By Emily Margosian, content specialist

For most, knowledge of hazardous plants is limited to the phrase, “leaves of three, leave them be.” However, despite widespread awareness of a few common culprits, many casual outdoor enthusiasts may find themselves with an itchy souvenir this summer following an encounter with a flower or shrub. “When confronted by a plant-induced skin rash, most clinicians immediately think of poison ivy, the most common and notorious cause of contact dermatitis in the United States,” says Gregory Juckett, MD. “However, a host of other common plants are capable of producing serious dermatologic reactions. Unfortunately, their potential for harm is almost unknown to the public and many physicians” (Postgrad Med. 1996;100(3):159-172).

With the potential to produce a wide array of clinical findings — ranging anywhere from mild inflammation caused by near-minute fibers to necrosis induced by certain types of sap (Dermatitis. 2009;20(2):63-78) — knowledge of plant exposures and their effects on the skin is a perennial area of focus for dermatologists as patients continue to seek time outdoors for work, fun, or relaxation. “From a professional perspective, our patients are more and more active in doing things outdoors — and when they come into contact with these various plants that cause rashes, they’re either going to go to their family doctor, or they’re going to come to us,” says Julian Trevino, MD, professor and chair of dermatology at Wright State University School of Medicine. “Oftentimes primary care doctors might not be familiar with some of these reactions, so I think it should be a part of our profession that we embrace. We are the doctors most likely to make these diagnoses; our patients look to us for good information on what to avoid, how to prevent exposure in the future, and how to manage their conditions.” In this piece, Dermatology World breaks down:

• Identification of potentially hazardous plant species in the backyard, backwoods, and beyond

• Common cutaneous reactions and route of exposure

• What patients should know regarding prevention and treatment

• Recognizing patients who develop immunologic responses through repeat exposure

Garden variety exposures

While the backyard can be a place of refuge, it’s also home to a variety of potentially hazardous plant activity. “I have a number of patients who own several acres of ranch, and do a lot of their own brush clearing,” says Rajani Katta, MD, a dermatologist in Bellaire, Texas and clinical professor of dermatology at the McGovern Medical School, University of Texas Houston. “So I’ll see a lot of patients who will develop reactions to things in their yard. Then at certain times of the year — especially spring — a lot of my neighbors are out there clearing up their tree branches and spreading mulch, and putting plants in the ground. So whenever there’s a lot of activity around your yard, that can be a high-risk time.”

Homegrown crops such as strawberry, garlic, and tomato can induce urticarial reactions in certain individuals, while traditional garden staples such as comfrey, borage, and rose hips can inflict mechanical injury upon unsuspecting victims by embedding fine hairs into the skin. Likewise, hot peppers, rhubarb leaves, and the bulbs of many popular spring flowers contain chemicals that frequently cause irritation to the skin. Handling tulip, daffodil, and hyacinth bulbs in particular should be avoided unless wearing gardener’s gloves due to the presence of calcium oxalate which can produce dermatitis localized on the fingertips, also known as “daffodil itch,” or “tulip fingers.” However, as “nearly every plant is a potential type I sensitizer for someone, somewhere” (Postgrad Med. 1996;100(3):159-172), discovering potential sensitivities may be a matter of trial and error.

Still, patients can follow some basic guidelines to avoid well-known (and painful) plant-induced reactions. As a rule of thumb, combined exposure to citrus fruit and sunlight should be avoided. “Phytophotodermatitis is caused by groups of plants that create a chemical called furocoumarin — which causes a photosensitivity type of reaction that can result in a dermatitis,” says Dr. Trevino. “Lemons, limes, clementines, oranges, and grapefruit all contain these light-sensitizing compounds.” Also sometimes known as “margarita dermatitis,” Dr. Katta says that the condition usually makes an annual resurgence around summertime as more patients cook or prepare food and drinks outside. “Every year in Houston, we see this from patients who have been out in their backyards making margaritas, or people who have gone to Mexico and been out on the beach with limes in their beer — the juice then gets on their hands and triggers the reaction.”

Typical presentation of phytophotodermatitis involves initial inflammation and redness, followed by darkening of the sun-exposed skin in areas where the juice came into contact — sometimes posing a diagnostic challenge for dermatologists due to the irregular patterns that can be produced. “Folks sometimes come into the dermatologist after the acute phase, and you have to work backwards to figure out why they have this bizarre rash. It can present as a linear or streaky rash corresponding to the manner in which the juice dripped on your skin,” says Dr. Trevino. “If someone comes in with a darker rash, particularly in the summertime, that looks like that, you might work backwards to discover that they were out in their backyard making guacamole or something with limes, where they had lime juice on their skin and then got some sun exposure.” Beyond citrus fruits, other garden-variety phototoxic plants can include celery, parsnip, dill, parsley, and rosemary.

Although patients may assume they know what’s living in their yard, Dr. Katta recommends erring on the side of caution, particularly when clearing weeds or other brush. “Even if you’ve had no problems in the past, you don’t necessarily know what kinds of new things may have crept in,” she says. “I was just talking to a patient one hour ago about this; if you’re using a chainsaw to cut branches and things are flying in the air — even if nothing directly gets into your eye — there have been lots of cases where people have chopped up not just poison ivy, but also things like stinging nettles which have then landed on their face. So not only is personal protective equipment important, but also personal protective behavior. Treat these plants with caution.”

Contact dermatitis from plants is typically categorized into two types: mechanical irritants and chemical irritants.

Mechanical irritants are usually comprised of plants’ more outwardly unfriendly defenses, including thorns, spines, glochids (small, hair-like barbs found on cacti), trichomes (very fine “hairs” found on various plants), and sharp-edged leaves.

While many plant chemical irritants are still being uncovered, common culprits include calcium oxalate, protoanemonin, isothiocyanates, bromelain, diterpene esters, alkaloids, naphthoquinone, and acids (Dermatitis. 2009;20(2):63-78).