Human subcutaneous dirofilariasis (HSD) is a zoonotic filariasis caused by an infection with one of the several species of worms belonging to the genus dirofilaria. Most documented cases are attributed to dirofilaria repens. The parasites are transmitted to man by zoo-anthropophilic blood sucking insects and an infection is manifested as subcutaneous nodules. We hereby report a case of subcutaneous dirofilariasis in front of the neck, in a 40-year-old female, clinically mimicking thyroglossal cyst. An excision biopsy showed the presence of an adult dirofilaria identified as D. repens with granulomatous inflammatory reaction.
Keywords: Dirofilariasis, human, neck, subcutaneous
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Srinivasamurthy V, Rao M S, Thejaswini M U, Yoganand. Human subcutaneous dirofilariasis. Ann Trop Med Public Health 2012;5:349-51
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Srinivasamurthy V, Rao M S, Thejaswini M U, Yoganand. Human subcutaneous dirofilariasis. Ann Trop Med Public Health [serial online] 2012 [cited 2020 Aug 7];5:349-51. Available from: http://www.atmph.org/text.asp?2012/5/4/349/102052
The genus dirofilaria includes various species that are natural parasites of dogs, cats, foxes and other wild animals. Although about 40 different species of dirofilaria are recognized, fewer than 6 are known to cause human infections, which can be found worldwide.  HSD has not been widely reported in India, but there is probably some incidence of human infection with D. repens in Kerala, and the disease is relatively common in Sri Lanka, which is geographically close to Southern India. 
Most of the documented cases of human dirofilariasis in India have had occular infections. Very few cases of subcutaneous dirofilariasis have been reported.  We report here one such case of subcutaneous dirofilariasis affecting the front of the neck without any other clinical manifestations.
A 40-year-old female, tailor by profession, presented with a nodule in the thyroid region since 8 months, increasing in size from 1 month. On examination, the nodule measured 2 × 1 cm, soft to firm in consistency, just above the thyroid cartilage. Skin over the swelling was normal. Cervical lymph nodes were not enlarged. Routine laboratory investigations, including an absolute eosinophilic count were within normal limits. A clinical diagnosis of thyroglossal cyst/lipoma was made. The lesion was excised under local anesthesia and sent for histopathological examination. Grossly, the specimen was grey-white, measuring 1.5 × 1 cm with a central cystic area. Multiple histopathological sections of the specimen showed a cyst, lined by a peripheral layer of granulation tissue with an intense inflammatory cell infiltrate composed of neutrophils, lymphocytes, occassional eosinophils and plasma cells along with foreign body giant cells. The center of the lesion showed cross sections of a nematode parasite [Figure 1], exhibiting a thick cuticle with fine external longitudinal ridges and a prominent circumferential muscle layer showing transverse striations [Figure 2]. Single intestine, two reproductive tubes and a gravid uterine segment were also recognized. Based on these features, the worm was morphologically identified as an adult female D. repens. A final diagnosis of subcutaneous dirofilariasis was made.
||Figure 1: Histopathology section showing dirofilaria repens (arrow) with surrounding inflammatory reaction. (H and E, × 100)
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||Figure 2: Cross section of dirofilaria repens adult worm consisting of thick multilayered cuticle, transverse striations, longitudinal wavy ridges and gravid uterus. (H and E, × 400)
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Human dirofilariasis is caused by D. repens, D. immitis, D. tenuis and D. urisi. In the USA, D. tenuis, a parasite of raccoons, is the species most commonly found in humans, whereas, D. repens, a parasite of dogs and cats, causes most cases of human dirofilariasis detected in Europe, Asia and Africa.  The first reported case of human occular filariasis in India occurred in Kerala in 1976, and subcutaneous dirofilariasis caused by D. repens was recorded in 2004 in the same region.  Even though, Southern India is considered endemic for dirofilariasis, there are few case reports from the Northern or Western parts of India.  A review of the literature showed about 402 cases of HSD, most of them reported from Southern and Eastern Europe, Asia and Sri Lanka. ,,
Man is an accidental host. D. repens larvae are inoculated by the bite of an infected insect and can invade a variety of tissues, where they mature into adult worms and die. The dead worms evoke a chronic inflammatory reaction with foreign body giant cells.  Almost all human infections by D. repens are localized to the upper half of the body, mostly in and around the eyes, although, it can occur in the extremities and thoracic wall as well.  The patients usually present with single migratory nodules measuring 0.5 to 2.5 cm in size, occurring in the exposed parts of the skin, which may or may not be tender. The important risk factors regarding human infections are mosquito density, warm climate with an extended mosquito breeding season, outdoor human activities and close contact with dogs. Even though, Culex, Aedes and Anopheles mosquitoes are the vectors for this parasite, information is lacking on which mosquito vector is involved in transmission of D. repens in Southern India.  D. repens is a nematode with a long thin filariform appearance. An average diameter of the adult worm is approximately 450 μ. These worms have 95 longitudinal ridges on an external cuticle, 2-5 chord nuclei per section and robust muscle cells. They have a rounded anterior end with buccal cavity. In contrast to the rounded short tail of female worms, the male worms have a coiled tail with several perianal papillae. 
The definite diagnosis of HSD can be made after surgical excision or biopsy. Blood eosinophilia or elevated serum IgE levels are rarely observed. Therefore, eosinophilic counts and measurements of total IgE are of limited value in screening for dirfilariasis in patients with subcutaneous nodules. 
In order to confirm the diagnosis of D. repens infection, DNA extraction followed by pan filarial polymerase chain reaction (PCR) may be performed. Microfilaria has never been reported in human blood. Eosinophilia occurs in <15% of the cases with D. immitis and rarely with D. repens.  In our case also, blood smears were negative for microfilaria, and there was no eosinophilia.
Surgical removal of the worm/nodule not only establishes the diagnosis in most of the cases, but also provides a definitive cure.  There is no need for chemotherapy, because microfilaremia is extremely rare. However, a few reported cases of meningoencephalitis, secondary to D. repens microfilaremia, were treated with albendazole and methyl prednisolone and showed good response.  It is emphasized that, both clinician and pathologist should have an increased awareness of this entity and include dirofilariasis in the differential diagnosis of subcutaneous nodules at any site. 
Human infection with D. repens has been increasing in India; many of them remain undiagnosed or unpublished. Medical awareness of an infection risk is essential for a correct diagnosis in the light of few reported cases of meningoencephalitis. The use of serologic analysis for D. repens somatic antigen merits further study as a diagnostic aid, which may improve the patient care.
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Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]