Human dirofilariasis: An emerging zoonosis.

Dirofilariasis is one of the zoonotic filarial infections inadvertently affecting the humans. It is caused by filarial nematodes of genus Dirofilaria, which naturally infects several domestic and wild animals, though canines are the principal reservoir hosts. There are about 40 recognized species of Dirofilaria and at least six of them i.e., Dirofilaria immitis, Dirofilaria repens, Dirofilaria striata, Dirofilaria tenuis, Dirofilaria ursi and Dirofilaria spectans are known to cause accidental infections in humans.[1] Mosquitoes belonging to the genera Aedes, Armigeres, Culex, Anopheles, and Mansonia species are reported to be involved in its transmission. Some species of fleas, lice, and ticks are also presumed to act as vectors.[2] The type of Dirofilaria species and the vector involved in spreading the infection seem to vary with the different geographical regions.

Though human dirofilariasis as a zoonotic infection is thought to be rare, cases are being increasingly reported in the past few years making it a case for consideration as an emerging zoonosis in many parts of the world.[3] The infection caused by D. repens is the most widely reported dirofilariasis with endemic foci in Eastern and Southern Europe, Asia minor, Central Asia and Sri Lanka.[3] Italy is known to be one of the worst affected country.[4] D. repens is also the main causative agent of subcutaneous dirofilariasis in Asia.[56] Within the Asian sub-continent, it is the Sri Lanka, which is the most endemic zone for this infection.[7] D. immitis infection is relatively uncommon and is reported from Malaysia.[7]

As humans are accidental 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. As natural transmittance of dirofilariasis is through microfilariae, which any way does not occur in humans, dirofilariasis cannot be transmitted from person-to-person.

Human dirofilariasis typically manifests as either subcutaneous nodules or as lung parenchyma disease. Patients infected with D. repens notice a subcutaneous lump in the affected area which most commonly includes; face and conjunctiva of the eye and sometimes chest wall, upper arms, thighs, abdominal wall and male genitalia. Ocular involvement is usually periorbital, orbital, subconjunctival, or subtenon infection.[8] Human D. immitis infection has been associated with the human pulmonary dirofilariasis and is usually asymptomatic. Those with symptoms have cough, chest pain, fever, and pleural effusion.[9]

In India though Dirofilaria cases are being reported occasionally, the number of cases is gradually increasing. While most of the cases reported from India are due to infection with D. repens, some of the D. immitis and D. tenuis infections have also been reported.[910] It is the South part of India, which is geographically close to Sri Lanka from where most of the cases of D. repens have been reported.[7] Though Kerala State in India seems to be the focus for human dirofilariasis, few cases have also been reported from States of Karnataka,[11] Assam[7] and Orissa.[6] The first cases of human ocular and subcutaneous dirofilariasis were reported from Kerala in 1976 and 2004 respectively.[512] Thereafter in 2009, D. repens infection involving lower part of the body was reported from Orissa, an Eastern part of India.[6] Three more cases with D. repens infection were reported in the very next year (2010) from Assam.[7] In the past 2 years also cases of solitary subcutaneous dirofilariasis with D. repens were reported in Kerala and Karnataka State.[1314] Pulmonary dirofilariasis with D. immitis was reported from India for the 1st time in 1989 by Badhe and Sane.[9] So far two cases of zoonotic filariasis due to D. tenuis have been reported and both cases were from South India.[14]

In the present issue of this journal, three cases of human subcutaneous dirofilariasis are being reported from Assam State. Three cases of ocular dirofilariasis were earlier described from the same Eastern Assam[7] indicating the presence of a wider spectrum of manifestations of this infection in this region. The subcutaneous Dirofilaria infections are usually associated with negligible to mild inflammatory symptoms occurring periodically as observed with the cases presented in the current report, which suggests that there might be the large number of cases unreported.

The identification of Dirofilaria worm is carried out by studying the fully matured adult worm.[7] Surgical removal of the worm and biopsy help in both diagnosis and treatment. Morphological examination has limitations in the identification of the exact species as a large number of zoonotic Dirofilaria species have been described that share morphologic features with D. repens.[15] The molecular tools that aid in species identification are not widely available. It is also possible that there are different strain variations of dirofilarial parasites as indicated by one of the reports of a case of D. repens infection with a subcutaneous gravid worm and the patient's concomitant meningoencephalitis and aphasia. Molecular analysis of the highly conserved mitochondrial 12S rRNA gene of D. repens in this case showed a 3% deviation from D. repens sequences deposited in public databases.[15]

The alarmingly increasing trend of dirofilariasis infection in the past few years points towards a need for proper and necessary action to be taken towards the control of this parasitic infection. Systematic epidemiological surveys, developing suitable molecular diagnostic tools for species identification and more intensive studies on vectors, natural hosts, and environmental factors will help in assessment of the exact prevalence of this emerging zoonotic infection and in devising appropriate control measures.