Giant hogweed (Heracleum mantegazzianum) is an invasive flowering weed of the family Apiaceae that typically reaches a height of 13 feet, with thick stems; large green leaves; and umbrella-shaped, flat-topped, radial clusters (umbels) of small individual white flowers1 (Figure 1). Because of the size and beauty of giant hogweed, it was widely planted in 19th century ornamental gardens in the United Kingdom and has since naturalized and spread throughout central Europe, Canada, and the United States.1,2 The plant most commonly is found in shady areas near rivers and woodlands.1
FIGURE 1. Giant hogweed (Heracleum mantegazzianum). © Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. Reproduced with permission.
Due to the invasive nature of the giant hogweed, its prevalence continues to grow, its eradication remains difficult, and reports of phytophotodermatitis are increasing in number and distribution. In fact, there has been widespread media coverage of the dangers of giant hogweed in the United Kingdom since 20161 and in the United States in 2018 and 2019.3-6
Transmission
Phytophotodermatitis is a type of nonimmunologic dermatitis caused by UV light reacting with a plant-based photosensitizing agent. In the case of giant hogweed, sap from the plant’s fruits, leaves, and stem contain furocoumarins or psoralens.7 Upon activation by UVA radiation, furan rings of these compounds create reactive oxygen species and intercalate with DNA pyrimidine bases, which results in cellular death, damage to successive skin layers, and reduced wound healing at the cellular level.8 This effect is intensified with increased percutaneous absorption of furocoumarin, which can result from high temperature, humidity, skin infection, lack of protective clothing, and moist conditions.9
The highest concentration of phototoxic compounds is found in giant hogweed from June through August,7 which, in combination with people increasing their outdoor activity in the summer, results in a greater prevalence and severity of H mantegazzianum phytophotodermatitis during summer months.
Presentation
Phytophotodermatitis caused by giant hogweed can range from burning and erythema to full-thickness chemical burns that require surgical debridement and skin grafting.8 After exposure to the offending agent, a harmful skin reaction can start within 15 minutes. After a latent period of approximately 24 hours, erythema, edema, and bullae can appear and generally peak by 72 hours.10 In addition to cutaneous injury, inhalation of giant hogweed traces can result in obstructive pulmonary symptoms. Eye contact can result in blindness.9
In addition to the rash caused by giant hogweed, a “weed-wacker dermatitis” or “strimmer rash” can be caused by the similar-appearing but smaller common hogweed (Heracleum sphondylium). Common hogweed is highly prevalent in the United States and often is confused with the larger giant hogweed because of tall stems and white, flat-topped flowers.
Management
Following contact with giant hogweed, a person should immediately avoid UV exposure and rinse the area with soap and water. UV radiation must be avoided for at least 48 hours. If erythema occurs, a topical steroid can be applied to the affected area; pain can be alleviated by a nonsteroidal anti-inflammatory drug.9