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Skin irritation due to plants
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Problems caused by plants may be mechanical and traumatic. Everyone has at some time had a thorn in their finger. If a splinter is left behind, this foreign body may be a source of infection, sometimes with unusual pathogens such as Sporothrix schenkii (sporotrichosis). Some cacti bear thousands of small sharp spikes, which trigger an uncomfortable and persistent dermatitis. In some plants there is a combination of mechanical and chemical factors. Consider nettles (Urtica dioica). This plant has countless hollow stinging hairs, filled with irritating chemicals such as formic acid. The point of these hairs breaks off after contact and the content enters the skin, with itching and irritation as the result. Many plants contain needle-shaped, razor-sharp crystals of oxalic acid, which can trigger skin irritation. They are rather like microscopic glass splinters. One of the infamous plants is the manchineel (Hippomane mancinella; fam. Euphorbiaceae), a tree that can grow to 15 metres. It bears apple-shaped fruits. The plant contains a greenish resin. Skin contact leads to severe dermatitis. If a drop gets into the eye there is severe conjunctivitis and keratitis, which may even lead to blindness. Eating the fruit can lead to bullous stomatitis, bloody diarrhoea and death. The plant was formerly widespread in the Caribbean, but now is only found sporadically, including in Florida. The plant was once mistakenly considered to be the cause of ciguatera intoxication.
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The resin obtained from the roots and rhizomes of Podophyllum peltatum (American mandrake or mayapple; fam. Berberidaceae or Berberis family) contains podophyllin, a highly irritating substance, used for the treatment of condylomata acuminata (see note in chapter on treponemata). The plant grows in the deciduous forests of Canada and the eastern USA. The raw resin contains podophyllotoxin, which is applied topically to the warts and washed off a few hours later. Any contact with the eyes or mouth must be avoided. If it is swallowed it can trigger severe systemic toxicity. The alkaloids in the plant form the basis for teniposide, an antitumoural agent that is used against testicular, breast and lung cancer.
