Human subcutaneous dirofilariasis of forearm an unusual presentation.

Human subcutaneous dirofilariasis (HSD) is a rare zoonotic filarial infection caused by filarial worms of the genus Dirofilaria. In view, recent rise in human Dirofilaria repens infections in several regions of the world, is considered as emerging zoonotic infection transmitted to man by zooanthrophilic blood sucking insects. Most of the documented cases of human dirofilariasis recorded in India are ocular infections and very few cases of subcutaneous dirofilariasis have been reported. We hereby present a case of subcutaneous human dirofilariasis of forearm and also emphasize on increased awareness of this entity for clinicians and pathologists in the differential diagnosis of patients with subcutaneous nodules.

What was known?

Most of human dirofilariasis are pulmonary or ocular infections

Introduction

Zoonotic infections have been reported in all parts of the world but human filariasis recognition gains importance as dirofilariasis in humans due to Dirofilaria repens infection is a fast emerging zoonosis in Kerala. The worm commonly known as dog heart worm, is widely dispersed and found in the tropics, subtropics, and temperate zones.[1]

The principal reservoir of D. repens is the dog and humans are accidental hosts with patent infections being extremely rare. Differential diagnoses of human subcutaneous dirofilariasis (HSD) include neoplasia and other granulomatous diseases and a definitive diagnosis usually requires surgical removal and examination of a lesion. Most of human dirofilariasis are pulmonary or ocular infections, but there have been a few reports of human dirofilariasis in unusual sites.[2]

Human dirofilaria has not been widely recognized in India, but there is probably a focus of human infections with D. repens in Kerala and the disease is relatively frequent in Sri Lanka. In India, cases have also been reported from Assam, Maharashtra, and parts of Karnataka.[34] We present this case because of very few cases of subcutaneous dirofilaria have been reported and most of the documented cases being dirofilaria associated with ocular infections.[5] Dirofilariasis should be considered as a differential diagnosis for migratory subcutaneous swellings and conjunctival nodules in Kerala and elsewhere in southern India.

Case Report

A 25-year-old male patient presented with painless mass in the left forearm for past 1 year and the mass was well-defined, nontender, and subcutaneous in location. On examination, the swelling was on the left side of his forearm and measuring approximately 3 × 2 cm [Figure 1]. His blood smear examination was negative for microfilaria. All other systemic examination and laboratory investigations were within normal limits. It was clinically diagnosed as sebaceous cyst and advised complete excision of the mass for definitive diagnosis. The excised mass was 2 × 2 cm and grey-white. The cut surface was grey-white with necrotic areas.

Figure 1

Forearm mass: Soft, nontender, subcutaneous swelling measuring 3 × 2 cm, on the flexor aspect of the left forearm

Hematoxylin and eosin stained sections showed cross-sections of nematode, which is surrounded by an inflammatory granulation tissue composed of eosinophils, lymphocytes, and occasional foreign body giant cells [Figure 2]. An outer surface of the nematode shows cuticle showing fine transverse striations and prominent longitudinal ridges [Figure 3]; with these features the worm was identified as an adult, female D. repens.

Figure 2

Histopathology section showing dirofilaria repens with dense inflammatory reaction (hematoxylin and eosin (H and E), ×100)

Figure 3

Cross-section of dirofilaria repens adult worm composed of multilayered cuticle, transverse striations, and longitudinal wavy ridges (H and E, ×400)

Discussion

There are about 40 recognized species of Dirofilaria and at least six of them, that is, D. immitis, D. repens, D. striata, D. tenuis, D. ursi, and D. spectans are known to cause accidental infections in humans.[6] Mosquitoes belonging to the genera Aedes, Armigeres, Culex, Anopheles, and Mansonia species are reported to be involved in its transmission. As humans are dead-end hosts of Dirofilaria and not the natural hosts, in most of the cases it is thought that the infective larvae injected through mosquito bites perish before attaining maturity.[3] Some species of fleas, lice, and ticks are also presumed to act as vectors.[7] D. immitis (the dog heartworm) is a worldwide filarial parasite of dogs and adult worms (up to 30 cm long) are usually located in the dog's heart. In heavy infections or when adult worms die, the parasites may be carried to the pulmonary vessels where they may produce emboli. The worms do not mature to adulthood in humans, but larval stages have been reported in cutaneous nodules (which may be confused with tumors) and have sometimes produced lesions in the lungs (coin lesions) or in breast tissue.[8]

The genus Dirofilaria has two subgenera: Dirofilaria and nochitella. While the nematodes in the subgenus dirofilaria have smooth cuticles and are typically found in the pulmonary arteries and the subgenus nochitella (of which D. repens is the species type) have longitudinal cuticular ridges and are usually found in subcutaneous tissue.[5] More important are the morphologic features seen in transverse sections of the worms. The cuticle is multilayered about 5-8 μm thick in living worms; the longitudinal ridges are low, smoothly rounded, and about 10 μm apart. On the inner surface of the cuticle in the lateral fields there is a conspicuous cuticular ridge that protrudes into the inner surface of lateral chords.[9]

The definite diagnosis of HSD can be made after surgical excision or biopsy. Blood eosinophilia or elevated serum immunoglobulin (Immunoglobulin)E 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, deoxyribonucleic acid (Deoxyribonucleic acid) extraction followed by pan-filarial polymerase chain reaction (PCR) may be performed. Microfilaria has never been reported in human blood with eosinophilia occurring in <15% of the cases with D. immitis and rarely with D. repens.[10]

In conclusion, zoonotic filarial infections will undoubtedly continue to be recognized in new geographical areas and in different sites in the host. Clinicians, pathologists, and parasitologists should have an increased awareness of this entity and also possibility of presentation of these infections both in currently clinical recognized and in unusual presentations.

What is new?

Subcutaneous human dirofilariasis rare and rise in focus of infection in southern India