Imported Fire Ant Envenomation: A Clinicopathologic Study of a Recognizable Form of Arthropod Assault Reaction

Background Skin reactions to the sting of the imported fire ant have characteristic clinicopathological features. Methods One case of experimental envenomation was prospectively followed during 48 hours, with biopsies. In addition, six cases from our laboratory were retrospectively evaluated histopathologically for the following features: spongiosis, exocytosis (and type of cells), pustule formation, erosion/ulceration, epidermal necrosis, scale/crust, papillary dermal edema, inflammatory dermal infiltrate (cell type, density, depth, distribution, shape), red blood cell extravasation, vasculopathy, vasculitis. Results The typical lesion follows a very distinctive clinical and histopathologic evolution over 48 hours, with the formation of a subepidermal pustule overlying a wedge shaped area of dermal collagen basophilic degeneration with scattered neutrophils. In the six cases retrieved from our files, the main features were a superficial and deep dermal, perivascular, periadnexal and interstitial infiltrate consisting of neutrophils, with basophilic degeneration of the collagen. A subepidermal pustule was noted in half of the cases. Conclusions In biopsies taken in a clinical setting, even in the absence of the characteristic subepidermal pustule, the diagnosis of imported fire ant sting can be suspected if there is a superficial and deep perivascular, periadnexal and interstitial infiltrate composed of neutrophils, with some basophilic denaturation of collagen.