Human dirofilariasis is an uncommon zoonotic disease where man is an accidental host.About forty species of Dirofilaria are identified,among which only a few give rise to human infection.1 Dirofilaria repens is the commonest species identified in India. The first reported case of human ocular dirofilariasis in India occurred in Kerala in 1976 and subcutaneous dirofilariasis was recorded in the same region in 2004.Most of the documented cases in India are of ocular dirofilariasis.Very few cases of subcutaneous dirofilariasis have been reported.2
A 65yr old female housewife from an agricultural background presented with a non tender nodule in the left temporal region besides the eyebrow of 2 months duration which had gradually progressed to the size of 2.5 x 2 cm. It was firm in consistency. Skin over the swelling was normal. A provisional diagnosis of epidermal cyst was made, excised and submitted for histopathological examination. Grossly, the specimen was grey white m 2.5 x 2 x 1.5 cm, cut section was solid grey white. On closer examination retrospectively revealed a thread like worm (Figure 1) Microscopic examination of Hematoxylin and Eosin stained sections showed granulation tissue with intense neutrophilic and eosinophilic infiltration, multinucleated giant cells, plasma cells. Cross section of a nematode parasite with a thick external cuticle, prominent circumferential muscle and cut section of intestine were recognized (Figure 1). Based on these findings the worm was identified as Dirofilaria repens. A diagnosis of Subcutaneous Dirofilariasis was made.
Complete blood counts and Peripheral smear examination did not show eosinophilia. Serum Ig E levels were normal. Ophthalmologic examination was requested to look for ocular parasite which was negative.
Human dirofilariasis is a zoonosis caused by animal filarial parasite Dirofilaria species. D. repens, d.immitis, D. tenuis and D.urisi are the known species causing human infection. D.repens, a parasite of cats and dogs is most commonly implicated in human dirofilariasis in Europe, Asia and Africa.1, 2 As with human filarial counterparts (Brugia, Wuchereria) Dirofilaria are transmitted by an arthropod intermediate hosts – various mosquito species including Aedes, Anopheles, Culex species, in which the microfilariae develop for about two weeks before reaching the infective stage. Dogs, wild canids and other mammals comprise the definitive host reservoir. After developing in subcutaneous tissue of the definitive host animal dirofilaria enter the blood supply and mature in the right side of the cardia and pulmonary arteries resulting in heart failure and pulmonary complications, explaining the lay labeling of dirofilaria as ‘Heartworm’’. Humans are accidental hosts and human infections does not progress to allow for development into sexually mature helminthes.3
In humans, the infective larvae enter subcutaneous tissues. Case reports describe dirofilaria isolated from numerous sites including skin, orbits, oral cavity, scrotum, and peritoneal cavity. If the worms enter venous circulation it may embolize to the pulmonary arteries resulting in infarction and formation of a granulomatous pulmonary nodule. Serological studies have implicated Dirofilaria as the cause of eosinophilic meningitis, arthritis and childhood asthma.4
Dirofilaria is a nematode with along thin filiform appearance. Average diameter of the adult worm is approximately 450 micrometer. These worms have longitudinal ridges on an external cuticle, 2-5 chord nuclei per section and robust muscle cells. They have a rounded anterior end with a buccal cavity. In contrast to the rounded short tail of female worms, the male worms have a coiled tail with several perianal papillae. Identification of the species of Dirofilaria relies upon its gross and microscopic characteristics. D.repens is identified by the presence of external longitudinal cuticular ridges and transverse striations which are absent in D. immitis.5 Worms isolated from human tissue appear to die prior to reaching sexual maturity. Pathology observed in dirofilarial infection is generally believed to be due to the host immune response to dying worms and its contents.1 In order to confirm the species, DNA extraction followed by pan filarial Polymerase chain reaction may be performed. 1, 2, 6 Surgical excision of the lesion is both diagnostic and therapeutic. Some advocate adding oral treatment with Albendazole, Diethylcarbamazine and Ivermectin.6
Human infection with Dirofilaria has to be in the differential diagnosis of subcutaneous nodules showing granulomatous reaction as it is one of the emerging zoonotic infection in India. Most cases are diagnosed retrospectively, when the histopathological sections of biopsy are viewed. As the number of reported cases are increasing it is imperative that pathologists, microbiologists, surgeons, ophthalmologists and veterinarians are aware of dirofilariasis. The development of specific and sensitive diagnostic tools for the most common specie D. repens and D immitis may help in early diagnosis.