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An otherwise healthy man in his 40s presented with a 3-month history of an asymptomatic nodule on his scalp. There were no systemic complaints, history of arthropod bite, immunosuppression, travel abroad, or contact with animals in the recent past. Physical examination revealed a skin-colored, firm, nontender, freely mobile 2 × 1.5-cm nodule with overlying normal skin on the right occipital region of the scalp (Figure, A). There was no lymphadenopathy. No other cutaneous lesions were present.
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B. Subcutaneous dirofilariasis
Subcutaneous dirofilariasis is a zoonotic infection caused by filarial nematode Dirofilaria, a natural parasite of dogs, cats, and wild animals. It is mostly reported in South America, Europe, Southeast Asia, and Africa.1,2 The dog parasite Dirofilaria repens is the most common agent for human infections acquired accidentally by the bite of mosquitoes of the genera Culex, Aedes, and Anopheles.1 Clinical forms include pulmonary, subcutaneous, and ocular disease.3 Subcutaneous dirofilariasis presents as an occasionally migratory nodule that may be painful or itchy, distributed on the face (especially periorbital and subconjunctival), chest wall, upper arm, thigh, and male genitalia.1- 3 As the adult female worm does not reach maturity in skin, microfilariae usually are not seen in blood. However, microfilariae have been reported in subcutaneous tissue.4 Diagnosis is usually established on macroscopic analysis of the parasite morphology or microscopic examination of histologic sections from the nodule. Excision of the nodule is both diagnostic and therapeutic. Adult worms have a multilayered thick-walled cuticle with longitudinal ridges and transverse striations, which appear as regularly spaced, round, elevated structures about 10 to 12 μm apart, and a well-developed muscular layer. Adult worms are 50 to 170 mm long and 370 to 650 μm wide.1 Polymerase chain reaction has been used in skin, eye, and pulmonary specimens for species confirmation and where histomorphology is inconclusive.5- 7 Peripheral eosinophilia is an inconsistent finding. Antibody detection using enzyme-linked immunosorbent assay is of little diagnostic value owing to low sensitivity and specificity.1 Slitlamp examination and chest radiograph should be performed to rule out ocular and pulmonary involvement, respectively.
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Corresponding Author: Geeti Khullar, MD, DNB, Department of Dermatology and Sexually Transmitted Diseases, Vardhman Mahavir Medical College and Safdarjung Hospital, New Delhi-110029, India (email@example.com).
Published Online: September 23, 2020. doi:10.1001/jamadermatol.2020.3527
Conflict of Interest Disclosures: None reported.
Additional Contributions: We thank the patient for granting permission to publish this information.
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