Mechanical Irritant Contact Dermatitis
MICD is reportedly the most common plant dermatosis. Mechanical irritation is frequently the result of physical trauma caused by direct contact of the skin with the irritant. The mechanical irritant is usually anatomically sharp and can scratch, impale, or lacerate the skin, inducing wounds and excoriations. The resultant MICD can be papular or vesicular and may be pruritic. Implicated parts of plants include leaves, stems, and germinating organs such as the flowers or bulbs. Sharp-edged leaves and leaf hooks such as those seen on the leaves of holly trees (family Aquifoliaceae) (Figure 1), agave plants (family Agavaceae) (Figure 2), and yucca plants (family Agavaceae) may damage the skin. Thorns are a well-known hazard of many varieties of plants. Widely cultivated decorative plants such as roses (family Rosaceae) (Figure 3) and bougainvilleas (family Nyctaginaceae) are classic examples. Spines and glochids are also a well-known hazard of cacti, such as Opuntia (family Cactaceae) (Figure 4). Stem and leaf hairs like those on comfrey and forget-me-not (family Boraginaceae) and on Bidens and cardoons (family Asteraceae) may also be irritants. Even barely visible irritant fibers located on a plant can be problematic, as are those on rose hips (family Rosaceae) and on tulip bulbs (family Liliaceae)[1,8] ( Table 2 ). Hairs, bristles, and barbs are often referred to collectively as trichomes or glochids.
Hairs (Trichomes) and Hooks
Penetration of the skin by hairs (trichomes), spines, and thorns can produce a papular irritant eruption. Many plants bear trichomes on their stems or leaves. Examples include plants in the Boraginaceae family, including the borage plant, which is often used as an herb in cooking. The borage plant contains many coarse stiff trichomes that can elicit an MICD when the plant is handled. Other irritant members of the borage family with similar trichomes include the herb comfrey (Symphytum spp) and the flowering plant forget-me-not (Myosotis spp). Cleavers (Galium aparine in the Rubiaceae family), also known as catchweed or stickyweed, is a common weed native to North America and Eurasia and famous for its ability to adhere to other objects, due to numerous trichomes. Some cleavers have pronounced hooked tips that resemble those on Velcro.
Hooks occur on a number of other plants, such as some tropical palms and bindi (Soliva pterosperma), an Australian weed. Spearlike "awns," which are bristlelike fibers on a head of grain, are present on cereal grasses, including barley (Hordeum vulgare). These must often be separated manually from the grain and can produce an eruption of pruritus and erythema on the hands. Dogwood trees (Cornus spp) have T-shaped trichomes that can independently cause erythema and pruritus on contact. Tulip bulbs may also result in MICD. Coarse dry fibers on the outer layer of tulip bulbs, termed "tecta," are notorious for causing skin damage. These tecta, along with chemical irritants and allergens (including calcium oxalate and tulipalin A, respectively) in the sap contribute to the common occupational dermatosis of "tulip fingers."
Spines, Glochids and Thorns
Cacti (family Cactaceae), which are among the most infamous mechanical irritant plants, have large prominent spines that cause reactions mimicking scabies or dermatitis from fiberglass. Interestingly, the smaller and nearly invisible barbed hairs (glochids) on these cacti are capable of causing cutaneous reactions that are more dramatic than those caused by the large cacti spines. The polka-dot cactus (Opuntia microdasys), for example, is common in homes and gardens and bears clusters of 100 to 200 glochids on each pad. These glochids grow in tufts on the areola of cactus pads and possess self-retaining barbs that make removal difficult. Implantation of these spines and glochids is an occupational hazard for growers.
The prickly pear cactus (Opuntia spp) is native to Central America but is widely cultivated in temperate areas. The cacti are large and form bushes and have thus been used in Africa and the Middle East as "burglar-proofing" fences. The cacti are also cultivated for their edible fruit. In addition to the readily apparent spines, (see Figure 4), both the pad and the fruit of Opuntia bear tufts of shorter hooked glochids, which cause mechanical cutaneous injury. The glochids may also be transferred to clothing and then to any part of the skin, including the genitalia, and interdigitally.[13,14] The oral mucosa and hard palate may be affected during consumption of the fruit. This eruption, like other cactus-induced MICD reactions, can closely resemble scabies and is colloquially referred to as "sabra dermatitis." The fruits are usually shaved prior to being sold (Figure 5), but they can still cause irritation.
Other plants known for their spines include Serenoa repens (family Arecaceae), a North American plant commonly known as saw palmetto (Figure 6). This palm gets its common name from the fine sharp spines on its stems that result in a saw-toothed appearance (Figure 7).
Beyond the initial MICD elicited by the aforementioned mechanical irritants, these spines, glochids, and trichomes are also known for resulting in foreign body granulomas when lodged in the dermis. The glochids of the cholla cactus (Opuntia lingularis) and spines of Opuntia acanthocarpa cause immediate pain after penetrating the skin, followed by erythema and swelling. If the spines remain in situ and are not excised, granulomas may develop over the course of months and persist indefinitely.
Thorns and wood splinters may also induce deep granulomas.[15,16] A series of five cases of granulomatous synovitis and inflammatory monoarthritis due to plant thorn penetration has been reported; surgical removal of the thorns resulted in a return to normal joint function in each case.
An often overlooked comorbidity of mechanical dermatitis from plants is the introduction of infectious organisms into the skin and subcutaneous tissue. Bacteria such as Clostridium tetani may be introduced by spines and thorns. Staphylococcus aureus can be commonly found on blackthorns (Prunus spinosa), which are used by some species of birds to impale their prey. Blackthorns are commonly used in hedgerows, and the brittle jagged tips of the thorns increase the ease of inoculation.Sporothrix schenckii infection, a deep fungal infection, may result in a nodular eruption at the site of inoculation, followed by involvement along the draining lymphatics (Figure 8). Inoculation may occur following contact with rose thorns, sphagnum moss, some grasses, and cornstalks. Atypical mycobacteria may also be transmitted following mechanical plant injury. Examples include Mycobacterium kansasii transmitted through contact with blackberry bushes, Mycobacterium marinum transmitted by cactus spines, and Mycobacterium ulcerans transmitted by tropical vegetation.[9,20] Last, vegetable fragments and barley awns have been noted to carry various Actinomyces species.
Dermatitis. 2009;20(2):63-78. © 2009 American Contact Dermatitis Society
Cite this: Irritant Contact Dermatitis from Plants - Medscape - Mar 01, 2009.