Teaching Case

A 13-year-old boy presented with a pruritic lesion above his right ankle. He was on a two-week vacation in South America with his family a month prior. Upon return, he noticed a small wound with serous discharge that progressively swelled and indurated. Clinical examination revealed an erythematous nodule with a central opening. Laboratory findings were unremarkable.

Ultrasound with a 14 MHz linear-array transducer was performed. Grey-scale ultrasound showed a tubular hyperechoic structure within the dermis surrounded by a hypoechoic rim and posterior acoustic shadowing (Figure 1). Doppler ultrasound showed punctiform signals in the structure and no hyperaemia of the surrounding tissue (Figure 2). After speaking with the child’s father, we found out he had similar lesions on his leg. An ultrasound was also performed on the father, and similar findings were seen (Figure 3).

Background

Cutaneous myiasis is a zoonosis caused by larvae of flies belonging to the order Diptera [1]. Predominantly associated with Dermatobia hominis in Central and South America, the endemic occurrence has been associated with living in rural areas and near tropical forests [1–3]. This species lays its eggs on an intermediary insect, which then carries them to the host [1,2]. Upon exposure to heat, the eggs hatch, and the larvae tunnel into the host’s skin, reaching the dermis [1–3]. An erythematous papule appears on the site of larval penetration, usually on the exposed parts of the skin: the scalp, legs and arms [4]. The papule grows with the parasite’s growth. Following a period of 5 to 10 weeks, the larva reaches a specific stage of maturity and exits the host to undergo pupation in the soil. Subsequently, lesions on the host typically resolve rapidly [1,2]. Though bacterial superinfection is possible, it is unlikely if the larva is alive, as it produces bacteriostatic substances [2].

Clinical Perspective

For clinicians in non-endemic countries, determining the patient’s travel history is crucial in diagnosing myiasis. Upon examination, one or more erythematous furuncular-like nodules with central pores can be seen (Figure 4) [1,2]. Reported symptoms are pruritus, pain, serohaemorrhagic discharge from the lesion, and sensation of movement under the skin (Figure 6 – Video) [1–3,5]. Laboratory findings are usually normal [1]. The most common clinical differential diagnoses include furuncle, arthropod bite, inflamed epidermoid cyst and cutaneous leishmaniasis [2].

Imaging Perspective

While the diagnosis of myiasis is usually made based on clinical findings, in instances of atypical presentation, ultrasound serves as a valuable tool in offering additional insights into the lesion.

Our findings align with existing literature. We demonstrated a hyperechoic, oval-shaped mass representing the larva, with a hypoechoic halo representing the cavity. The larva had a characteristic acoustic shadowing [3,5].

If the larva is viable, spontaneous movements can sometimes be seen [3,4,6]. Another characteristic finding is the “bubbling sign”, in which fluid movement in the fistulous tract above the larva can be observed. This happens due to occlusion of the opening through which the larva breathes with US jelly, thus causing its asphyxiation [5].

We demonstrated Doppler signals in the larva, indicating the movement of its internal fluids, which is another sign of parasite viability [6]. There was no hyperaemia or fluid collections in the surrounding tissues.

Outcome

In our case, the clinicians’ initial differential diagnosis was an abscess or cutaneous leishmaniasis. Ultrasound provided insights into the lesion that led to the diagnosis of myiasis. Although the larvae leave their host independently, both the father and the son had the larvae surgically removed (Figure 5) to alleviate the symptoms and prevent possible superinfection and invasion of the species in a non-endemic area [1].

Take Home Message

• Clinical diagnosis can be difficult in non-endemic regions and cases with atypical presentation.
• Ultrasound with a high-frequency linear probe shows the characteristic appearance of the larvae and can confirm the diagnosis.
• Ultrasound can show the viability of the larvae.

All patient data have been completely anonymised throughout the entire manuscript and related file.