|Year : 2014 | Volume
| Issue : 2 | Page : 86-89
Human dirofilariasis, "small bite big threat" to public health
Sulekha Bhageerathi, Jyothi Rajahamsan, Manjusree Shanmugham
Department of Microbiology, Government Medical College, Thiruvananthapuram, Kerala, India
|Date of Web Publication||14-Nov-2014|
Department of Microbiology, Government Medical College, Thiruvananthapuram, Kerala
Source of Support: None, Conflict of Interest: None
Dirofilariasis is an emerging zoonotic disease caused by Dirofilaria spp., a parasite carried by domestic dogs, cats and other wild animals. Human infection is accidental. Blood sucking arthropods transmit the infection to man. It usually presents as a nodular lesion in the lung, subcutaneous tissue, peritoneal cavity or eyes. We report a few cases of human dirofilariasis presented as subcutaneous swellings. In all the cases the larvae extracted were identified as Dirofilaria repens, based on the morphological features. The purpose of this article is to highlight the fact that human dirofilariasis is on the rise in Kerala, a geographical area that has a large mosquito density and high incidence of dirofilariasis in the canine population.
Keywords: D. repens , Emerging zoonosis, Human dirofilariasis
|How to cite this article:|
Bhageerathi S, Rajahamsan J, Shanmugham M. Human dirofilariasis, "small bite big threat" to public health. J Acad Clin Microbiol 2014;16:86-9
| Introduction|| |
Dirofilaria is a filarid parasite widely prevalent in carnivores (dogs, cats, foxes, raccoons and bears). Mosquitoes can transmit the infection to man. Dirofilaria repens and Dirofilaria immitis are the two species of dirofilaria commonly associated with human infection. D. repens is the major species reported from ocular and cutaneous dirofilariasis in Asia and India. Dirofilaria tenuis, a parasite of raccoons, is also associated with ocular lesions in man. , Dirofilaria ursi a parasite of bears, is also reported from human dirofilariasis.  D. immitis is commonly known as dog heartworm which resides in the right ventricle and pulmonary artery of dogs. It rarely causes human infections.
| Case report|| |
A 2-year-old girl presented with an abscess on her finger.
While doing incision and drainage (I and D), a creamy white thin worm of size 10.2 cm × 0.2 mm was extracted in October 2013.
A 50-year-old lady, otherwise healthy, presented with a painful swelling on the upper abdominal wall. Initial diagnosis was acute cholecystitis.
Laboratory investigation showed marked eosinophilia (38%) and a high erythrocyte sedimentation rate (ESR) (30 mm/hr). Ultrasonography of abdomen, revealed a worm in the lesion. Exploration of the lesion was done in November 2013 and a worm of size 11.5 cm × 0.25 mm was extracted.
A 13-year-old boy presented with a subcutaneous swelling of 16 mm × 3 mm size in the calf region of right leg. Laboratory investigation showed slight eosinophilia (10%). Excision of the swelling was done in April 2014 and a worm of size 10.5 cm × 0.2 mm was extracted.
An 11-year-old girl presented with a swelling on the medial aspect of left knee joint. Laboratory investigation showed moderate eosinophilia. Ultrasonography revealed a worm in the lesion. Exploration was done in April 2014 and a thin white worm of size 14 cm × 0.3 mm was extracted.
Worms extracted from all the cases were examined in our microbiology laboratory. All worms looked alike, creamy white, thin, uniformly cylindrical thread or twine like in appearance [Figure 1] and of size ranging from 10.2 cm × 0.2 mm to 14 cm × 0.3 mm.
Microscopically (10×), the worm showed slightly curved and pointed cephalic end, which was unarmed. The caudal end was rounded. It has a short esophagus and a long intestinal tube, with a patent anus at the posterior end, features suggestive of Dirofilaria [Figure 2] and [Figure 3].
On the glycerine wet mount, the cuticle of each worm was devoid of ridges or boggy swellings and was smooth and with 2-3 distinct layers of muscularis. Muscles were separated into dorsal and ventral bands and transverse striations with prominent longitudinal ridges were seen [Figure 4]. These features were suggestive of D. repens, which were absent in D. immitis.,
|Figure 4: Glycerine wet mount showing transverse striations with longitudinal ridges|
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Based on the size, morphological appearance, clinical condition and the geographic location, all worms were identified as D. repens.
| Discussion|| |
We have also received a few cases of dirofilariasis from conjunctival nodules for identification from health centres near Thiruvananthapuram district. Though human dirofilariasis is rare, the number of cases that are reported from various parts of India, particularly from Kerala, has been increasing over the last decade. The definitive host for D. repens and D. immitis are dogs but other animals also have been reported as reservoirs (cats, bears and raccoons). The adult female worm releases microfilaria into the circulation, which are ingested by the vectors (mosquitoes, ticks and fleas) during their meal of blood. An accidental human infection results in subcutaneous lesions anywhere in the body. In response to the infection, an inflammatory granulomatous reaction develops. The adult worms do not reach maturity in humans and do not produce microfilaria.
Wu CC et al. reported an incidence of canine dirofilariasis in 307/1228 (25%) dogs at Taiwan, and 503/1228 (41%) in mid-Taiwan.  Dirofilaria spp., Acanthocheilonema spp. and Brugia spp. are the commonly reported filarid parasites in Indian dogs. Lucy Sabu et al. reported a higher incidence of dirofilariasis in dogs, in Kerala, in Calicut (13.4%) and in Trichur (24.2%). 
The first case of human dirofilariasis was reported by Addario in 1885 from Milan, Italy and was described as an infection by filaria conjunctivae. Human cases have been reported from Southern and Eastern Europe, Sub-Saharan Africa, Asia particularly Sri Lanka, Malaysia and India. , Italy has the highest prevalence of human dirofilariasis (66%) followed by France (22%). In India cases have been reported from Assam,  Kerala, ,,, Tamil Nadu  and Karnataka.  Available literature shows that in India, there is a wide variation in age groups affected, ranging from 14-70 years. In Srilanka, infection with D.repens is reported to be a common zoonosis, where children younger than nine years are most likely to be affected. 
Dirofilaria repens and D. immitis are the two species commonly associated with human infections. A total of 397 cases of subcutaneous dirofilariasis caused by D. repens have been reported from 30 different countries, according to a review of world literature.  D. repens was the major species associated with ocular and subcutaneous dirofilariasis reported from Asia as well as India. ,, Two important species of genus Dirofilaria are reported to be involved in ocular infections namely, D. tenuis and D. repens.
In cutaneous dirofilariasis, the patient seldom seeks medical help. Most subcutaneous nodules are benign, unless the nodules are situated at a lymphnode or adjacent to major blood vessels. Only few cases present with systemic symptoms. Eosinophilia is also not usually present. The diagnosis is established by biopsy, however careful inspection of the entire tissue may be needed to find out the parasite. In order to confirm the Dirofilaria spp., DNA extraction followed by pan-filarial polymerase chain reaction (PCR) may be performed. 
Infection with D. immitis is rare in man. More than 50% of cases will be asymptomatic and show coin lesion only on routine chest radiography, while others presented with cough, chest pain or hemoptysis, most likely due to pulmonary infarction. In some instances, lung infiltrates that resolve into nodules are noted. Eosinophilia occurs in 15% of cases. The diagnosis is made with certainty only by biopsy. Although serological tests are available, their sensitivity and specificity are not adequate to rule out the potential life threatening conditions.  As subcutaneous dirofilariasis due to D.repens can mimic various benign and malignant lesions, histopathological examination remains a gold standard to confirm the diagnosis.
Recent studies of the relationship between D.immitis and D.repens and the symbiotic bacteria of the genus Wolbacteria, and some proteomic analysis of worms has resulted in a profound shift in understanding of filarial biology, the pathologies that they inflict in different hosts and the issues related to the treatment. Based on the electron microscopy and the molecular studies, Wolbacteria belongs to the order Rickettsials, genus Wolbacteria. The presence of these intracellular bacteria in abundance in the larvae and hypodermal cords of adults of both gender and genital organs of female worms are suggestive that these are essential for larval development and long term survival of the adults in their host. Adult worm can live over seven years and microfilariae live as long as two years. The environmental temperature is the key factor that determines the length of development in the mosquitoes.  Extraction of the worm or complete excision of the lesion is the treatment of choice for human dirofilariasis. As microfilaremia is extremely rare, chemotherapy is not recommended. ,
Many of the infections remain undiagnosed and unpublished, hence there is a need for increased awareness about the infections, and the diagnostic serological tests which would improve the prevalence rate and patient care.
The incidence of human dirofilariasis is on the rise in several states of India, particularly in Kerala  . It could be attributed to the ecological niche suitable for the breeding of the vector species,  warm climate and the higher incidence of dirofilariasis in canine population.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]