Introduction

Actinomycosis is a chronic, suppurative infection caused by anaerobic or microaerophilic bacteria, primarily Actinomyces israelii, a gram-positive filamentous organism that is part of the normal flora of the oral cavity, gastrointestinal tract, and female genital tract (1). It typically presents in cervicofacial, thoracic, and abdominopelvic regions, with inguinal involvement being exceptionally rare (2).
Actinomycosis has a notorious reputation for mimicking malignancy due to its ability to form mass-like lesions with infiltrative borders, necrosis, and lymphadenopathy (3, 4). Clinically and radiologically, these features can closely resemble those of carcinomas, lymphomas, or tuberculosis, especially in regions rich in lymphatic tissue such as the inguinal canal (5).
Pelvic and abdominal actinomycosis is often associated with intrauterine device (IUD) use in females, but it can also occur in males and non-IUD users when there is mucosal breach or trauma (6, 7). Inguinal actinomycosis may follow trauma, surgery, or hematogenous spread, and the resultant lesion often presents as a slowly enlarging, painless mass.
Diagnosis is challenging and often delayed due to the nonspecific nature of clinical features and imaging findings. Even advanced imaging techniques such as computed tomography (CT) and positron emission tomography–CT (PET–CT) frequently mistake actinomycosis for malignancy, given the irregular mass formation and lymphadenopathy with central necrosis (8). Biopsy remains the gold standard, with histopathological identification of sulfur granules and actinomycotic colonies confirming the diagnosis (9).
Misdiagnosis may lead to unnecessary radical surgeries under the assumption of malignancy, especially when preoperative suspicion is low (10). Early recognition and appropriate antibiotic therapy can achieve full resolution without the need for invasive intervention. This case report aims to raise awareness of inguinal actinomycosis as a differential in atypical lymphadenopathy cases. This case report has been reported in line with the Surgical Case Report (SCARE) criteria.

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Case Presentation

A 65-year-old male presented to the general surgery unit at a tertiary care teaching hospital in Tamil Nadu with complaints of a gradually progressive, painless swelling in the right inguinal region persisting for over 3 months. There was no associated history of fever, night sweats, weight loss, trauma, gastrointestinal, or genitourinary symptoms. The patient denied any significant comorbidities. He was a non-smoker with no history of alcohol use. His body mass index was 22.4 kg/m2, indicating normal nutritional status. He had no prior history of tuberculosis, diabetes mellitus, or immunosuppression and was not on any long-term medications. His past surgical history included a right below-knee amputation performed several years ago following trauma. Local examination revealed a 3×3-cm firm, non-tender swelling in the right inguinal region with surrounding induration and multiple palpable lymph nodes (Figure 1). The overlying skin appeared normal, with no discharging sinuses or pus points, and an old surgical scar was present, consistent with the prior right below-knee amputation site.

 

Figure 1.

Clinical, radiological, and intraoperative findings of right inguinal actinomycosis: (a, b) preoperative images showing a right inguinal swelling; (c, d) contrast-enhanced computed tomography demonstrating necrotic inguinal–external iliac lymphadenopathy; (e) intraoperative view of the excised necrotic inguinal mass.

As part of the evaluation, routine blood tests, chest radiograph, electrocardiogram, and echocardiography were performed and found to be within normal limits. Imaging studies including ultrasonography and contrast-enhanced CT of the abdomen revealed multiple conglomerated necrotic lymph nodes in the right inguinal region, extending into the right external iliac area and anterior abdominal wall, with central necrosis and associated soft tissue thickening suggestive of an infective etiology. Venous Doppler of the right lower limb demonstrated an echogenic thrombus extending from the right common femoral vein into the superficial femoral vein, along with diffuse subcutaneous edema. Arterial Doppler revealed diffuse atheromatous changes and monophasic flow in the proximal anterior and posterior tibial arteries.
The initial differential diagnoses included cutaneous tuberculosis (scrofuloderma), deep fungal infection, lymphoma, metastatic malignancy, and soft tissue sarcoma, based on the nodal necrosis and infiltrative pattern on imaging. These diagnoses were systematically excluded as follows: tuberculosis was considered less likely given a negative Mantoux test, absence of acid-fast bacilli on Ziehl–Neelsen (ZN) staining, and lack of constitutional symptoms (fever, night sweats, weight loss); fungal infection was excluded on the basis of negative special stains (periodic acid-Schiff and Grocott methenamine silver) on histopathology and the absence of relevant travel or exposure history; lymphoma was considered unlikely given the absence of systemic B symptoms, a normal chest radiograph, and the histopathological picture showing suppurative granulomatous inflammation without atypical lymphoid proliferation; metastatic malignancy was excluded by the absence of any identifiable primary lesion on clinical and radiological evaluation, and normal tumor marker profile; and soft tissue sarcoma was ruled out by histopathology, which showed no spindle cell proliferation or pleomorphic features. A histocytological cell-block preparation (a method in which aspirated material is processed as a tissue fragment to enable histological sectioning and immunostaining) from the lymph node fine-needle aspirate showed granulomatous lymphadenitis with suppuration. Histopathological findings and clinical correlation ultimately pointed to a rare diagnosis: inguinal actinomycosis—a chronic granulomatous infection most often caused by A. israelii, which is known to mimic malignancy both clinically and radiologically.
The patient was managed with intravenous antibiotics including ceftriaxone, metronidazole, and amikacin, followed by oral doxycycline (100 mg twice daily) for a total antibiotic course of 6 weeks. It is acknowledged that high-dose intravenous benzylpenicillin (penicillin G) is the recommended first-line agent for actinomycosis; however, it was not available at our institution at the time of management. Ceftriaxone was used as a beta-lactam alternative with documented activity against Actinomyces species. Metronidazole and amikacin were included empirically to provide broader anaerobic and gram-negative cover, given the initial diagnostic uncertainty and the polymicrobial nature of the infection. Supportive medications included a proton pump inhibitor and calcium supplementation. Multidisciplinary evaluations were conducted with cardiology, pulmonology, and nephrology teams to assess surgical risk and systemic involvement. He was discharged in stable condition on day 14 of admission with oral doxycycline and instructions for outpatient follow-up. At the 4-week follow-up review post-discharge, the patient demonstrated objective improvement with a significant reduction in swelling size (from 3×3 cm to <1 cm), resolution of surrounding induration, and intact wound status with no signs of recurrence or sinus formation. He reported good adherence to the antibiotic regimen. Repeat ultrasonography at 8 weeks confirmed near-complete resolution of the previously noted conglomerated lymph nodes. No recurrence was detected at the final follow-up visit at 3 months post-treatment.
Histopathological examination of the excised right inguinal swelling revealed a skin-covered soft tissue mass measuring 4.5×3×2 cm. The cut surface showed gray–yellow to gray–brown areas with focal cystic changes. Microscopic evaluation demonstrated skin hyperplasia, acanthosis, and papillomatosis, with focal ulceration. Subepithelial tissue showed sinus tracts lined by stratified squamous epithelium, areas of necrosis, and dense mixed inflammatory infiltrates comprising neutrophils, lymphocytes, plasma cells, and eosinophils. Numerous epithelioid histiocytes, neutrophilic abscesses, and characteristic basophilic colonies surrounded by eosinophilic radiating clubs—consistent with the Splendore–Hoeppli phenomenon—were seen, confirming the presence of actinomycosis colonies. Inflammatory infiltration extended into adjacent muscle fibers. Gram staining confirmed gram-positive filamentous organisms, supporting the diagnosis. The final impression was chronic suppurative granulomatous inflammation with sulfur granules, consistent with actinomycosis (Figure 2).

 

Figure 2.

(a) The section shows skin lined by a stratified squamous epithelium with acanthosis. The subepithelium shows the sinus tract lined by stratified squamous epithelium with dense inflammatory cells mainly neutrophils, lymphocytes, plasma cells, and eosinophils. Occasional foci show neutrophilic abscesses. A few foci demonstrate the Splendore–Hoeppli phenomenon. (b) The section studied shows the Splendore–Hoeppli phenomenon along with inflammatory cell infiltrates composed of neutrophils, lymphocytes, and a few plasma cells with congested blood vessels. (c) Gram stain shows a positive filamentous structure consistent with actinomycosis.

This case highlights the diagnostic challenge posed by inguinal actinomycosis, especially in presentations that closely mimic malignancy or tuberculosis. In such scenarios, a high index of clinical suspicion, along with appropriate imaging and tissue diagnosis, is essential to establish the correct diagnosis and guide effective treatment while avoiding unnecessary surgical procedures or misdiagnosis.

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Discussion

Actinomycosis is a rare, chronic, granulomatous infection caused by A. israelii, a filamentous gram-positive anaerobe that normally colonizes the oral cavity, gastrointestinal tract, and genitourinary system. Its invasive nature allows it to cross tissue planes and cause mass-like lesions with surrounding fibrosis, necrosis, and sinus tract formation. In this context, it frequently mimics malignancies or other chronic inflammatory conditions, which leads to diagnostic confusion and, at times, unnecessary surgical interventions.

 

Table 1.

Summary of published cases of inguinal and rare-site actinomycosis mimicking malignancy

F, female; FNAC, fine-needle aspiration cytology; GEJ, gastroesophageal junction; M, male; NR, not reported; TB, tuberculosis.

The inguinal region is an exceptionally rare site for actinomycosis, with only a handful of cases documented in the literature (Table 1). In our case, the patient presented with a slow-growing, painless, firm swelling in the right inguinal region without constitutional symptoms. This clinical picture, combined with imaging findings of necrotic lymphadenopathy and soft tissue thickening, strongly raised suspicion for malignancy or tuberculous lymphadenitis—common diagnoses in this anatomical region in endemic areas.
Historically, Pradhan et al. reported a similar diagnostic dilemma in a case of maxillary sinus actinomycosis, which was initially mistaken for carcinoma based on clinical and radiological findings but was eventually confirmed by histology (5). Likewise, Biller et al. described esophageal actinomycosis masquerading as cancer at the gastroesophageal junction, where biopsy played a key role in avoiding extensive surgical resection (3).
Specifically in the inguinal region, Fiane documented a case of actinomycosis with features overlapping those of neoplasia or chronic abscesses, emphasizing the difficulty of diagnosis without tissue confirmation (2). Like our case, this patient had no systemic signs of infection, which further delayed the consideration of an infectious etiology.
Radiological findings are often misleading. In our patient, contrast-enhanced CT of the abdomen revealed multiple necrotic lymph nodes in the right inguinal and external iliac regions, with extension into the anterior abdominal wall. These findings raised high suspicion for either metastatic malignancy or tubercular lymphadenitis. Eskarous et al. described a similar challenge in diagnosing abdominal actinomycosis, where CT showed omental masses initially interpreted as malignancy until histopathology revealed the true infectious nature (4). Baykal et al. found that even PET–CT scans demonstrated high metabolic uptake in pulmonary actinomycosis, often mistaken for lung carcinoma (8). This highlights the fact that neither CT nor PET–CT is diagnostic for actinomycosis, and imaging must be interpreted cautiously.
Biopsy remains the gold standard. Our patient’s fine-needle aspiration cytology (FNAC) with histocytological cell-block analysis showed granulomatous lymphadenitis with suppuration, raising suspicion for actinomycosis or tuberculosis. The absence of acid-fast bacilli on ZN staining and a negative Mantoux test helped rule out tuberculosis. Histological identification of sulfur granules or colonies, although not always present, is a definitive marker of actinomycosis. Pillappa et al. stressed the diagnostic importance of surgical biopsy in a case of esophageal actinomycosis, which was mistaken for cancer on imaging and endoscopy (9).
Therapeutically, high-dose intravenous benzylpenicillin (penicillin G) is the drug of choice for actinomycosis, typically administered for 2–6 weeks before transitioning to oral amoxicillin. Doxycycline serves as an effective alternative in penicillin-allergic patients or in resource-limited settings. In our case, penicillin G was not available at the institution, and ceftriaxone was used as an evidence-supported beta-lactam substitute. Metronidazole and amikacin were added empirically given the initial diagnostic uncertainty. Our patient responded well to this combination regimen of intravenous ceftriaxone, metronidazole, and amikacin, followed by oral doxycycline. Bhuskute and Shinde reported similar successful outcomes in pelvic actinomycosis cases treated conservatively with antibiotics without the need for radical surgery (10). Early medical management significantly reduces the risk of overtreatment and surgical morbidity.
Another important point to consider is the risk of unnecessary surgery. In our case, a preoperative diagnosis allowed us to avoid excisional biopsy or lymph node dissection. Multiple other reports in the literature—including those by Scribner et al. (2000) and Saramago et al. (2019)—have emphasized that unrecognized actinomycosis can lead to extensive surgical procedures, especially in the pelvis, due to its resemblance to gynecological or colorectal cancer (6, 7).
Histopathological confirmation in our case was crucial. The presence of sinus tracts, dense mixed inflammatory infiltrates, and the characteristic Splendore–Hoeppli phenomenon—where colonies of Actinomyces are surrounded by eosinophilic clubs—are classic findings. This histological hallmark, along with the demonstration of gram-positive filamentous organisms on special stains, solidified the diagnosis. Similar findings were documented in previous studies, including those by Pillappa et al. and Bulut et al., where misdiagnosed mass-like lesions revealed actinomycosis only upon detailed histopathological and microbiological examination (1, 9). This case emphasizes that in ambiguous soft tissue masses, especially those mimicking malignancy, a thorough histopathological workup with appropriate staining is indispensable for accurate diagnosis. While the mimicry of actinomycosis is an established concept, several features of this case add specific value to the literature: the occurrence in an elderly male without an IUD or obvious immunosuppression in the unusual inguinal location; the presence of concurrent deep vein thrombosis and peripheral vascular disease creating additional diagnostic complexity; the structured documentation of systematic exclusion of multiple differentials; and the detailed follow-up confirming complete clinical and radiological resolution with conservative antibiotic therapy. Together, these elements make this case a useful clinical reference for managing atypical inguinal masses in resource-limited settings where PET–CT or advanced molecular testing may not be readily available.

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Conclusion

Inguinal actinomycosis is a rare and often overlooked differential diagnosis for chronic inguinal swelling. Its ability to clinically and radiologically mimic malignancy or tuberculosis presents a significant diagnostic challenge. Our case highlights the importance of considering infectious etiologies in patients with necrotic lymphadenopathy, especially when biopsy reveals granulomatous inflammation with suppuration. Early recognition and appropriate antibiotic therapy can lead to complete recovery and prevent unnecessary surgical interventions. Clinicians should maintain a high index of suspicion for actinomycosis in atypical presentations to ensure timely and accurate management.

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Data Availability
The data supporting this case report are available from the corresponding author upon reasonable request.

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Author contributions
All authors equally contributed to conceptualization, data curation, formal analysis, methodology, supervision, and in writing, reviewing & editing of the original draft.

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References​​

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2. Fiane A. [Inguinal actinomycoses]. Tidsskr Nor Laegeforen. 1989; 109(26): 2658.

3. Biller JJ, Cho R, Zagorski S. Actinomyces infection mimicking esophageal cancer. Cureus. 2021; 13(8): e17266.

4. Eskarous H, Pingili A, Venugopal D. Abdominal actinomycosis mimicking malignancy: a case report. IDCases. 2021; 25: e01252.

5. Pradhan S, Datta NR, Prasad KN, et al. Actinomycosis mimicking carcinoma of the maxillary sinus. Indian J Cancer. 1993; 30(1): 1-4.

6. Saramago SM, Cominho JC, Proença SSM, et al. Pelvic actinomycosis mimicking pelvic malignancy. Rev Bras Ginecol Obstet. 2019; 41(7): 463-6.

7. Scribner DR Jr, Baldwin J, Johnson GA. Actinomycosis mimicking a pelvic malignancy: a case report. J Reprod Med. 2000; 45(6): 515-8.

8. Baykal H, Ulger AF, Çelik D, et al. Clinical and radiological characteristics of pulmonary actinomycosis mimicking lung malignancy. Rev Assoc Med Bras. 2022; 68(3): 372-6.

9. Pillappa R, O’Brien TF, Sullivan JL, et al. Esophageal actinomycoses mimicking malignancy. Ann Thorac Surg. 2016; 101(5): 1967-70.

10. Bhuskute N, Shinde R. Actinomycosis mimicking gynecological malignancy: imaging patterns in seven cases. J Med Sci. 2018; 5(2): 57-63.