Description

A woman in her 30s presented with pruritic pustules on her toes following an encounter with fire ants (figure 1A). Initial evaluation of the stings using reflectance confocal microscopy (RCM) revealed an inflammatory infiltrate that extended the full thickness of examined skin, from stratum granulosum down to reticular dermis (figure 1B). Three days after the stings occurred, they were treated with clobetasol, a topical corticosteroid (figure 2A). RCM was performed again 5 days after treatment and showed reduced density of neutrophils and resolved spongiosis (figure 2B).

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Figure 1

(A) Pruritic pustules (yellow arrow) from fire ant bites on left hallux and other toes. (B) In vivo reflectance confocal microscopy reveals dramatic neutrophilic spongiosis (yellow arrow) 3 days following bite.

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Figure 2

Evolution of neutrophilic spongiosis after treatment with clobetasol. (A) RCM of pustule 3 days after bites occurred, before treatment. (B) RCM of pustule 8 days after stings occurred and 5 days after treatment. RCM, reflectance confocal microscopy.

Solenopsis invicta, also known as ‘red imported fire ants’, is an invasive species that can be found in the southeastern USA. Although native to central South America, they have become pests in other parts of the world and are known to cause painful stings.1 Their venom consists of a non-allergenic alkaloid component which causes an initial, painful skin reaction and a proteinaceous component that induces an allergic immune response.2 3 The alkaloid component contains cis and trans isomers of 2-methyl, 6-n-alkyl piperidine compounds and makes up 95% of the venom.4 The remaining 5% of venom consists of protein components that are responsible for acute immunosensitisation in addition to a delayed-hypersensitivity reaction.5 Although not completely characterised, the proteinaceous portion seems to consist of various enzymes including phospholipases, metalloproteinases, hyaluronidases and neurotoxins.4 6

Clinically, the initial reaction from a sting is a 25–50 mm dermal flare, with wheal development within minutes and pustule development within hours.4 7 These pustules transform into clear fluid filled vesicles within 4 hours and then cloudy fluid filled vesicles within 8 hours, indicating infiltration of inflammatory cells. The development of pustules occurs after 24 hours. Pathophysiology of the allergic response involves venom allergens processed by antigen-presenting cells and subsequent T cell activation.6 Neutrophils are recruited to the sting site via inflammatory cytokines.7 Eosinophils are seen in later-phase reactions, most likely due to a Th2-mediated mechanism.2 4 Although histopathological examination has demonstrated that neutrophils are involved in the physiological reaction to fire ant stings4 7 this has not been demonstrated in vivo, to our knowledge. Here, we use RCM to demonstrate this.

RCM is a relatively new, non-invasive approach being used in clinical dermatology. It can be used in vivo to obtain cellular level information about a skin lesion and potentially eliminate the need for a traditional skin biopsy. It works by back-scattering light from the target tissue via a low-energy laser.8 The varying refractive indices of different cellular components will produce varying brightness. This imaging method can penetrate up to 250 nm, meaning we can visualise the papillary dermis and upper reticular dermis.7 The lens is placed directly on the skin with use of an immersion oil and can produce images comparable to using a 30× objective.8 However, there are some limitations to RCM including depth of imaging, cost and quality of images on areas with thickened stratum corneum.8 Overall, this imaging modality can be used by practitioners to evaluate and diagnose a variety of skin conditions with relative ease.