Live worm at uncommon site-mimicking soft tissue tumor: a case of subcutaneous dirofilariasis

A 46-year-old male presented with complaint of painless hard insideous, gradually increasing swelling in right naso-orbital region measuring approximately 1.5 cm since 2 months. (Figure 1)  It was non-tender on palpation. Negative aspiration and mild eosinophilia on blood workup. Patient was then referred to radiology department, underwent ultrasonography of the lesion.

High-frequency linear probe ultrasonography showed a small 1.5 cm diameter cystic oval well-defined lesion. The lesion had a thin echogenic wall, 3- 4 mm thick. The contents of this cystic lesion revealed coiled structure with parallel echogenic walls, which showed active movement within the nodule. It was then diagnosed as a suspected case of subcutaneous nodule at an atypical site due to live parasitic giving an impression of ‘dancing larva’ on ultrasonography.(Figure 2,3)

Background

Human dirofilariasis caused by nematode belonging to genus Dirofilaria is of clinical significance and has two distinct presentations; subcutaneous dirofilariasis caused by D.repens, D.tenuis etc and pulmonary dirofilariasis caused by D.immitis. Other species of Dirofilaria are of less importance to humans. Zoonotic filariasis due to D. repens is prevalent in several regions of the world, mainly reported from Europe, Africa and Asia.[1] The first reported case of human ocular dirofilariasis in India occurred in Kerala in 1976 and subcutaneous dirofilariasis caused by D. repens was recorded in 2004 in the same region.[2]

Dogs are the primary hosts and reservoirs of D. repens, vector-borne transmission by mosquitoes,  while humans are accidental hosts and dead ends in Dirofilaria infestation.[3] The worms found in humans cannot attain maturity and are hence unable to express larvae in the blood stream. Since there are no microfilariae in the blood stream, antibiotics are ineffective.[4]

Clinical Perspective

Subcutaneous form of the disease is presented as a firm, painless, pruritic and well defined nodule under the skin. It can be visible with naked eyes or found by palpation, located in the hypodermis or deeper, measuring a few centimeters in length, with mild redness surrounding. Diagnosis is usually based on a high clinical suspicion in patients from endemic areas. A definitive diagnosis is secured on isolation of the worm. High-resolution ultrasonography is the imaging modality of choice, as live motile worms can be visualised in real time.[5]

Imaging Perspective

Ultrasound scan of the affected soft tissue reveals a continuously moving worm seen as thin parallel hyperechoic lines, surrounded by a thick hypoechoic area representing the coiled up worm and the surrounding granuloma, respectively.[6] CT imaging is not specific and may simulate an abscess since most commonly it would show a heterogeneous soft tissue mass with peripheral enhancement.[3] MRI is best utilized for assessment of any deep extension into the muscles and joints. The worm is usually described to be seen within the cyst with adjacent inflammation and fibrosis. The worm appears hypointense on both T1 and T2 weighted images but may show irregularly bordered hyperintensity on T2 or STIR images. [4,7]

Outcome

The nodule was then excised completely under general anesthesia, the live worm wriggled out of the nodule when the wall was incised. (Figure 4) The sample was then sent for parasitological examination. The results confirmed presence of Dirofilaria repens having characteristic thick cuticle with longitudinal muscle layer. (Figure 5,6)

Take Home Message / Teaching Points

Subcutaneous dirofilariasis imitates various benign and malignant lesions .Ultrasound is a primary noninvasive diagnostic tool to identify live adult worm and may help in pre-operative diagnosis of subcutaneous dirofilariasis and is helpful in distinguishing it from other differential diagnoses.