[Presentation of Case]
A 50-year-old woman, previously healthy, presented to an outside hospital because of cough, rhinorrhea, and fever for one week. There was no sputum, and she denied headache, chest discomfort, abdominal pain, symptoms of genitourinary system, myalgias, or arthralgias. There was no history of weight loss or decrease of appetite. She did not smoke, consume alcohol or betel nut, or use illicit drug. She used to be a merchant who lived in Taipei city, and had not travel outside Taipei city recently. She recalled no contact with sick people or animal, but there were dogs in her neighborhood. In her family, there were no hereditary diseases or malignant diseases. On examination, the patient appeared in no distress. There was no neck stiffness or regional lymphadenopathy. Examinations of the heart, lung, and abdomen were unremarkable. A chest X-ray (figure 1A) showed a nodule in the right lower lung. The computed tomography of the chest showed a 1-cm homogeneous nodular lesion, with smooth margin, in the right lower lung (figures 1B and 1C) The laboratory examinations reported negative for cryptococcal antigen and the serum CEA level was 3.41 ng/ml. The whole-body bone scan reported no metastatic bone lesion, and the magnetic resonance imaging of the brain detected no lesion. She underwent video-assisted thoracoscopic surgery with wedge resection to remove the nodule with grades 3 and 4 lymph node dissection. Grossly, the nodule, measured 1.0*0.8-cm, appeared to be a yellowish, elastic tumor, with mild pleural adhesion and pleural retraction. (figure 2A) The pathology reported granulomatous inflammation with necrosis and presence of parasitic roundworm in hematoxylin-and-eosin stain and highlighted by Periodic acid-Schiff stain and Grocott's methenamine silver stain (figures 2B, 2C, and 2D), which were highly suggestive of human pulmonary dirofilariasis. The lymph nodes were unremarkable. She recovered well after the surgery. She reported no further symptoms in the follow-up clinic visits.
Figure 1A. A standing chest X-ray of the patient.
Figure 1B. An axial cut of the computed tomography of the chest, showing a nodular lesion in the right lower lung.
Figure 1C. A coronal cut of the computed tomography of the chest, showing a nodular lesion in the right lower lung.
Figure 2A. Gross appearance of the nodule.
Figure2B. Hematoxylin-and-eosin stain, 100X.
Figure2C. Hematoxylin-and-eosin stain, 400X. A roundworm with cuticle (arrow), smooth muscle wall (arrow head), and guts.
Figure2D. Grocott's methenamine silver stain. It shows that the roundworms are in the lumen of a blood vessel.
[討論]
本病例為心絲蟲 (Dirofilaria) 在人類身上造成感染症的案例，以單一肺結節為表現。單一肺結節在臨床診斷上一向是個難題，其鑑別診斷從感染性 (如：肺結核；器質化肺炎 (organizing pneumonia)；膿瘍；黴菌感染如麴菌症 (aspergillosis)、隱球菌症 (cryptococcosis)、組織胞漿菌症 (histoplasmosis)；寄生蟲感染如阿米巴原蟲症 (amoebiasis)、囊型包蟲症 (echinococcosis)、心絲蟲症 (dirofilariosis)；麻疹；土壤絲菌 (Nocardia)、非結核分支桿菌 (non-tuberculous mycobacteria)、肺囊蟲 (Pneumocystis jirovecii)等肺部感染；敗血性血栓 (septic embolus) )、發炎性 (如：類風濕性關節炎 (rheumatoid arthritis)、Wegener’s 肉芽腫 (Wegner’s granulomatosis)、顯微性多血管炎 (microscopic polyangiitis)、類肉瘤病 (sarcoidosis) )、良性腫瘤 (如：過誤瘤 (hamartoma)、軟骨瘤 (chondroma)、纖維瘤 (fibroma)、脂肪瘤 (lipoma)等)、惡性腫瘤 (如：肺癌、肺類癌 (primary pulmonary carcinoid)、轉移癌、畸胎瘤 (teratoma)、平滑肌瘤 (leiomyoma) )、血管性 (如：動靜脈畸形 (arteriovenous malformation)、肺血管梗塞 (pulmonary infarct)、肺動脈血管瘤 (pulmonary artery aneurysm)、肺靜脈曲張 (pulmonary venous varix) )、淋巴性 (如：淋巴癌等 )、到先天性病灶 ( 如：支氣管囊泡 (bronchogenic cyst)、游離肺 (lung sequestration)......等 )，有五花八門的可能性 [1]，其診斷需依賴臨床症狀、影像來做初步的評估。依據 Gould 的建議 [2]，當發現病患有大於 8 mm 的單一肺結節，要評估惡性腫瘤的可能性時，需考慮病患是否有惡性腫瘤的風險，如年紀、是否抽煙、是否有其他惡性腫瘤、肺結節的大小、位置、邊緣平滑與否、此結節在功能性檢查 (如，正子電腦斷層攝影) 上的表現，來評定病人屬於低度、中度、或高度風險。若病患年紀大、有抽煙、有其他惡性腫瘤病史、肺結節較大、位於上肺葉、邊緣不規則或呈針刺狀(spiculated)，則風險較高。若屬於極低度風險，可考慮以電腦斷層在3至6個月、9至12個月、及18至24個月時追蹤。若屬於低度至中度風險，可考慮先以非手術的方式取得組織切片，以輔助下一步的治療規劃，若屬於高度風險，或組織切片有懷疑惡性腫瘤的可能，則建議以外科手術的方式切除。
心絲蟲體型長且細，約20-30公分，白色，屬於圓蟲，或俗稱線蟲，為圓形動物門(Nematoda)下的一員，其中最常在人類身上造成疾病的心絲蟲有三種：D. immitis、D. repens、和 D. tenuis。心絲蟲在自然界中的自然宿主 (natural hosts)為狗及野生的犬科動物，如D. immitis、D. repens 較常寄生的狐狸、狼，另外D. tenuis 則較常寄生於浣熊。人類經由蚊子叮咬而被傳染，但人類僅為心絲蟲的偶發宿主(accidental hosts)，也就是心絲蟲無法在人體內成熟或繁殖 (figure 3) 。人類不會直接傳染給人類，人與人之間也不會透過蚊子傳染 [3]。在狗身上，心絲蟲好寄生於右心和肺動脈，造成心臟衰竭，另外也會寄生於其他器官如肺臟、肝臟、腎臟，造成器官損傷。在人類， D. immitis 的感染最常造成無症狀的肺部肉芽腫性結節，而 D. repens、和 D. tenuis 的感染最常見的症狀則是皮下、結膜的肉芽腫性結節。在全球的分布，美洲以 D. immitis 為主，歐洲以 D. repens 為主，非洲、亞洲則是 D. immitis、 D. repens 都有，台灣亦有個案報告 [4]。在臨床表現方面，肺部感染心絲蟲的患者常無症狀，或有咳嗽、咳血、胸痛、發燒、肋膜積水等症狀，胸部X光上常見『錢幣型病灶』(coin lesion)，即小且圓的結節陰影。但也曾經有發現 D. immitis 在腦、眼睛、睪丸的個案報告 [5]。因為 Dirofilaria 造成之肺結節常和其他鑑別診斷難以鑑別，如惡性腫瘤等，因此最常由肺結節切除手術的組織切片診斷出。目前並沒有任何血清學方面的檢驗可以幫助診斷人類的心絲蟲症。治療方面，以手術切除，在組織病理診斷的同時，也就治療完成，不需要其他特別的藥物治療。
心絲蟲症在人類致病並不常見，且其肺部之單一結節病灶常與其他鑑別診斷難以區分，以至於病患在經過種種臨床、檢驗、影像的評估後，多半還是需要手術切除來排除惡性疾病，未來若有更方便的血清學檢驗可以幫助診斷，也許可以嘉惠更多手術或麻醉高風險病患。
Figure 3. D. immitis life cycle (節錄自http://www.cdc.gov/parasites/dirofilariasis/biology_d_immitis.html)
參考文獻
- Patel VK. A practical algorithmic approach to the diagnosis and management of solitary pulmonary nodules algorithmic approach to solitary pulmonary nodules. Part 1: radiologic characteristics and imaging modalities. Chest 2013;143:825.
- Gould MK, Donington J, Lynch WR, et al. Evaluation of individuals with pulmonary nodules: when is it lung cancer? Diagnosis and management of lung cancer, 3rd ed: American College of Chest Physicians evidence-based clinical practice guidelines. Chest. 2013;143:e93S-120S.
- http://www.cdc.gov/parasites/dirofilariasis/
- Tsung SH et al. Human pulmonary dirofilariasis in Taiwan. J Formos Med Assoc. 2003 Jan;102:42-5.
- Theis JH. Public health aspects of dirofilariasis in the United States. Veterinary Parasitology 2005; 133:157-180.
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Case Discussion
繼續教育考題