Paederus dermatitis: causes, symptoms, diagnosis, and treatment

Paederus dermatitis is a dermatitis caused by the venom secreted by paederus.


Causes

Paederus is a small, ant-shaped flying insect with a body of 0.6 - 0.8cm in length, with black head, orange chest and abdomen, frontal abdomen covered by black elytra, 3 pairs of feet, 2 posterior spines, and short hairs all over the body. This insect lives in dark places such as damp grass, rice fields, vegetable gardens or rotten woods during daytime. They emerge at night and have phototaxis, flying in groups at night, especially in places with fluorescent lights. They reproduce from April to September, and overwinter as adults.

Figure 1 paederus

Their spines can secrete venom. When crawling, the tail is upturned, and there is often a small drop of translucent liquid at the end, which is the venom secreted by the insect. In summer and autumn, the insects fly in the room at night and fall on the human skin to bite or release venom when compressed, which can cause dermatitis. However, the insects usually do not release venom when crawling on the skin, and when the insects are slapped or crushed, the venom contaminates the skin, leading to skin lesions.


Signs and Symptoms

The skin lesions often occur on the exposed areas such as the face, neck, chest, back, and extremities. When the insect invades the skin, there is creeping or foreign body sensation. If there are scratches or the insect is crushed, punctate, funicular, flaky redness and swelling and pruritus occur in 2 - 4 hours due to the stimulation of the venom. Gradually, there are burning pain. Vesicles occur on the skin in 12 hours, mostly transparent, and some develop into pustules or grayish black necrosis. Bright red, punctate or flaky papules or vesicles may occur around the skin lesions, and may develop into bright red erosion if scratches.

Figure 2 Paederus dermatitis

If the eyelid is invaded, the eyelid is red and swollen. If the external genitalia are scratched by the hand contaminated by the venom, local flaky erythema may occur.

Few patients with severe skin lesions and inflammation often present systemic symptoms such as headache, dizziness, and fever, and superficial lymph nodes may be swollen.

The disease persists for 1 - 2 week and heal after dryness and decrustation, leaving pigmentation or superficial scars.


Diagnosis

The disease can be diagnosed on the basis of clinical presentations.


Treatment

Topical calamine lotion, 1% - 2% alum solution, or 1:5000 potassium permanganate solution is appropriate.