Pulmonary coin lesion caused by Dirofilaria immitis – a report of two cases with a minireview of the literature

Introduction

Helminthic infections in humans are prevalent worldwide, but roundworms are only rarely detected in the lung. Human pulmonary dirofilariasis (HPD) is a rare zoonotic disease, most commonly caused by the filarial nematode Dirofilaria immitis. The infection is usually asymptomatic and presents on routine chest X-ray as a solitary pulmonary nodule suggestive of malignancy [1].
We present 2 patients with pulmonary dirofilariasis. They are the first documented cases of this uncommon disease in Slovenia.

Material and methods

An asymptomatic, peripheral, well-circumscribed mass was discovered by chest X-ray in the right upper lobe of the 42-year-old and 71-year-old men. Both patients underwent a preoperative clinical examination. Fine-needle aspiration biopsy, and bronchoscopy with bronchial and transbronchial biopsy were unsuccessful. Both nodules were clinically suspicious for malignancy resulting in excision. Frozen sections were performed in both cases. The remaining tissue obtained at surgery was stained with H&E, Weigert Van Gieson, periodic acid-Schiff (PAS), Ziehl-Neelsen, and Grocott silver stainings.

Results

Patient 1

A 71-year-old retired driver with a 30-year history of smoking one pack of cigarettes per day presented with repeated respiratory infections, high temperature (38.5–39°C), shivers, malaise, and productive cough for 4 months before admission. The patient denied haemoptysis, chest pain, or hoarseness. His peripheral blood revealed a slightly elevated erythrocyte sedimentation rate and leukocytosis without anaemia or eosinophilia. A subsequent chest X-ray showed solitary, peripheral, well-defined pulmonary opacity in the right upper lobe, an oval pulmonary nodular lesion in the posterior segment of the right upper pulmonary lobe, measuring around 15 mm (Fig. 1). The lesion was below the pleural surface with ribbon-like extensions into the surrounding lung parenchyma. Computer tomography was not performed. Fine-needle aspiration biopsy disclosed squamous metaplasia with necrotic debris in the background. Transbronchial biopsy revealed chronic catarrhal bronchitis with focal metaplastic changes of bronchial mucosa with moderate dysplasia and inflammation. A wedge-shaped resection of the upper right lobe was performed. Frozen sections showed an acellular necrotic area and peripheral inflammation suggesting tuberculoma. In the resected lobe, a well-circumscribed nodule was found, measuring 15 mm in diameter. On cut section, the nodule appeared granular grey-yellow and slightly softened in the central portion.
Microscopically, there was subtotal necrosis surrounded by fibrous pseudocapsule including few multinuclear giant cells, suggesting the diagnosis of tuberculosis (Fig. 2A). Differential diagnosis also included echinococcosis due to preserved partially fragmented laminated structures. Evaluation of infarcted tissue and the embolized lung vessel, however, disclosed cross-sectioned helminth. Parasite, better seen with Weigert Van Gieson staining, measured 170–260 µm in diameter (Fig. 2B). The observed circular muscular layer, central intestinal tube, pair of reproductive organs (uterus), and smooth laminated chitin-like cuticle without longitudinal ridges confirmed the diagnosis of female Dirofilaria immitis.

Patient 2

A 41-year-old male was admitted for routine medical examination and a chest X-ray. The patient denied chest pain, dyspnoea, wheezing, fevers, night sweats, or weight loss. He also denied alcohol abuse or smoking. Lungs were clear to auscultation. The chest radiograph revealed a solitary non-calcified subpleural nodule in the apical region of the right upper lung lobe, measuring 15 mm in diameter. Fine-needle aspiration biopsy and TTB were both non-specific, showing only a few inflammatory cells and fibrous tissue. The lesion was considered as a benign solitary fibrous tumour of the pleura, and surgical resection was not performed. The patient was asymptomatic and followed-up and re-examined every 3 months with chest X-ray. Nine months later, the nodule suddenly enlarged to 25 mm in diameter, prompting surgical removal. Frozen section was performed during surgery, suggesting pulmonary tuberculosis due to abundant central necrosis surrounded by fibrous pseudocapsule and few Langhans’ giant cells. In the centre of the resected tissue, there was a well circumscribed, soft, greyish nodule with a reddish rim. Microscopical examination demonstrated haemorrhagic infarction with dirofilarial embolization of pulmonary vessel (Figs. 3A, B), similar to Patient 1.

Discussion

Pulmonary dirofilariasis is caused worldwide by Dirofilaria immitis and occasionally by Dirofilaria repens. Humans are accidental and dead-end hosts of Dirofilaria organisms, which are transmitted from dogs and other carnivores to humans by mosquitoes infected with the third-stage larvae [1, 2].
The age span of patients is wide, at 8–80 years, with an average age of 51–58 years. Males are involved in 59–70%, females in 30–40%. More than half of all patients are asymptomatic; the remaining patients present with cough, fever, chest pain, haemoptysis, and arthralgias. In almost 90% of reported cases, lesions are solitary, noncalcified, well circumscribed, and peripheral. Approximately three-quarters of nodules are in the right lung, with a predilection for the lower lobe. Sub-pleural lesions are encountered in 68% of all cases and may be associated with pleural effusion [1, 3, 4].
Because most of the injected larvae perish at the site of inoculation, the parasite usually cannot be identified in the blood. Dashiell described the first report of HPD due to Dirofilaria immitis in 1961 [5].
Dirofilaria includes more than 40 species, but only a few infect humans. In 2016 around 400 cases worldwide and only about 30 cases of Dirofilaria immitis infection of humans in Europe had been reported, which were demonstrated as lung or intravascular or heart tumorous lesions [6–12]. Until now only a few more cases of HPD in Europe have been reported [13, 14]. This confirms that in Europe, pulmonary dirofilariasis is very rare medical entity.
Infections in humans have been reported in both temperate and tropical regions of Asia, Australia, Japan, North, Central, and South America, as well as in Southern Europe [15, 16]. Most cases reported from Northern Europe were thought to be acquired in southern countries. The highest prevalence is reported in river valleys and humid areas where the environmental conditions are more favourable for the breeding of vectors [16]. However, in some studies from large endemic regions, as in the Canary Islands, the reported seroprevalence in women is 6.7% and in men 6.1%, and the highest seroprevalence in humans corresponded to the areas where the highest canine prevalence is reported [17].
According to the recent review performed by Anvari et al. in 2020, the prevalence of Dirofilaria immitis in dogs for various continents are as follows: Australia 2.68%, Asia 12.07%, America 11.60%, Europe 10.45% , and Africa 7.57% [18].
The critical factor affecting the prevalence of this infection is the weather, geographical distribution, density and age of dogs, number and distribution of vectors, and diurnal periodicity of microfilaraemia.
The life cycles of all Dirofilaria species are similar. The female mosquitoes (Culex, Aedes, Anopheles, Myzorhynchus) serve as a vector and as an intermediate host after ingesting a bloody meal from an infected dog [19]. Approximately 60 species of mosquitoes have been shown to support complete larval development [20]. Mansonia, fleas, louses, and ticks have also been vectors. Besides dogs, the definitive hosts may be cats, wolves, coyotes, jackals, muskrats, foxes, bears, otters, beavers, and sea lions.
The adult worms reside in the dog’s pulmonary artery and in the right ventricle. The developing embryo in the pregnant female worm hatches from its eggshell and sheds into the blood as a living first-stage L1 larva (microfilaria) [21]. The circulating microfilariae (L1) are ingested by a female mosquito, where they further develop in Malpighian tubules (MT). For normal development, the optimal environmental temperature should remain above 27°C. Below the threshold temperature of 14°C, the cycle is halted [22].
During the second larva stage (L2), after the first moult, the internal organs of the worms are formed. The second moult occurs at 11–12 days. After 2 moults in MT lasting 10–17 days, L3 develop, resembling miniature adults. L3, also called infective larvae, are injected by mosquito’s bite into the skin of the animal or human host. The L3-stage larvae reside in the subcutaneous and fatty tissue with a formed cuticle [23]. The cuticle has important absorptive, secretory, and enzymatic activities.
Three to six days after infection, L3 moults to the fourth larval stage (L4) [24]. The larvae in the dog’s subcutaneous tissue mature over the period of 60–120 days until the final moult into the immature L5 occurs [19, 22]. Thereafter, L5 migrate through blood vessels to the right ventricle and pulmonary arteries where the adult worms reach sexual maturity in about 6 months in the animal host [25].
The final size of the female parasite may be 25–30 cm in length (male worms are only 12–16 cm long) and 1–2 mm in diameter [22].
In humans, most larvae do not mature but are arrested and perish in the subcutaneous tissue. Only a few may reach the right heart, fail to achieve sexual maturation, and die. They are swept with the blood into the right pulmonary artery where they obstruct some of the small lung vessels, causing small pulmonary infarctions. These are often seen as solitary subpleural nodules (‘coin lesions’) with an average diameter of 2 cm (0.7–4.0 cm) [4, 8]. Histology reveals infarcts where the necrosis is surrounded by a fibrotic rim and inflamed granulation tissue. The necrosis is of coagulative type in about two-thirds of cases and caseous in the remaining one-third. Surrounding infiltrates consist of plasma cells, histiocytes, lymphocytes, numerous eosinophils, and scattered Langhans’ giant cells with some neutrophils. Charcot-Leyden crystals, microcalcification, and cholesterol clefts are seen in 27%, 22%, and 17% of the nodules, respectively. Adjacent lung parenchyma shows a desquamative interstitial pneumonia-like reaction in 66% and bronchiolitis in 46% of cases. Patchy organizing pneumonia and numerous foci of reactive metaplastic squamous epithelium lining adjacent airways are detected in 34% of cases. Half of the reported cases show focal vasculitis involving muscular arteries [4]. Anatomy of the worm is better appreciated in silver (Gomori methenamine) or PAS-stained histologic sections. Wiegert Van Gieson staining is very helpful in highlighting the worm’s structures and the lung vessel emboli [19].
Morphologic characteristics of D. immitis, as well a histologic differential diagnosis of other, more common parasitic infestations, are summarized in Table I [26–30].
Awareness of this entity is important because clinical and radiographic presentations of HPD often mimic lung neoplasm, particularly as 70% of patients with dirofilarial lesions are heavy smokers. Dirofilariasis should be included in the differential diagnosis of sub-pleural “coin-like” lesions. Besides primary pulmonary carcinoma or haemorrhagic pulmonary infarction, differential diagnosis may also include benign tumours (hamartoma, solitary fibrous tumour of pleura), solitary lung metastasis, necrotizing granuloma (tuberculoma), Wegener’s granulomatosis, rheumatoid nodule, fungal lesion, etc. [2].
In multiple pulmonary nodules, diagnostic pitfalls associated with HPD include patchy organizing pneumonia and follicular bronchiolitis. Necrotic tissue may even suggest idiopathic bronchiolitis obliterans organizing pneumonia, extrinsic allergic alveolitis, lymphoma, or metastatic process [4].
Although eosinophilia is commonly associated with parasitic infestations, mild peripheral eosinophilia is found in only 7.5–20% of patients with pulmonary dirofilariasis [1, 3, 31]. Infrequent eosinophilia is due to a very small amount of released antigen [19]. In HPD, surgery can be avoided if serological tests with anti-dirofilarial antibodies are positive and if imaging studies and bronchoscopy rule out malignancy or mycobacterium infection [31].
The serologic tests (complement fixation tests, enzyme-linked immunosorbent assay – ELISA, indirect immunofluorescence, indirect hemagglutination test) have, in general, low specificity [32]. An improved specificity with ELISA has been shown with a fusion protein of 35 kDa and a native polypeptide of  22 kDa (Di22) [33, 34]. Biopsy or surgery is requiring for conclusive diagnosis, but more recently molecular methods based on polymerase chain reaction and sequencing has improved diagnostic accuracy [35, 36].

Conclusions

Humans are dead-end hosts in the life cycle of Dirofilaria sp., and surgical treatment in cases of pulmonary dirofilariasis is unnecessary. The clinicians should be alert to the possibility of dirofilariasis, particularly in individuals who have travelled to endemic areas, as documented in our patients. Because most of the patients are asymptomatic, we believe that HPD is underestimated due to unreported or undiagnosed cases.
The authors declare no conflict of interest.

References