Contact-Poisonous Plants of the World

Contact-Poisonous Plants of the World

©M. Rohde
1988-2006

Introduction

Many plants are highly poisonous when ingested, this is common knowledge. It is remarkable, however, that simply touching certain plants can also be a serious health hazard. Stinging Nettles are rather harmless in this respect, but there are much more dangerous contact-poisonous plants in many parts of the world, especially in the tropics. They can cause severe pain, rashes, blisters or leave scars. Some trees are reported to be so powerful that even raindrops falling from them can irritate the skin. Other plant species can cause blindness through the smoke of burning wood or from rubbing the eyes after touching the leaves.

This document gives a concise overview of those contact-poisonous plants that may be of interest for travellers. The first part briefly introduces the active principles, effects, treatment and geographical distribution. The second part lists about 35 important plant species and describes them in detail.

Information about this interesting subject is usually scattered across many different sources, like scientific works about dermatology or botany, regional field guides, travel literature, or magazines. The rare scientific literature on the subject usually lists thousands of plant species and describes in medical detail the effects on workers who were exposed to the same species for years. Only a small number of those plants is dangerous after an occasional contact, however, which means that information relevant for travellers is hidden amid lots of other data. Available information has been selected and densified to be presented here.

Latest update: 2 October 2006.

Contents

Active Principles
Effects and Measures
Geographical Distribution
Guide to Contact-Poisonous Plants
Reference
Index

Active Principles

Plants make use of different techniques to scare away unwanted visitors. The plants in the guide below are marked with symbols to indicate the techniques:

The most obvious defence mechanism a plant can use is Active principle: mechanical

mechanical, for example prickles, thorns, or barbs. Less visible but also purely mechanical are the sharp edges of certain grasses which can cause unpleasant cuts. Bamboos also belong to the botanical family of grasses, and some bamboo species bear thin bristles on the surface which can penetrate the skin and cause itching and irritation. There is an exceptional lot of mechanically active plants, and although injuries of the skin can cause secondary infections when dirt enters the human body, the mechanical principle is not subject of this document. Only the malicious and widespread Opuntia with its almost invisible, easily breaking off barbs is included in the list below.

The Active principle: chemical

chemical agents of some plants are poisons that can enter the skin without mechanical action. When the sap of some species gets onto the skin surface it can lead to painful skin irritation or irreversible damages. Some species can even cause temporary or permanent blindness if a person touches broken parts of the plant and then the own eyes. Throwing such plant material into a fire can also be dangerous as the smoke can irritate the skin or also lead to blindness. Typical representatives of this principle belong to the Euphorbiaceae family. There are many members of other botanical families, however, that act similar. Another kind of plants like the Upas tree contain sap that is not skin-irritating but can be fatal if a very small amount of it gets into the bloodstream, for example through minor skin injuries.

The Active principle: phototoxic

phototoxic principle occurs in a number of plants of which the best example is the Giant Hogweed. Phototoxic poison acts chemically, but only if the skin is exposed to sunlight at the same time.

A more sophisticated principle is the Active principle: mechanical-chemical

mechanical-chemical one: Plants penetrate the skin of the victim mechanically and introduce a poisonous chemical. The result is often immediate burning sensation of the skin. The best known representative of this kind of plants is the Stinging Nettle. Its closely related species are native to all parts of the world. Most mechanically-chemically acting plants belong to the botanical families Urticaceae and Euphorbiaceae, some of them being much more powerful than the Stinging Nettle. Under an electron microscope, fragile hollow needles become visible with nettle cells at their bases, filled with liquid poison. When touched, the needles break off, leaving oblique tips which can enter the human skin like syringes and release the poison.

Electron microscope image of a stinging hair of Urtica dioica. Image: Y. Sell.

Electron-microscope image of a stinging hair of Urtica dioica. Image: Y. Sell.

Another powerful principle, the Active principle: allergizing

allergizing principle, is found mainly in members of one botanical family called the Anacardiaceae. Infamous examples are the Toxicodendron species native to North America. The first encounter with one of them usually does not have any noticable consequences. But it allergizes the victim, and any subsequent contact can cause gradually more severe skin irritation.

Effects and Measures

After skin contact with one of the plants described here, typical symptoms are burning, redness, itching, swelling, or development of blisters. Chemical contact poisons are often oily substances which do not evaporate and thus can contaminate equipment for years. The poisons can be spread across the skin by scratching or they can get into the eyes when the eyes were rubbed with hands. People do react differently to the poisons and the effect of contact poisons is often increased when a person is exhausted or sweating. Washing with water and soap is generally a good idea. If stinging hairs or bristles are on the skin, they should first be removed with adhesive tape or tweezers. In severe cases a doctor must be consulted.

Geographical Distribution

For travellers, the danger of contact-poisonous plants depends on the region visited. The more harmful allergizing Toxicodendron species are native to North America, while the powerful mechanically-chemically active Dendrocnide species are mostly found in Australia. These are regions with highly efficient prevention and information distribution. However, it is the tropics where the most powerful contact-poisonous plants occur in great variety.

Plants which are not native to a region but have been introduced are marked with an asterisk (*).

North America Cnidoscolus stimulosus Cyanophyta (in water) Heracleum mantegazzianum Laportea canadensis Metopium toxiferum (Southern USA) Toxicodendron spp. Urtica spp.	Europe Cyanophyta (in water) Heracleum mantegazzianum Opuntia spp. (Southern Europe)* Urtica spp.	North and East Asia Cyanophyta (in water) Toxicodendron vernicifluum Urtica spp.	Australia and Oceania Anacardium occidentale* Cyanophyta (in water) Dendrocnide spp. Excoecaria agallocha Laportea spp. Mucuna spp. Opuntia spp.* Urtica spp.
Central and South America Anacardium occidentale Croton ciliato-glandulosus Cyanophyta (in water) Dieffenbachia spp. Excoecaria agallocha Hippomane mancinella Hura crepitans Jatropha urens Metopium toxiferum Mucuna spp. Opuntia spp. Urera baccifera Urtica spp.	Africa Anacardium occidentale* Cyanophyta (in water) Excoecaria agallocha Laportea spp. Mucuna spp. Opuntia spp.* Smodingium argutum Urtica spp.	South and South-East Asia Anacardium occidentale* Antiaris toxicaria Cyanophyta (in water) Excoecaria agallocha Girardinia leschenaultiana Gluta renghas Hippomane mancinella* Holigarna arnottiana Hura crepitans Laportea spp. Melanorrhoea spp. Mucuna spp. Opuntia spp.* Semecarpus spp. Swintonia floribunda Tragia spp. Urtica spp. (in mountains)

Guide to Contact-Poisonous Plants

By family names in botanical order, and alphabetically by scientific names within each family.

Disclaimer: The plant species listed here are toxic. This also applies to others which are not presented here. The information in this document have been compiled with care and accuracy. Correctness cannot be guaranteed, however.

Microcystis spp., Anabaena spp., Lyngbya spp. etc.

Image: environment.act.gov.au
Search Google for images: genus or species

Microcystis spp., Anabaena spp., Lyngbya spp. etc.

Blue-Green Algae

Different families of order Cyanophyta

Synonyms: Cyanobacteria (en), Blaualgen (de)

Occurrence: Worldwide, especially in warm regions, in seawater as well as in freshwater.

Description: These algae sometimes make water seem green in colour or sometimes form a blue-green scum on the surface. A bad (musty-earthy) smell may indicate the presence of these algae.

Effects: Chemical through algal toxins. Marine cyanobacterial dermatitis ("swimmers' itch" or "seaweed dermatitis") is a severe inflammation of the skin that may occur after swimming in water containing blooms of certain species of marine cyanobacteria. The symptoms are itching and burning within a few minutes to a few hours after swimming in an area where fragments of the cyanobacteria are suspended. Visible dermatitis and redness develop after 3 to 8 hours, followed by blisters and deep desquamation.

Measures: Washing of skin and eyes. Corticosteroid ointment. Antihistamine orally may be effective to relieve itching.

Reference: Mitchell/Rook; WHO Guidelines