common name



Acokanthera oblongifolia (Hochst.) Codd (= A. spectabilis

(Sond.) Hook. f.) Plumeria species


Calotropis gigantea (L.) Ait. f. C. procera (Ait.) Ait. f.


Euphorbia cotinifolia L. E. gymnonota Urb. E. lactea Haw. E. lathyris L. E. marginata Pursh E. milii Ch. des Moulins E. myrsinites L. E. tirucalli L.

Excoecaria agallocha L. var. orthostichalus Muell. Arg. Grimmeodendron eglandulosum (A. Rich.) Urb. Hippomane mancinella L. Pedilanthus tithymaloides (L.) Poit.

Sapium hippomane G.F.W. Mey. S. laurocerasus Desf. Stillingia sylvatica Gard. Synadenium grantii Hook. f.

Ranunculaceae Caltha palustris L.

Clematis species (e.g., C. virginiana L.) Pulsatilla patens

Mill. (= Anemone patens L.) Ranunculus species (e.g., R. acris L.)

Century Plant, Maguey

Bushman's Poison, Wintersweet Frangipani

Crown Flower Algodon de Seda

Poison Spurge, Carrasco

Candelabra Cactus Caper Spurge, Mole Plant Snow-on-the-Mountain Crown-of-Thorns Creeping Spurge Pencil Cactus Blinding Tree Poison Bush Beach Apple, Manzanillo Slipper Flower

Hinchahuevos Queen's Delight African Milkbush

Pasque Flower Marsh Marigold Virgin's Bower Buttercup, Crowfoot


Daphne mezereum L. Dirca palustris L.


Leatherwood, Wicopy related urushiol-like compounds from diverse sources [e.g., from mango (Mangifera indica) or cashew nut (Anacardium occidentale)] produce identical reactions in patients sensitized to urushiol.

Diagnosis includes the use of patch testing, in which a single or several known allergens are applied to the skin and a reaction is sought and is

TABLE 8. Anacardiaceae Producing Allergic Contact Dermatitis Botanical name

Common name

Anacardium occidentale L.

Comocladia species (e.g., C. dodonaea (L.) Urban) Cotinus coggygria Scop. (= Rhus cotinus L.) Mangifera indica L.

Metopium toxiferum (L.) Krug & Urban Schinus terebinthifolius Raddi

Toxicodendron diversilobum (Torr. & A. Gray) Greene (= Rhus diversiloba Torr. & A. Gray)

  1. pubescens P. Mill. ( = T. toxicarium (Salisb.) Gillis; Rhus toxicodendron L.; R. quercifolia (Michx.) Steudel)
  2. radicans (L.) Kuntze (=Rhus radicans L.)

T rydbergii (Small) Greene

Cashew, Maranon Guao

Smoke Tree Mango

Poisonwood, Cedro Prieto Brazilian Pepper Tree, Florida Holly Western Poison Oak

Poison Ivy Western Poison Ivy Eastern Poison Oak

Poison Sumac generally confirmatory. Although more advanced testing is available, this is a common initial screen for contact allergens. Occasionally, irritant dermatitis may result and be misinterpreted as ACD, so expert interpretation is needed. The risk of patch screening by this method is that sensitization to any of the tested compounds may occur just from the testing alone, so often the most strongly sensitizing plants are excluded from testing.

The primary therapy, of course, is avoidance of the known allergen. Because this is difficult or impossible in some situations, the use of barrier protection may provide a sufficient impediment to dermal exposure. Barriers include the use of clothing or of barrier creams that can be applied if an exposure is anticipated. Treatment of the ACD once it has occurred is generally symptomatic, with the use of analgesics, antipruritic medications (e.g., hydroxyzine, diphenydramine), occasionally corticosteroids (generally topically applied), and rarely immune modulating agents (e.g., tacrolimus ointment). Desensiti-zation may be attempted for patients with severe reactions or unavoidable exposures.


This is a relatively uncommon clinical entity, in which certain compounds increase the sensitivity of the skin (photodermatitis) to ultraviolet light (e.g.,

TABLE 9. Representative Plants Producing Phytophotodermatitis

Botanical name

Common name

Compositae (Asteraceae)

Achillea millefolium L.

Yarrow, Milfoil, Milenrama

Anthemis cotula L.

Dog Fennel, Mayweed


Ficus carica L.


F. pumila L.

Creeping Fig, Creeping Rubber



Agrimonia eupatoria L.



Citrus aurantiifolia (Christm.) Swingle


Dictamnus albus L.

Gas Plant, Burning Bush

Pelea anisata H. Mann


Ruta graveolens L.

Rue, Ruda

Umbelliferae (Apiaceae)

Ammi majus L.

Bishop's Weed

Anthriscus sylvestris (L.) Hofmann

Daucus carota L. var. carota

Queen Anne's Lace, Wild Carrot

Daucus carota var. sativus Hoffm.


Heracleum lanatum Michx.

Cow Parsnip

H. mantegazzianum Sommier & Levier

Giant Hogweed, Wild Rhubarb

H. sphondylium L.

Cow Parsnip

Pastinaca sativa L.


sunlight) (Table 9). Classically, psoralen, a furocoumarin derived from celery and other plants, enters the skin either directly by contact or via the systemic circulation following ingestion. In the skin, the psoralen is activated by sunlight to produce oxidant skin damage, which manifests as burning, erythematous skin in sun-exposed areas, which may blister severely (i.e., sunburn). Interestingly, psoralens may be administered therapeutically to patients with dermal disorders such as psoriasis to increase the sensitivity of the skin to therapeutic ultraviolet light.


Given the ubiquity of plants and the constant interaction of humans with plants, many dermatological disorders are often attributed to a contemporaneous plant exposure. However, mimics of plant dermatitis are common and may be missed due to the often-simultaneous nature of the two exposures. For example, pesticides, fungicides, insects, and soil products may each induce dermatitis that is often indistinguishable from one of the foregoing syndromes.

Without intense investigation or advanced medical testing, this link may be missed and the patient advised incorrectly to avoid a certain plant exposure. More consequentially, the patient may not be aware of the dermatitis trigger. The best method by which to distinguish pseudophytodermatitis from phyto-dermatitis is by having the necessary clinical suspicion and attentiveness to the exposure. Even then, this link is often difficult to make.

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