Pulmonary actinomycosis: the great masquerader

Abstract

Actinomycosis is a rare but treatable disease. Thoracic manifestations are varied and can mimic malignancy or tuberculosis. We report the case of a 54-year-old man who presented with a persistent cough and radiological evidence of right upper lobe lung mass. Conventional computed tomography guided biopsy and bronchoscopy proved to be non-diagnostic. Thoracotomy and histopathologic examination of the tissue confirmed actinomycosis infection. Excellent clinical and radiologic responses were noted following treatment with penicillin V. Despite a high clinical suspicion, the diagnosis can prove to be challenging.

BACKGROUND

Pulmonary actinomycosis is a rare infection and can mimic a variety of malignant and chronic suppurative lung conditions. This can lead to delayed or even missed diagnoses, as it is often unsuspected. Increased awareness among physicians may lead to timely diagnosis thus preventing unnecessary morbidity.

CASE PRESENTATION

A 54-year-old man presented with a 4 week history of non-productive cough, night sweats and generalised weakness. He did not report weight loss or fever. Eighteen months before presentation, he was diagnosed with stage III (T3N2M0) rectal adenocarcinoma for which he had low anterior resection followed by chemotherapy and radiation which he completed 6 months ago. He had a 30-pack year smoking history and worked on a production line in an automotive factory.

INVESTIGATIONS

Examination revealed crepitations at the right upper lung zone. Complete blood count, renal and liver function tests were within normal limits. His carcinoembryonic antigen had minimally risen from a baseline of 5 ng/dl to 12 ng/dl. Chest radiograph showed a mass in the right upper lobe (fig 1A). Computed tomography (CT) scans of the chest revealed an irregular density in the apical portion of his right upper lobe (fig 1B). This lesion had an intermediate increased standardised uptake value (SUV) of 7.5 on positron emission tomography. A tuberculin skin test was negative.

Figure 1. (A) Chest posterior-anterior (PA) radiographic view showing right upper lobe mass indicated by arrow.

(B) Chest computed tomography scan showing right upper lobe irregular density. (C) Lung biopsy specimen showing Actinomyces colonies and granuloma formation with surrounding inflammatory infiltrate. Haematoxylin and eosin ×40. (D) Left (panel 1): Lung specimen gram stains (×40) revealing Gram positive Actinomyces colonies. Right (panel 2): Lung specimen silver stain (×40) revealing Actinomyces colonies.

CT guided fine needle aspiration was negative for malignancy and non-diagnostic. No endobronchial lesion was visualised on bronchoscopy and bronchoalveolar lavage did not reveal any malignant cells. No acid-fast bacilli or fungal particles were demonstrated and bacterial and fungal cultures were negative. Right thoracotomy for wedge resection of the lesion was undertaken.

Histopathologic examination of the lung biopsy (fig 1C,D) showed a localised chronic organising pneumonitis, multifocal micro-abscesses and granulomas with multiple filamentous and branching Gram positive rods. No malignant cells were seen nor did acid-fast bacilli (AFB) isolate. Mycobacterial and fungal cultures were negative. The diagnosis of actinomycosis was confirmed by growth of Actinomyces naeslundi on microbiological anaerobic cultures.

DIFFERENTIAL DIAGNOSIS

• Primary Lung cancer

• Lung metastasis from previous colon cancer

• Tuberculosis

• Fungal infection

TREATMENT

The patient was treated with oral penicillin V for 12 months.

OUTCOME AND FOLLOW-UP

Clinical improvement was noted within a few weeks of starting treatment. A chest x ray obtained after 8 weeks showed normal lung parenchyma.

DISCUSSION

Actinomyces comprise a group of anaerobic or micro-aerophillic branching filamentous bacteria existing as commensals in the human oral cavity, gastrointestinal tract and female genitalia.¹ Actinomyces israelii is the most common cause of human disease in the group comprising 14 other types. Disruptions in the mucosal barrier or the presence of devitalised tissues are prerequisites for causing an infection in these otherwise low virulent organisms. Actinomycosis infections are invasive and destructive with a predilection for the jaw, lung, ileocecal junction and pelvis. Haematogenous spread to the liver and central nervous system has also been reported. Chronic suppurative lesions with histopathologic evidence of granulomas containing neutrophils and sulfur granules with Actinomyces colonies are the hallmark of actinomycosis. However, sulfur granules have been found in other infections such as nocardiosis and botryomycosis.² Actinomyces have been isolated in combinations with other co-pathogens such as Enterobacteriaceace, Fusobacterium, Bacteriodes, Staphylococcus and Streptococcus species.³ This polymicrobial nature of the disease has been posited to create a suitable anaerobic milieu by inhibiting the host defence mechanisms, thus promoting the dissemination of actinomycosis infection.⁴

Thoracic involvement accounts for approximately 15–50% of the cases. Aspiration of contaminated oropharyngeal or gastrointestinal secretions has been incriminated in the pathogenesis of pulmonary actinomycosis.⁵ Haematogenous or lymphatic spread from distant sites is rare.⁶ Poor oral hygiene, dental procedures and diabetes mellitus are potential risk factors.

In the patient described the right upper lobe lung mass with the background history of colon cancer was highly suspicious of malignancy. Most non-invasive diagnostic modalities proved ineffectual in providing a diagnosis. Thoracotomy and wedge resection led to diagnosis of actinomycosis. An immunocompromised status from recent chemotherapy coupled with poor dentition and probable underlying chronic obstructive lung disease due to extensive smoking history may have served as predisposing factors.⁴

The clinical symptoms in pulmonary actinomycosis are generally non-specific. Radiological resemblance of pulmonary actinomycosis to a spectrum of lung diseases may lead to delayed diagnosis or misdiagnosis.¹ Intermediate to high SUV values on positron emission tomography (PET) scans may further confound the diagnosis.⁷ Resection of the lung lesion may be imperative for diagnostic and therapeutic purposes.⁸ An extended course of high dose penicillin G remains the cornerstone of treatment.⁴ Prognosis is excellent with prompt diagnosis and treatment.

Pulmonary manifestations of actinomycosis remain well described in the literature. However, its rarity and declining prevalence in the developed countries makes it an often forgotten diagnosis. We would like to remind clinicians to consider actinomycosis in their differential diagnoses of pulmonary lesions in the appropriate clinical setting.

LEARNING POINTS

• Thoracic actinomycosis is a rare but highly treatable disease.

• It is most commonly misdiagnosed as lung cancer and tuberculosis and may require thoracotomy and resection to confirm diagnosis.

• With early diagnosis and proper treatment the prognosis is excellent.