Tropical Parasitology

: 2021  |  Volume : 11  |  Issue : 2  |  Page : 125--127

The first case report of subcutaneous dirofilariasis caused by Dirofilaria repens in Thailand

Jerapas Thongpiya1, Suchada Kreetitamrong2, Theerasak Thongsit3, Tanaporn Toothong4, Sunsanee Rojanapanus4, Patsharaporn Techasintana Sarasombath1,  
1 Department of Parasitology, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand
2 The Office of Disease Prevention and Control 5th, Ratchaburi, Thailand
3 Kanchanaburi Provincial Public Health Office, Kanchanaburi, Thailand
4 Division of Vector Borne Disease, Department of Disease Control, Ministry of Public Health, Nonthaburi, Thailand

Correspondence Address:
Patsharaporn Techasintana Sarasombath
Department of Parasitology, Faculty of Medicine Siriraj Hospital, Mahidol University, Adulyadejvikrom Building, 7th Floor, 2 Wanglang Road, Bangkoknoi, Bangkok 10700


Dirofilariasis is a rare zoonotic disease which is commonly caused by two Dirofilaria species; Dirofilaria immitis and Dirofilaria repens. Humans are accidental dead-end hosts of the parasites, and the infection is mainly asymptomatic. Here, we report the case of a 54-year-old Thai woman who experienced a painful left shoulder nodule and eosinophilia for 1 month. An excisional biopsy of the nodule revealed a degenerated filarial nematode compatible with adult females of the Dirofilaria species. Molecular identification of the partial 12 mt rRNA gene of the worm confirmed that the causative species was D. repens, a zoonotic filariasis that causes subcutaneous dirofilariasis in dogs and cats. To the best of our knowledge, this was the first reported case of subcutaneous dirofilariasis caused by D. repens in Thailand. This increased concerns about zoonotic filariasis from natural animal reservoirs in Thailand.

How to cite this article:
Thongpiya J, Kreetitamrong S, Thongsit T, Toothong T, Rojanapanus S, Sarasombath PT. The first case report of subcutaneous dirofilariasis caused by Dirofilaria repens in Thailand.Trop Parasitol 2021;11:125-127

How to cite this URL:
Thongpiya J, Kreetitamrong S, Thongsit T, Toothong T, Rojanapanus S, Sarasombath PT. The first case report of subcutaneous dirofilariasis caused by Dirofilaria repens in Thailand. Trop Parasitol [serial online] 2021 [cited 2022 Oct 1 ];11:125-127
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Full Text


Human dirofilariasis is a rare zoonotic disease caused mainly by Dirofilaria belonging to two species; i.e. Dirofilaria repens and Dirofilaria immitis.[1],[2] Although Dirofilaria infection in dogs and cats is common, especially in several regions of Thailand, human infection is rare and only a few cases had previously been reported.[3],[4],[5],[6] In prior cases, ocular dirofilariasis was reported in all instances.[4],[5],[6] Moreover, molecular identification of the responsible species was performed in only one case in 2018.[6]

Dirofilaria infects various domestic and wild animals but primarily affects canines which serve as major natural reservoir hosts.[1],[2] In animals, the main clinical presentations of D. repens and D. immitis infection are subcutaneous and cardiopulmonary dirofilariasis, respectively.[1],[2] However, the infection in humans can be found in several anatomical sites which are distinct from those commonly associated with each species.[1] Therefore, identification of the causative species is crucial for understanding disease epidemiology, prevention, and control. Here, we report the first case of human dirofilariasis in Thailand which presented with a subcutaneous mass, caused by D. repens.

 Case Report

A 54-year-old woman began to experience a painful palpable mass on her left shoulder for 1 month. She noticed that the mass was progressively becoming larger and had caused her increasing pain for 1 week. She denied any history of fever or any other systemic abnormalities. She lived in a home located near a mosquito-dense woodland area in Tha Muang district in Kanchanaburi Province and had no travel history in the past year. She was healthy and had no history of underlying disease. She owned a cat 6 years ago and currently lived with a pet dog and five stray dogs wandering around the house. A physical examination revealed a 1-cm tender and slightly movable nodule on her left shoulder with no axillary lymphadenopathy. A complete blood examination was performed which revealed 14.6% eosinophilia with an absolute eosinophil count of 1146 cells. No blood microfilaria was observed. An excision of the nodule revealed a cystic nodule with irregular fibro-fatty tissue measuring 2.1 cm × 2.3 cm × 1 cm. Histological findings of the mass revealed a circumscribed cystic lesion surrounded by a fibrous capsule with intense eosinophilic lymphoplasmacytic granulomatous reactions with central necroinflammatory debris and a degenerated coiled nematode worm [Figure 1]a. The transverse section of the worm was compatible with the filarial nematodes, and it had a thin cuticle with irregular spacing of longitudinal ridges. Meanwhile, a transverse section revealed two uterine tubes with developing cells, which were compatible with early adult Dirofilaria females [Figure 1]b. Filarial species identification was further confirmed by a polymerase chain reaction (PCR)-targeted the partial mitochondrial 12S ribosomal RNA (mt 12S rRNA) using the protocol as previously described.[7],[8] The PCR products were cloned into the T-Vector pMD20 and subjected for sequencing. The mt 12S rRNA sequence was then blasted with an NCBI reference-sequence database which was 99.12% identical to Dirofilaria sp. “hongkongensis” (GenBank accession number KY750550.1) and 99.12% identical to D. repens (GenBank accession number GQ292761.1). The sequence was submitted to the NCBI database under GenBank accession number MW051483. After excision of the nodule, no additional treatment was performed. During the following visit, the lesion was completely resolved.{Figure 1}


Dirofilariasis is one of the zoonotic filarial infections caused by filarial nematodes belonging to the genus Dirofilaria. More than forty species of Dirofilaria naturally infect a wide variety of domestic and wild animals; however, only some species are known to infect humans including D. repens, D. immitis, Dirofilaria tenuis, Dirofilaria ursi, Dirofilaria subdermata, Dirofilaria spectans, and Dirofilaria striata.[9] The transmission of Dirofilaria occurs through mosquito bites of flies belonging to the genera Aedes, Culex, Anopheles, and Mansonia.[9] The Dirofilaria species that causes human dirofilariasis is closely related with the geographic region in which the infection is acquired and the site of the worm in the human body.[2] Two Dirofilaria species, i.e. D. immitis and D. repens, commonly cause most human infections.[1] While D. immitis causes both canine and feline cardiopulmonary filariasis, D. repens is a common subcutaneous parasite found in both dogs and cats.[1] In humans, clinical presentations of human dirofilariasis are mostly related to the causative species; D. repens usually causes subcutaneous and ocular dirofilariasis, whereas D. immitis is responsible for human pulmonary dirofilariasis.[1] However, this is not always the case since subcutaneous dirofilariasis can also be caused by D. immitis and pulmonary dirofilariasis can be caused by D. repens and vice versa.[1] In the past, D. immitis was prevalent in domestic canines and felines as well as humans in tropical and temperate regions across the world, while D. repens was exclusive to the Old World regions.[1] However, there has been a change recently in the pattern of distribution and prevalence of dirofilariasis as more cases of the disease in nonendemic regions have been reported.[1],[9] Thus, identification of the causative Dirofilaria species is crucial for disease control strategies as well as raising awareness of a possible emerging zoonosis.

In this study, we report the first case of human subcutaneous dirofilariasis caused by Dirofilaria, which is closely related to D. repens in Thailand. Previously, only three cases of human dirofilariasis caused by D. repens were reported, and in all those cases, ocular dirofilariasis was present.[4],[5],[6] Although D. immitis and D. repens infection in animals, especially in domestic dogs and cats in Thailand, is prevalent throughout the country, human infection is considered rare.[3],[8],[10],[11] This may be due to the fact that the infection in humans may be transient, misdiagnosed, overlooked, or not reported.[1],[12] Moreover, patients with subcutaneous dirofilariasis may have clinical features that are not typical, and the lesion may resolve itself without notice.[1],[12]

Reports about zoonotic filariasis have been increasing lately throughout the world, including Thailand.[1],[9],[13],[14] This presents a possible scenario of an emerging zoonosis in process. A possible cause for the increased incidence of human dirofilariasis may be due to climate change and global warming.[1],[9],[12] It has been shown that an increase in temperature can speed up the length of larva stage 3 (L3) developmental period in mosquitoes, shorten the developmental period, alter seasonal transmission, increase the mosquito feeding patterns, and expand breeding regions.[1],[12] The emergence of zoonotic filariasis in Thailand had raised concerns about initiating proper control strategies to eliminate filarial infection in reservoir animals including dogs and cats. Moreover, care must be taken regarding mosquito eradication in order to reduce the rate of filarial transmission.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

This study was financially supported by the Internal Departmental Funding, Department of Parasitology, Faculty of Medicine Siriraj Hospital, Mahidol University.

Conflicts of interest

There are no conflicts of interest.


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