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Notes on the Anacardiaceae

ANACARDIACEAE is the plant family [class, order, family, genus, species] to which poison-oak belongs. My comments are in brackets.  CB

ANACARDIACEAE, pp.63-97 and 95 in Mitchell and Rook

The 600 species in the 60 genera of this family include plants which probably cause more dermatitis than those in all other plant families combined. Most occur naturally in the tropics.

The resin-canals of the leaves do not open to the surface, and a leaf must therefore be injured to release the allergenic principles. The leaves of some species are covered with fine hairs; if these are broken the irritant sap exudes. The fruit of most species is a drupe with a resinous mesocarp. In some species containing irritant and sensitizing sap, the fruit at maturity may be eaten with safety [Poison-oak is such a plant; it and certain birds, crows for example, have learned to trade food, the mesocarp, for seed dispersal. The greenish immature fruit is protected from premature consumption by corrosive urushiol-containing resin-canals in the mesocarp. As the fruit matures the resin-canals become visible as multiple green ducts running from stem to blossom end. The fruit then turns creamy white and the urushiol turns black, signaling that the fruit is safe to eat.]

The noxious substances in this family are mono- or dihydric phenols or monohydric phenolic acids, having an unbranched C15 C17 C19 partially unsaturated alkyl side chain [Urushiol, the noxious substance in poison-oak, is 8,11dienyl pentadecyl catechol. The side chain is C15 (pentadecyl), with double bonds at the 8-9 and 11-12 positions. The catechol portion of the molecule is 1,2 dihydro-, 3 C15, a dihydric phenol.]

The sap (latex) of each species contains in various proportions a mixture of phenols, some of which may be confined to a single or a few species, and others of which are widely distributed throughout the family. Moreover, chemically distinct substances may be immuno-chemically related, so that cross-reactions are frequent [The noxious substance in other ANACARDIACEAE are:]

Anacardium occidentale, Cashew nut Tree, a native of Brazil

      Cardol: 1, 5 dihydro-, 3 C15 H27 phenol

      Anacardic Acid: 1 monohydro-, 2 carboxyl, 3 C15 H27

Semicarpus heterophylla

                Renghol: 1, 2 dihydro-, 3 C15 H29

Semicarpus vernicifera

Semicarpus travancorica

Holigarna arnottiana

Rhus succedanea


      Laccol: 1, 2, dihydro-, 3 C17 H31

Gluta renghas

      Glutarenghol: 1, 2, dihydro-, 3 C17 H33

Rhus vernicifera




       Urushiol: 1, 2, dihydro-, 3 C15 H27 (3-n-Pentadecyl 8,11-dienylcatechol)

As a general practical principle cross-sensitization must be considered to be general within this family [That is, if one gives you a rash all the others will too.]

The clinical features of the [allergic] dermatitis they causeshare certain distinctive characteristics. [In higher doses urushiol also produces chemical burns.

[First,] In previously unsensitized individuals, exposure is followed by a latent period of about nine to fourteen days before dermatitis appears.

In sensitized individuals, symptoms develop from a few hours to several days after contact, with itching at the sites of contact, usually the fingers, hands, forearms and legs. Papules appear, often in linear pattern, and soon evolve into vesicles or bullae. In severe cases the eyelids, other parts of the face, the genitalia and the trunk maybe affected. Except in regions well protected by hair, oedema may be considerable, and is particularly so in the eyelids and the male genitalia.

In most noxious species the allergen is widely distributed throughout the plant. Digging the soil in which roots are growing may cause dermatitis. The pollen and flowers of Toxicodendron are not allergenic [poison-oak depends on insect pollination; whereas mangoes rely on wind pollination and have irritating pollen] nor is smoke from the burning plant, unless the smoke contains incompletely burned plant material. [Id have to disagree; smoke is evidence that combustion is incomplete, and firefighters have a lot of skin trouble.]

[Chronic plaques of thickened irritated skin develop in some individuals. Most commonly these are at sites of repeated exposure, often a wrist] In dermatitis constitutional symptoms are [un] usual, [except for fever, which occurs whenever the rash is extensive.

Some examples of  ANACARDIACEAE other than poison-oak  are the following:

Anacardium occidentale, the cashew nut tree:

A native of Brazil.

Unprocessed nuts are responsible for dermatitis amongst dock workers who unload them. Downing, JG (1940) Dermatitis from Cashew-Nut Shell Oil J. Indust. Hyg.  22 169. Museum has. 90% of these men kept working, and became more comfortable after three weeks; this is an example of topical hardening. On follow-up examination many of them had thick, cracked skin.

Improperly prepared cashews can cause acute gastroenteritis. Dark-colored cashews should be discarded.

Rengas or Renghas trees:

In the timber trade Rengas and Renghas refer to the heartwoods of ANACARDIACEAE of genera Gluta and Melanorrhoea. The hard durable wood of Melanorrhoea has also been sold as Singapore mahogany.

The heartwood of Gluta is blood red, with darker bands; it is hard, durable, easy to turn, and beautiful. However the sawdust irritates the mouth, nose and throat; and furniture made from it causes dermatitis. The sap slowly penetrates polish, even if the wood is seasoned. Two years of curing and a polyurethane lacquer finish are recommended. The woods are obtainable only on very special order, and the species have been inadequately collected, because of the irritancy of their saps.

One species of Gluta grows in Madagascar; the other twelve species are native to Indomalaysia. The twenty species of Melanorrhoea are from Southeast Asia, the Malay Peninsula and Borneo.

In Malaya acute rengas dermatitis with malaise and sometimes fever is not uncommon in estate laborers, gardeners and woodcutters. The fruit juice causes dermatitis; two companies of soldiers developed dermatitis after wading in water contaminated with their fruits. Occasionally a scantily clad person who scrambled through broken Rengas branches dies from the dermatitis.

Forestry workers are cautious working on these trees because the sap and sawdust causes blistering burns. Sometimes they anoint themselves with sesame oil, in an attempt to avoid the dermatitis. Also the felled trees may be left in the jungle for decay and white ants destroy the sapwood, before they are dragged out.

These are rain trees. It is unwise to sit beneath them because raindrops carry poison from the canopy.

Their white sap turns black on exposure to the air.

The name Rengas is also given to several large jungle trees in three other genera of ANACARDIACEAE, whose sap also turns black. In vernacular rengas, hangus, rangus, ruengas, ingas, angas and ligas are used for trees the sap of which produces sores of the skin. Most of these are Gluta and Melanorrrhoea.

The three species of Lithraea are leading causes of dermatitis in South America. L. caustica is endemic to Central Chile from Coquimbo to Arauco, where it grows on hills and sunny plains. The lumber from this tree, Aroeira or litre, is used for furniture in Brazil and Chile; this furniture remains allergenic, and has sensitized 10% of the population in Southern Brazil. It is dangerous to sleep under the tree because of leaf exudates, especially in summer. The sap is caustic, and the dried wood is allergenic. [Lithrin is the name of the irritant in these plants.]

The forty species of  Mangifera, mango, are natives of Southeast Asia and Indo-Malaysia. Many species are now cultivated, or have become naturalized, in tropical regions around the world because of their importance as a source of food and timber. M. indica produce the grocery store mango.

In Hawaii during its fruiting season it is the leading cause of plant dermatitis. Unlike poison-oak mango pollen is irritant and allergenic; sniffing the blossoms provokes hard sneezing, and wind-blown pollen causes dermatitis. There are allergens in the fruit peel. Also stem sap gets on the peel during harvesting; the resulting spots appear black, bleached or varnished. Biting the unpeeled fruit causes rashes of the face around the mouth; rarely hives and shock occur.

Urticaria and anaphyllaxis are uncommon reactions other ANACARDIACEAE. However degranulation by urushiols is evidence that in many sensitive people there is an immediate component: Shelley, Walter B and Resnik, Sorrel S (1965) Basophil Degranulation Induced by Oral Poison Ivy Antigen Arch Dermat 92 August.

Mangoes are less sensitizing overall than the other sensitizing ANACARDIACEAE, and people growing up in areas where mangoes  grow are less easily sensitized to other ANACARDIACEA (Mitchell).

Mangoes are safe to eat and drink if contact with the peel and stem is avoided.

Metopium toxiferum, poison wood, is widely distributed in Florida and the Caribbean. The glossy leaves of young trees inspire transplantation to gardens, and use in wreaths and bouquets. In Florida the trees often grow so close to pine trees that they escape destruction when the land is cleared. The plant has caused dermatitis when branches brushed clothes on a clothesline.

Many of the 200 Rhus species cause dermatitis. Indians complain the smoke from burning them makes them swell and blister.

R. coriaria is used for dying hides in Southern Europe. R. jugglandifolia is common in Costa Rica, where its local name, hinchador, means the sweller; this shrub is the right height to brush the face and hands. In Africa scratches and pricks from the sharp-tipped twigs of R. pyroides are extremely painful, and burn like fire; hence the name pyroides.

Chinese lacquer is obtained from R. succedanea and R. vernicifera. Tsuta urishi is preferred by the Japanese.

The bark [is cut] just deep enough to reach the wood, and the latex collected in bamboo tubes; or the sap is collected from felled trees, under water. Exposure to air is controlled because air darkens the latex, and dark latex makes inferior lacquer. It was used in China by 2255 BC, and lacquer dermatitis was described by the Chinese in 453 BC.

Lacquering was introduced to Japan about 250 BC. By 701 AD farmers were required to have lacquer plantations, and paid their taxes in lacquer.

The lacquer is hard, waterproof, heatproof, an acid and alkali proof (Anderson). It takes a high shine, which is durable. In Japan it has had myriad uses: embossed wallpaper, furniture, rust proofing cans, canes, lampshades, bracelets, boxes, rifles, bar countertops, bowls, trays, and toilet seats (Mitchell, Anderson, Gillis).

The risk to Japanese tappers is reduced by the fact that they are a special caste of highly skilled workmen. For example they know there is a volatile poison in the freshly collected latex, which they eliminate by stirring it in open vessels for several hours.

All parts of Schinus terrebinthifolius contain a volatile substance, which is most problematic at blossoming time. Sneezing and asthma occur in some people when near the plant.

The ink from Semecarpus anacardium, the Indian marking nut tree, is soluble in alcohol, but not in water. Dhobis, Indian laundry men, mark the insides of shirt collars, bras, belts, vests, and underpants; the mark can be cut out or covered with adhesive plaster.

The black, tarry  residue of Semecarpus australiensis, the tar trees sap, causes severe dermatitis.

Semecarpus cuneiformis is the most common cause of contact dermatitis in the Philippines. It is another of the rain trees; rain dripping off the tree causes dermatitis.

The South African Smodingium arguutum is a potent sensitiizer, in contrast to Mangifera. Its sap is irritant in high concentrations. In lower concentrations it is only an allergen.

Mangoes have irritant pollen; since pollen is particulate, it lodges in the upper respiratory tract and causes sneezing. The substance in Schinus vaporizes, so it gets all the way to the bronchioles, sometimes producing breathing trouble. The precipitation of asthma in only some people may be just irritant; people differ tremendously in their vulnerability to asthma.

Perhaps mangoes are less sensitizing because the allergen is sometimes ingested. That would invoke T8 cloning. [T8 cloning is induce decreases sensitivity; T4 cloning increases sensitivity.]

Poison-oak is not tall enough to frequently lie or walk under. However, since runoff through poison-oak causes dermatitis, and black spots are always present, caution is warranted around the wet plants.

With regard to the variable effects of Smodingium at different concentrations, another example is primin, the allergenic quinone of Primula obonica, common name cyclamens. It is the most commonly sensitizing plant in England, and now commonly sold in the US; 9% of English housewives are allergic to it.  At 1:100 primin is always irritant; at 1:1000 it is often irritant; at 1:10,000 it actively sensitizes; and at 1:50,000 dilution it elicits reactions only in sensitized people.

80% of the plant material that is bad to touch is just irritant. An example is oxalic acid crystals in the barbs of stinging nettle. The metabolic cost of producing and storing that much oxalic acid is large. The metabolic cost of producing a few molecules of primin, dispensed by fragile hairs; or urushiol, is less.

To summarize, many ANACARDIACEAE have evolved resin canals, containing 3-n-C15-17 phenols. These phenols turn into quinines when exposed to air. These quinones are haptenes, capable of inducing delayed, cell-mediated allergic reactions in molecular quantities.

All content copyright Dr. Curt Beebe. Please do not use without permission.