Microbiology Case Study: a 53 Year Old Man with a Black Spot on His Shoulder

Case History

A 53 year old man presents to urgent care with a primary complaint of an area of erythema and tenderness around a small black spot on his left shoulder, shortly after returning from Ecuador. He does not report any fevers, chills, or drainage from the affected area. The patient reported that he occasionally felt the area moving. An occlusive Vaseline dressing was applied to the central black spot, and the organism shown below emerged from the wound.

Laboratory Identification

The parasite shown above is a human botfly larva, Dermatobia hominis. The clinical history is strongly suspicious for a botfly infection, and the patient himself suggested the diagnosis.

Dermatobia hominis is identified in large part by its relatively unique presentation combined with identification of the larvae in tissue. Laboratory identification of genus and species involves comparing morphological structures including the anterior and posterior spiracles, mouthparts and cephalopharyngeal skeleton, and cuticular spines. Travel history can also be helpful for genus or species-level identification.

Discussion

The lifecycle of human botflies begins when the female botfly lays her eggs on a mosquito. Once a mosquito feeds on a host, the botfly larva drop onto the host and burrow into the skin. They may remain in that location for up to 10 weeks before dropping off the host into soil to pupate and continue the life cycle.

The human botfly is found in North America, ranging from Mexico to Paraguay and northeast Argentina. Cases in the US are primarily in travelers returning from the botfly’s native range. Measuring the incidence of infection in travelers can be difficult due to the nature of the disease. Experienced travelers may be able to remove the larva at home. In other cases the botfly larva may leave the host before the patient seeks medical care.

Testing for the presence of these larva is easy as they require oxygen coming in through a hole in the skin. Cover the lesion with a thick layer of sterile Vaseline gauze and wait approximately 5-15 minutes for the organism to emerge. Surgery is usually not required as the larva is most often removed intact. Antibiotics should be given following removal of the parasite to prevent secondary infections.

-Britt Boles, MD is a 1st year Anatomic and Clinical Pathology Resident at the University of Vermont Medical Center.

-Christi Wojewoda, MD, is the Director of Clinical Microbiology at the University of Vermont Medical Center and an Associate Professor at the University of Vermont.

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