Processionary Caterpillars: A Differential Diagnosis for Scabies

This week, a report was filed with the Midi-Pyrénées Regional Health Agency following the early morning onset of skin symptoms (small bumps, red patches around the wrists, chest, back, neck, and face, and between the fingers and toes) in 25 children who had arrived the previous evening for a summer camp stay. The sudden and simultaneous onset in several individuals, particularly children, initially pointed toward a diagnosis of scabies. However, the epidemiology of this event (the temporal and spatial distribution of cases) should have led to a different consideration of the cause of these symptoms. After confirming the absence of other cases among children not participating in this camp at their home elementary school, the hypothesis of histamine-type food poisoning (symptoms typically appearing within an hour of eating) was ruled out. Given the symptoms, another strong hypothesis that was also raised was the occurrence of allergic reactions following exposure to processionary caterpillars. After investigation, this hypothesis was ultimately accepted, as a common exposure for all these children was identified. The teachers in charge of the group of children confirmed a stop at a highway rest area where nests of processionary caterpillars were observed; this exposure could have occurred either through direct contact with the caterpillars, their nests, or the ground beneath the trees where nests were visible. As a reminder, mere contact with the spines is sufficient to trigger this type of allergic reaction. The caterpillars’ spines are present in the nests or are dispersed into the surrounding environment when the caterpillars die. These spines are harpoon-shaped and easily attach themselves to the skin, mucous membranes, and sometimes the respiratory tract. In this instance, a parasitological diagnosis of scabies was quickly confirmed.