Botanical Dermatology

Irritant dermatitis can affect virtually anyone depending on the person's skin barrier function and the potency and duration of the irritant stimulus. Irritant dermatitis ranges from mild erythema to hemorrhagic bullae and necrosis. After several episodes of irritation, a person is often at risk for allergic sensitization. Mechanical irritation accounts for the majority of unpleasant reactions due to plant exposure.²²

MECHANICAL DERMATITIS

Skin penetration by macroscopic spines or thorns causes a papular irritant reaction that may resemble scabies or fiberglass dermatitis. Trichomes or glochids (in cacti) are hairy outgrowths of single cells or small groups of cells that often cause even more severe reactions than the larger thorns.¹⁶ Damage is caused by actual physical penetration into the skin. As with the urticating plants, irritant hairs and thorns defend against plant-eaters.

Many plant families have been implicated. A recent article reviewed many Australian plants that cause mechanical dermatitis.¹⁶ Common ivy (Hedera helix, Araliaceae) possesses many leaf-borne stellate hairs that readily detach as the leaves age. Some members of the family Compositae possess prominent irritating appendages such as thistles (multiple genera) and prickly lettuce (Lactuca serriola). Members of the Boraginaceae contain stiff, sharp hairs on the leaves and stems: borage (Borago officianalis), Amsinckia spp., and Echium spp. Rose hips (the fruit of roses) in members of the family Rosaceae possess irritant hairs used in itching powders (Figure 6). ¹⁵ 

Cacti

Of course members of the family Cactaceae are the most infamous irritants. While cacti possess large spines, the smaller glochids cause more notorious dermatologic problems. Glochids are stiff, barely visible, hairs that grow in tufts on the areolae of cactus pads. They possess self-retaining barbs that make extraction from the skin difficult.²⁴ As an example, Opuntia microdasys ('Polka Dot cactus'), a house and garden favorite, bears disarmingly-appearing fluffy clusters of 100-200 glochids on each of their pads. The related 'Beaver-tail cactus' (Opuntia basilaris) grows naturally in the Mojave and Colorado deserts.

  Figure 2: (Opuntia spp.) Prickly pears with spines and glochids.

Prickly pears, including the species Opuntia ficus-indica (the Indian or Barbary fig), causes 'Sabra dermatitis'. This pruritic, papular eruption occurs among prickly pear pickers and those who unwarily stumble into burglar-proof hedges of this native Mexican plant. The fruit contains the highest concentration of glochids and is a staple part of the diet in the Middle East where the eruption is easily confused with scabies.²⁵ 'Sabra' is from the Hebrew word for 'Native of Israel' and is also colloquially applied to the fruit. ¹⁵

^{Figure 3: Sacbies versus scabies in circle of Hebra which both simulate 'Sabra' dermatitis.}

A notorious member of the family Rubiaceae (Galium aparine) causes 'goose-grass' dermatitis due to the hooked prickles on the fruit, stems, and leaves (Figure 4). The prickles attach fruits (containing seeds) to animals, thus naturally increasing the range of the plant.²⁶

Figure 4: Galium aparine (Catchweed bedstraw). Note the hooked prickles on the fruit, stems, and leaves. While mechanical irritants to man, they help the plant expand its range.

Members of the family Moraceae include figs (Ficus spp.) and mulberries (Morus spp.). Numerous fine, abrasive bristles cover the leaves and fruit of certain species. ¹⁵ Many true grasses of the family Poaceae (Gramineae) possess fine hairs, prickly spikes, and even cutting leaf edges. Australian natives of the family Proteaceae, Genus Grevillea possess terminal, sharp points on their leaves that cause a prickling sensation on contact, as during pruning.

While buttercups of the family Ranunculaceae are remembered more for their irritating chemicals, they also possess fine bristles. Coarse fibers on tulip bulb tunics (tecta) cause the irritant component of 'tulip fingers.' This reaction to tulip bulbs (genus Tulipa, Liliaceae) is primarily an allergic contact dermatitis. ¹⁵

Of course, the well-known rose (Family Rosaceae, Genus Rosa) can cause penetrating injuries. All thorns can cause foreign body granulomata if a portion remains in the skin. Thorns can cause even more severe reactions such as tenosynovitis, chronic arthritis, and exuberant periosteal reactions if implanted deeper into bones and joints.²⁵

Verbascum thapsus (Family Scrophulariaceae), commonly known as 'mullein' or 'flannel-plant', grows throughout the central plains of the United States, but it was introduced from Eurasia.⁹ The woolly hairs on leaves have been used cosmetically for their rubefacient effects.²⁶ Members of religious groups that forbade the use of cosmetics rubbed their upper cheeks with the leaves to create an erythematous glow. In Ancient times, the Romans used the plants bundled together as torches.

Figure 7: Verbascum thapsus

A recent report implicates 'California glory,' Fremontodendron cultivars (Sterculiaceae) as a hazard. This attractive, yellow, flowered shrub is native to California and has become widely available in Europe. Small, irritant hairs shaped like sea urchins cover the shrub, and they can induce a florid mechanical dermatitis.²³

Inoculation of microorganisms

Clostridium tetani can be introduced by spines and thorns, Staphylococcus aureus by blackthorn injury (Prunus spinosus, Rosaceae), and Sporothrix schenckii by grasses, sphagnum moss, and rose thorns. Atypical mycobacteria such as Mycobacterium kansasii (blackberries), M. marinum (cactus spines), and M. ulcerans (spiky tropical vegetation) can also be unwanted plant hitch-hikers that add infectious insult to injury. ^{15 , 16, 24}

Treatment

Many intriguing methods of removing glochids have been recommended. Warm wax, glue, sticking plaster, or cellophane tape is applied to the affected area. Then, the glochids are quickly ripped off. ^{15 , 24} Facial gels and masks have been used similarly.^{24, 25} In perhaps the only controlled study (in rabbits), glochids (of Opuntia ficus-indica) were most effectively removed by first removing the larger clumps with tweezers and then applying glue to the affected area with gauze on top. After the glue dried, the gauze was grasped and peeled off. This resulted in removal of 95% of implanted spines.²⁷