UPMC Dermatopathology "Case of the Month" Presentations
UPP - Department of Dermatology, Dermatopathology Unit
Christina Wahlgren, MD, Grace Lee, MD and Jukic Drazen, MD
JANUARY 2008 CASE OF THE MONTH
DISCUSSION & DIAGNOSIS
Diagnosis
Furuncular myiasis via D.hominis
Discussion
Dermatoid hominis or the human botfly is the most common cause of furuncular myiasis in Central and South America.1 As an obligate parasite, D. hominis must live off of a host in order to survive. Despite its name, human infestation is accidental and livestock mammals are the typical host.2 The life cycle of D. hominis is unique in that the fly does not directly deposit its eggs on the host, but lays its eggs on the abdomen of a blood-feeding insect such as the mosquito. When the mosquito or other insect takes a blood meal from the mammal, the increase in ambient temperature allows the eggs to hatch and the larvae enter the host via the puncture site, a hair follicle or chew through intact skin.3 Once embedded in the skin, the larva survives by ingesting the surrounding tissue and grows multiple concentric rows of backward projecting spines securing its position in the tissue. Throughout the infestation, the larva breathes through a ventilation hole that is seen clinically as a central punctum or sinus tract.4 The larva develops over a time period of 4-14 weeks at which point it exits the skin to pupate in the soil. In two to three weeks, an adult fly emerges.2
Signs and symptoms of infestation occur approximately 24 hours after the insect bite.5 Lesions frequently appear on the scalp, face, neck, arms and hands and can be singular or multiple. Initially, the host develops a 2 to 3mm pruritic papule at the puncture site, but with time, the area becomes increasingly erythematous and indurated, often draining serous or serosanginous fluid through the central punctum resembling a furuncle.2-6 Patients may complain of localized pain and a crawling sensation as the larva feeds and burrows deeper into the tissue. Secondary infection is unusual owing to the bacteriostatic properties of materials produced in the gut of the larva5 and systemic symptoms such as fevers, myalgias and malaise typically do not occur. At times the spiracles may be visible in the central punctum and air bubbles will be generated if the lesion is submerged in water.4 Ultrasound may be used to detect the larva in the tissue,7 but usually history and physical exam are sufficient to make the diagnosis.
Extraction of the larva is curative and numerous treatments ranging from watchful waiting to surgical intervention have been used successfully. Occlusive materials such as petrolatum, beeswax, bacon or superglue6, 7, 9, 10 may be used to suffocate and force the larva to the surface, although this is best in early stages of infestation when the larva are still located superficially in the tissue2. Other methods including subcutaneous lidocaine injections, liquid nitrogen and ivermectin have been used to expel larvae.6,11,12 Surgical intervention is often reserved for those who have failed conservative treatment, however, this is the preferred method by many.2,4,9,13 The surgery consists of complete excision of the larva and its burrow, debridement and healing via secondary intention. For reasons listed above, systemic antibiotics are typically not necessary.
Although not life threatening, early recognition and treatment of D. hominis myiasis is becoming increasingly important as travel to endemic regions continues to rise. In one study, cutaneous myiasis was reported as the fourth most common travel associated skin disease in tourists visiting tropical countries.14 The diagnosis is made based on travel history and physical exam and surgical extraction of the larva is curative.
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