A 71-year-old lady reported to the emergency with a history of a fall and was referred for CT brain. Retrospective history revealed recurrent swelling and itching over the left temporal region for the last 6 months.
No trauma-related abnormality was seen on CT. However, a thick-walled cystic lesion was noted in the subcutaneous plane on the left temporal region of the scalp (Figure 1).
Ultrasound of the lesion showed a cyst with actively mobile, tubular serpiginous structures, having echogenic walls showing a “filarial dance” sign (Figures 2 and 3).
Dirofilariasis is a zoonotic condition occurring when humans serve as accidental hosts to Dirofilaria immitis, Dirofilaria repens, and Dirofilaria tenuis. Dirofilaria immitis is the commonest Dirofilaria globally [1]. Dogs serve as the definitive host for the parasite, with the mosquito acting as the intermediate host. Microfilariae present in the dog’s peripheral blood are ingested by the mosquito, where they develop into infective larvae that can then be transmitted to incidental hosts, such as humans.
The infection is endemic in continents like Asia, Africa, and Europe. In Asia, this infection is commonly caused by Dirofilaria repens and is seen in Sri Lanka and southern states of India [2]. The initial cases of human ocular and subcutaneous dirofilariasis in India were recorded in the state of Kerala, occurring in 1976 and 2004, respectively [3].
Clinical Perspective
Of the 782 cases documented worldwide, one-third were orbital lesions. The majority of reported cases involved exposed regions of the head and neck, accessible to mosquitoes [4]. Occasionally, the parasite has been detected in deeper locations, including peritoneum, omentum, breast, and liver.
Many infected subjects are asymptomatic, and a strong clinical suspicion is essential for making a diagnosis. Subcutaneous nodules are the most common initial symptom, but those can be indicative of various conditions like lipoma, fibroma, foreign body granuloma, soft tissue sarcoma, metastasis, and other parasitic infections, including onchocerciasis or cysticercosis. Serologic tests are unreliable due to the worms’ lack of reproduction in the human body, causing insufficient parasitic burden [5].
Imaging Perspective
Ultrasound, with its real-time imaging capabilities, can definitively diagnose the presence of the parasite if it is alive—visualised as a motile, folded tubular structure exhibiting parallel echogenic stripes within a cyst. Even when the worm is dead or non-motile but maintains structural integrity, ultrasound remains the definitive diagnostic modality, with no alternative differentials [6].
CT findings are non-specific and may show heterogeneous soft tissue mass with peripheral enhancement, simulating an abscess.
Outcome
Surgical removal of the worm is the definitive treatment. However, antiparasitic treatment with ivermectin and diethylcarbamazine may be advised if secondary lesions are suspected in deeper parts such as the chest or abdomen [7]. Usually, clinical symptoms will disappear after the parasite is removed. Our patient was treated conservatively with diethylcarbamazine (tablets) and showed symptomatic improvement.
Take Home Message
Ultrasonography is a useful non-invasive diagnostic tool in making a reliable diagnosis of subcutaneous dirofilariasis, and identifying live adult worms. Specific imaging findings for dirofilariasis can be observed using ultrasound, and other possible differential diagnoses can also be excluded.
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Dirofilariasis
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Based on the provided CT brain images, there is no obvious sign of acute intracranial hemorrhage or large-scale infarction. A dense soft tissue lesion with relatively clear boundaries is observed in the subcutaneous layer of the left temporal region. Post-contrast images reveal areas of uneven enhancement.
Concurrent reference to ultrasound imaging shows a round or nearly round low-echo lesion in the subcutaneous tissue of the left temporal region, with heterogeneous internal echoes and possible tubular or cord-like structures. Surrounding soft tissue exhibits mild edema, and no significant blood supply signal is noted. Overall findings indicate a primarily subcutaneous lesion with no apparent skull destruction or intracranial extension.
Based on the patient’s imaging findings and the six-month history of having a “subcutaneous lump with itching,” along with local epidemiology and previous literature, the following potential diagnoses should be considered:
Considering the patient’s age, clinical presentation (local itching, recurrent subcutaneous nodules), imaging characteristics (tubular or cord-like structures observed under ultrasound), and history of improvement with treatment, the most likely diagnosis is: Subcutaneous Dirofilariasis (Dirofilariasis).
For further confirmation, surgical excision and pathological examination can be performed to identify the specific parasite species and to rule out other possible lesions.
Treatment Strategy:
1) Specific Pharmacotherapy: Commonly used antiparasitic agents include diethylcarbamazine (DEC) or ivermectin.
2) Surgical Treatment: If the lesion is localized and causes a significant cosmetic or symptomatic impact, surgical excision can be considered. Post-excision, the tissue and parasite can be examined for definitive diagnosis.
3) Symptomatic Management: For local itching, topical agents or oral antihistamines may be used. Anti-inflammatory medications can be considered if necessary.
Rehabilitation and Exercise Prescription (FITT-VP Principle):
Since this condition primarily involves subcutaneous parasitic infection, if pain or itching occurs at the lesion site, attention should be given to the wound or local skin condition. Overall rehabilitation exercise should focus on low to moderate intensity. Example guidelines:
• Frequency (F): 3–5 times per week, adjusted according to individual physical capacity and disease status.
• Intensity (I): Initially low to moderate (RPE scale around 4–6). Activities such as comfortable walking or slow cycling are recommended.
• Time (T): 20–30 minutes per session, gradually extending to 45 minutes if well tolerated.
• Type (T): Emphasize aerobic exercises (walking, Tai Chi, cycling) and light resistance training, avoiding strenuous or high-impact activities.
• Progression (P): If the condition stabilizes and tolerance improves, consider gradually increasing exercise intensity or duration. Progress should be gradual to avoid fatigue or skin irritation.
• Volume & Pattern (V & P): Combine with daily activities to maintain at least 150 minutes of exercise per week, which can be divided into segments.
During rehabilitation training, closely monitor local skin status. If skin lesions or incisions do not heal well, reduce activity intensity or pause exercise and consult a physician for re-evaluation.
This report is a reference analysis based on available information and cannot replace in-person diagnosis or the opinion of a qualified physician. If there is any change in your condition or if you experience any discomfort, please seek medical attention promptly.
Dirofilariasis
