Tungiasis and myiasis — SAQ

Stem

A 34-year-old man presents three weeks after returning from a beach holiday in Brazil with a painful, furuncle-like nodule on his scalp that has not responded to two courses of oral flucloxacillin. On examination there is a 1 cm erythematous nodule with a central punctum from which a small amount of serosanguineous fluid weeps, and the patient describes a sensation of movement within the lesion. He also has a serpiginous, intensely pruritic track on the sole of his right foot.

Questions

a) What are the two most likely diagnoses, and what organisms cause each? (2 marks)

b) What bedside and clinical features distinguish furuncular myiasis from a bacterial furuncle, and cutaneous larva migrans from larva currens? (3 marks)

^[1]c) Outline the stepwise management of the scalp nodule, including the technique, the principle behind it, and the key procedural pitfall to avoid. (3 marks)

d) Give the first-line drug treatment for the foot lesion, with dose and route, and two situations in which an alternative agent would be preferred. (2 marks)

Model answer pointers

• a) Furuncular myiasis (Dermatobia hominis, human botfly — Central/South America) and cutaneous larva migrans (animal hookworm larvae — tropical beaches).

^{[1] [2]}

• b) Myiasis: furuncle-like nodule with a central breathing pore, sensation of movement, serosanguineous discharge, no response to antibiotics, travel to the neotropics. CLM vs larva currens: CLM migrates 1 to 2 cm/day, serpiginous and scaly, on feet/buttocks; larva currens is faster (several cm/day), urticarial, perianal, and recurrent (Strongyloides autoinfection).
• c) Occlude the punctum with thick petrolatum or bacon fat to cut off the larva's oxygen supply; wait 3 to 24 hours; extract the larva intact with forceps; surgically excise if occlusion fails. Never simply incise like an abscess (releases the larva, contaminates the wound) and never rupture the larva (cuticular spines anchor it; contents cause a foreign-body granuloma).
• d) Oral albendazole 400 mg daily for 3 to 5 days, or oral ivermectin 200 mcg/kg single dose. Prefer topical thiabendazole 10 to 15% for localised disease, pregnancy, or young children; avoid albendazole in the first trimester of pregnancy.

References2ShowHide

1. [1]Leung AKC, Barankin B, Hon KLE. Cutaneous Larva Migrans Recent Pat Inflamm Allergy Drug Discov, 2017.PMID 28078983
2. [2]Calvopina M, Ortiz-Prado E, Castañeda B, et al. Human myiasis in Ecuador PLoS Negl Trop Dis, 2020.PMID 32084134