Dermatitis linearis refers to an acute skin condition caused by exposure to the potent toxin paederin, found in certain species of the subfamily Paederina in the Staphylinidae family of beetles. Paederus beetles are relatively slender and can be uniquely identified by their size (1.5 mm wide and 7–10 mm in length) and coloring (black head with red, russet or orange thorax). Humans are exposed to paederin when the beetle is crushed and smeared across the skin, resulting in a characteristically linear lesion. Paederus beetles causing dermatitis linearis have a variety of local names, including Nairobi eye in Africa, ‘tomcat’ in Southeast Asia, and whiplash rove beetle in Australia.
Climate has a prominent effect on the activity patterns of Paederus, and sequentially epidemics of dermatitis linearis. Beetles are most active during the rainy season, after unusually wet weather patterns, and during hot and humid weather. Paederus beetles most commonly inhabit crop fields but can also be found in marshes and riverbanks. Beetles migrate to residential areas for a variety of reasons.
Dermatitis linearis manifestation frequently mimics symptoms of other skin conditions and therefore can be misdiagnosed and mistreated by healthcare professionals. Unless the presence of the beetle is specifically recalled or the specimen is captured, individuals may not understand the source of newly manifesting symptoms. Even in regions where beetles are common, the local population and travelers may remain unaware of the conditions associated with beetle presence. The most frequently reported sensations at the symptom onset of dermatitis linearis are intense burning, stinging and itching. Lesions are most commonly found on the face, neck, trunk, and forearms. Kissing lesions are observed where affected skin is in apposition with another skin surface, such as the crook of the elbow or the armpit. Though hands are usually involved in the initial crushing of the beetle, the thick skin on the palms of the hand and soles of the foot seem to be less affected by paederin, due to the histological architecture of the dermis in those areas. Paederin on the skin may be further transferred to other regions of the body, including the delicate skin surrounding the genitals and the eyes, or in some cases directly transferred to the eye itself.
As the exposure is chemical in nature and symptoms typically resolve over time, treatment is traditionally considered palliative. If noticed immediately (minutes after crushing), individuals should generously flush the exposed area with soap and water. Wet compresses may be applied to the area in aim of diluting the concentration of the toxin, in addition to providing pain relief for burning and itching. Topical corticosteroids are effective in alleviating symptoms and swelling as well. In addition to topical remedies, antihistamines are commonly administered to reduce the inflammatory response to paederin.
Individuals can reduce their exposure to Paederus beetles by not sitting directly under lights at night. Changes such as closing doors and windows at night, removing vegetation immediately surrounding buildings, and sweeping away any dead beetles found indoors also lessen the odds of exposure. If beetles do land on the skin, they should be brushed or blown off, rather than crushed. Housing modifications can also be made to reduce the exposure to the beetles. Nets can be draped around sleeping areas and under lights in order to catch stray beetles. Additionally, targeted use of pesticides may reduce beetle populations in unwanted areas.
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