Case PresentationA 26-y-old hiker on the Colorado Trail near Salida, Colorado, in late July noticed erythematous papules, burning, and itching on the dorsa of both thumbs and second fingers on Day 14 of a 25-d trek. The hiker used alcohol-based hand sanitizer during this time but recalled not having had the opportunity to wash with soap for up to 4 d at a time. She had worn neither sunblock nor gloves on her hands. Over the next week of trekking, the rash worsened and became more painful, eventually resulting in bilateral dorsal hand swelling that extended proximal to the wrists. At this point, the pain was 6/10 in severity, enough to prevent the hiker from sleeping. The papules evolved into vesicles and tense bullae, measuring up to 10 mm in diameter, the largest being at the lateral aspects of the second metacarpophalangeal joints (Figure 1, Figure 2, Figure 3). The only available medications were ibuprofen and cetirizine, neither of which alleviated the symptoms. On Day 7 of symptoms, the hiker trekked with a botanist, who thought the rash was typical of exposure to cow parsnip (Heracleum maximum) and identified this plant on the trail (Figure 4). When the hiker reached an urgent care clinic on Day 9 of symptoms, a physician prescribed a 9-d course of prednisone for contact dermatitis. As the blisters drained and dried, scabs remained, finally healing by Day 21, leaving hyperpigmented macules. The hyperpigmentation faded but was still visible 5 mo later.

Figure 1Hand at Day 7 showing dorsal swelling and early bullae. Photo by Cathleen Rueckeis and used with permission.

Figure 2Hand at Day 11. Photo by Cathleen Rueckeis and used with permission.

Figure 3Hand at Day 12. Photo by Cathleen Rueckeis and used with permission.

DiscussionCow parsnip, or H maximum, is also known as Heracleum lanatum, American hogweed, Satan’s celery, Indian celery, Indian rhubarb, or pushkie.1A mystery. Cow parsnip phytophotodermatitis. Cow parsnip is a native plant found in Alaska, Canada, and throughout the contiguous United States except on the Gulf Coast from eastern Texas to Florida. It thrives in sunny to partially sunny areas and prefers moist soil, such as that found in meadows, river banks, and ditches. The plant grows up to 2 m tall with broad leaves and umbels of small white flowers that bloom in early summer (Figures 4 and 5). It is a forage species for many large mammals and is used in indigenous medicine and cuisine. The oil of cow parsnip, found in all parts of the plant, contains furanocoumarins that cause the rash known as phytophotodermatitis when exposed to the sun. The distribution of the hiker’s rash was related to the use of trekking poles that likely spread the plant oils between the hiker’s hands. The poles also caused the radial aspects of the second digits and the medial aspects of the first digits to be exposed to direct sunlight for prolonged periods of time.A cytotoxic reaction in the skin takes place when furanocoumarins are exposed to ultraviolet A (UV-A). Ultraviolet B is more responsible for sunburn during the middle of the day and more susceptible to being blocked by cloud cover than UV-A. UV-A radiation is strong throughout the daylight hours and can penetrate clouds even in conditions that may not prompt awareness of sun exposure. When exposed to UV-A radiation, furanocoumarins form bonds with nucleobases, such as thymine, resulting in cross-links between DNA strands that disrupt DNA synthesis.2Del Río J.A. Díaz L. García-Bernal D. Blanquer M. Ortuño A. Correal E. et al.Furanocoumarins: biomolecules of therapeutic interest. The resulting cell destruction causes blistering similar to thermal burns (Table 1).3Herb of grace: an unusual cause of phytophotodermatitis mimicking burn injury.

Table 1Plants implicated in phytophotodermatitis

Phytophotodermatitis is a clinical diagnosis based on presentation in a patient with UV-A exposure and possible contact with a culprit plant. Knowledge of local flora can be helpful in diagnosis. The severity of the rash is directly related to the amount of exposure to both sap and sun. Casual contact, such as picking a flower stem, may result in uncomfortable erythema, while more intense contact associated with land clearing, for example, may result in full-thickness skin damage that can require skin grafting. Most patients present like the patient we describe, with pain, erythema, swelling, and tense bullae beginning 24 to 48 h after contact. Mild irritation may be noted after only a few hours. As the skin heals, localized hyperpigmentation or, less commonly, hypopigmentation appears, which may persist for months. Phytophotodermatitis is more common in the summer when people spend more time outside for work and recreation, more skin is exposed, and the blooming plants are more attractive. Also, skin is more permeable in warm temperatures. Phytophotodermatitis is commonly misdiagnosed as contact dermatitis or as Rhus dermatitis from Toxicodendron species such as poison ivy, poison oak, or poison sumac. Burning and swelling help differentiate phytophotodermatitis from Rhus dermatitis, which typically begins with itching as the only symptom. The rash is always in sun-exposed areas, so mild cases may be attributed to sunburn. Rashes in sun-exposed areas may also be caused by lupus, polymorphic light eruption, or drug-related photosensitivity.4Maniam G. Light K.M. Wilson J. Margarita burn: recognition and treatment of phytophotodermatitis. Dermatitis in covered areas should prompt consideration of other diagnoses, including chemical burns and bullous impetigo.Treatment is usually supportive with comfort measures, protection of affected areas from additional sun exposure, and wound hygiene (Table 2). Systemic and high-potency topical corticosteroids can be used to reduce inflammation in severe cases. High-potency topical steroids should be applied only to intact skin and used for no longer than 3 wk.

Table 2Prevention and treatment of phytophotodermatitis

NSAID, nonsteroidal anti-inflammatory drug.

Washing generously with soap after potential exposure to furanocoumarin-containing plants may help remove the plant oils and prevent or attenuate skin damage. Skin should be washed as soon as possible after exposure or onset of symptoms. Cellular damage begins as soon as oils are absorbed and exposed to sunlight. Clothing and sunblock effective against UV-A, such as zinc oxide or avobenzone, can also prevent reactions and mitigate continued cellular damage after exposure. In addition to protection against sunburns and skin cancer, prevention of phytophotodermatitis is another reason to educate the public about sun protection in the outdoors.

Phytophotodermatitis should be in the differential diagnosis of blistering rashes on sun-exposed skin. While citrus plants, especially limes, are said to be the most common cause of phytophotodermatitis worldwide, cow parsnip, wild parsnip, and giant hogweed are more common offenders in North America among hikers and other outdoor enthusiasts and persons clearing land.5Flanagan K.E. Blankenship K. Houk L. Botanical briefs: phytophotodermatitis caused by giant hogweed (Heracleum mantegazzianum). Increased awareness of plants that can cause phytophotodermatitis may help us live more happily in the outdoors and should help clinicians care for patients who fall victim to sap and sun.