Eur J Cardiothorac Surg 1999;16:475-477
© 1999 Elsevier Science NL
Pulmonary presentation of Dirofilaria immitis (canine heartworm) in man
Kishan Narinea,
Barbara Brennanb,
Ian Gilfillana,
Andrew Hodgea
a Department of Cardiothoracic Surgery, Fremantle Hospital, Alma St, Fremantle, Perth 6010, Australia
b Department of Pathology, Fremantle Hospital, Alma St, Fremantle, Perth 6010, Australia
Corresponding author. Tel.: +61-8-9431-3104; fax: +61-8-9431-2915
e-mail: kishan.narine{at}health.wa.gov.au
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Abstract
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Canine heartworm or Dirofilaria immitis can occasionally infect man. We present the case of a 36-year-old Balkan woman referred to us for a thoracoscopic biopsy of a well defined pulmonary mass. The latter was thoracoscopically resected and proved to be due to Dirofilaria immitis on histopathological examination. To our knowledge this is the second reported case in Western Australia, the first being that reported by Brine et al. [1]. The prevalence of the disease in Australia was recognized in 1969 in one study in the state of Queensland, where 12% of 761 dogs at one veterinary clinic and 20% of 296 dogs at another were shown to be infected. Dirofilaria immitis resides in the right ventricle and pulmonary arteries of dogs [2]. The microfilariae are released into the blood stream and are transmitted to secondary hosts by mosquitoes.
Key Words: Dirofilaria immitis • Canine heartworm
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1. Case report
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Our patient is a fit 36-year-old woman who has been living in Australia for several years. She had presented to her general practitioner with pleuritic chest pain of several weeks duration. A subsequent chest X-ray, revealed a well defined pulmonary opacity in the upper lobe of the right lung. She had no significant past medical history. Her blood biochemistry revealed no anemia (hemoglobin 140 g/l, red blood cell count 4.4x10 /l). nor eosinophilia (0.08x10 /l). A CT scan of her thorax (Fig. 1) confirmed the presence of a focal lesion in the right upper pulmonary lobe. There were no other pathological findings. Two attempts at diagnostic fine needle biopsy were unsuccessful due to insufficient sampling for histopathological examination. The patient was subsequently referred to us for thoracoscopic biopsy of the lesion.
At thoracoscopy we found a well demarcated hard nodule located in the apex of the right upper lobe. Macroscopic examination revealed that the lesion measured 20x25x20 mm. The apex was adherent to the chest wall at that site.
Thoracoscopic wedge resection was performed using automatic stapling apparatus. The nodule with approximately 1 cm of lung tissue surrounding it was removed. Microscopic sections (Fig. 2) showed the presence of degenerate helminthes with a diameter between 120 and 180 µm, present within arteries in an infarcted area of lung and surrounded by a granulomatous reaction. These were in keeping with Dirofilaria immitis. At 6 months follow-up our patient was doing well and showed no recurrence of the disease at that time.
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Fig. 2. Histological section showing profiles of Dirofilaria (arrow) in the lumen of an artery (arrow head) within the infarcted lung. (Magnification 200x) (Hematoxylin and eosin stain).
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2. Comment
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Dirofilarial infections are well known to veterinarians [3]. D. immitis is a filarial nematode whose primary host is the dog. The disease is transmitted by mosquitoes to humans who are secondary hosts. Although rare the incidence of Dirofilarial infection appears to be increasing [4]. Moreover the similarity of the disease to tuberculosis and the difficulty in its diagnosis suggests that the true incidence is higher than that reported. While the majority of reports to date concern transmission from dogs, human infection from cats, raccoons, bears, and porcupines have been reported. In areas where parasitaemias in the dog are prevalent the incidence can be expected to be higher.
In the infected dog the adult female resides in the right ventricle where it releases thousands of microfilariae into the circulation. The vector mosquito that feeds on an infected dog transmits microfilariae to other dogs or to secondary hosts upon whom they feed. In the dog the microfilariae migrates into the subcutis and muscle sheath where they mature and are subsequently transported to the heart. A similar transmission pattern has been proposed for humans. After reaching the human heart the worm is washed into the pulmonary artery resulting in embolisation of one of the tributaries.
Worms isolated from humans until now have been consistently undeveloped. This would suggest that the human environment is not suitable for propagation of the parasitic cycle implying that the worms do not achieve a gravid form in man. Exceptions to this have been reported. Boreham et al. [2] suggested that while D. immitis does not normally survive in the subcutis of man it may sometimes develop into a mature worm. In addition, Beaver et al. [5] reported a gravid form of D. immitis in an immunocompromised patient suffering from lymphoid leukemia.
The majority of patients with dirofilariasis are asymptomatic. When present the most common symptoms are coughing, chest pain, haemoptysis, eosinophilia and fever. The presentation is typically a spherical, subpleural infarct due to embolisation of the worm into a branch of the pulmonary artery. There is usually a self limiting granulomatous reaction which has been suggested to be due to toxins released from the dead worm. The focal infarct appears as a coin lesion on CXR. While other species of Dirofilaria have been reported in other locations in man D. immitis has been reported primarily in the lung. Extrapulmonary locations of Dirofilaria have been reported by Takeuchi et al. [6] in the heart and vena cava, and by Beaver et al. [7], in the index finger. Moorhouse [8] in 1978 reported two cases of intra ocular location from which immature filariids were isolated. In addition extrapulmonary infections with immature worms retrieved from the orbit and peritoneal cavity of two patients respectively and which were morphologically indistinguishable from D. immitis have been reported [9].
Diagnosis of the disease preoperatively is difficult. Attempts to identify infected patients including serology and skin tests have not been successful. While they may be used these tests are controversial as they do not evaluate the pulmonary lesion itself. The latter still necessitates excision or open biopsy.
Radiological, cytological and bronchoscopic examinations have not been reported to give definitive diagnosis. Preoperative diagnosis is further complicated by the suggestion that Dirofilaria can reside in human tissue for years without eliciting any host response. The latter was challenged, however by Santamaria et al. [10] who reported the use of enzyme linked immunosorbent assay to identify a strain of Dirofilaria with reactivity similar to that of D. immitis rabbit immune serum and which was present in the subcutaneous tissue of two patients.
The disease while not posing a significant threat to humans is medically important because of the differential diagnosis with other well defined pulmonary lesions such as tuberculosis, fungal infections, carcinoma and hamartomas. In cases of coin lesions which are difficult to evaluate preoperatively, one should be prudent when planning extensive thoracotomies. Diagnostic thoracoscopy for accessible lesions would seem to be the intervention of choice in these cases.
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Acknowledgments
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The authors wish to thank Mrs. Martia Harness, Dept. of Cardiothoracic Surgery, Fremantle Hospital, for her help in preparing this manuscript.
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References
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Brine J.A.S., Finlay-Jones L.R., Charters A.D. Pulmonary dirofilariasis – a case in Western Australia. Med J Aust 1971;2:1238-1241.[Medline]
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Boreham P.F., Atwell R.B. Dirofilariasis in man. Boca Raton, FL: CRC Press, 1988:217-234.
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Beaver P.C., Orihel T.C., Leonard G. Pulmonary dirofilariasis: restudy of worms reported. Am J Trop Med Hyg 1990;43(2):167-169.
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Takeuchi T., Azami K., Kobayashi S., Masuda M., Tanabe M., Miura S., Osakawa M., Murai T. Dirofilaria immitis infection in man: report of a case of the infection in heart and inferior vena cava. Am J Trop Med Hyg 1981;30(5):966-969.
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Beaver P.C., Brenes R., Ardon J. Dirofilaria from the index finger of a man in Costa Rica. Am J Trop Med Hyg 1986;35(5):988-990.
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Moorhouse D.E. Dirofilaria immitis: a cause of human intra-ocular infection. Infection 1978;6(4):192-193.[Medline]
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Santamaria B., DiSacco B., Muro A., Genchi C., Simon F., Cordero M. Serological diagnosis of subcutaneous dirofilariosis. Clin Exp Dermatol 1995;20(1):19-21.[Medline]
Received February 17, 1999;
received in revised form June 15, 1999;
accepted June 26, 1999.
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