A mysterious skin rash outbreak over Pescadores Islands in Taiwan Strait: An airborne contact dermatitis by Euproctis taiwana revealed by ultrastructure mapping

Background A mysterious skin rash (papular lesions) outbreak broke out for unknown reasons in Taiwan’s Pescadores Islands in mid-autumn 2021, causing much anxiety in the population already nervous about the ongoing COVID-19 epidemic. Objectives We conducted a field survey for possible culprit agents. Methods Skin samples were taken with rapid-glue adhesives from the affected skin. Electron microscopy from the skin samples as well as caterpillar seta was performed. Additional neurotransmitter analysis by mass spectrometry (MS) was conducted on the skin samples. Results A field survey found a large increase in caterpillars, Euproctis taiwana, in September 2021, the warmest September in recent years. Electron microscopy performed on superficial skin samples (3–6 cells thick) of affected and healthy skin (n = 5 and 3, respectively) and caterpillar setae revealed ultrastructural similarities between setae spurs (0.15–0.2 μm) and tiny pits (0.2 μm) on the affected skin as well as similar vegetative debris (0.75 μm) on both setae and affected skin. MS showed large increases in tyrosine, serine, and histamine, known mediators of arthropod insult. Conclusion The co-occurrence of E. taiwana population growth, excessive warming, and strong monsoon winds may lead to this unusual autumn outbreak in late September. The lifting of COVID-19-related travel restrictions against earlier that month probably exacerbated it.