Abdominal actinomycosis as a mimic of intestinal cancer: Case report

Case report

A 73-year old woman with a past history of oophorectomy three decades earlier presented as an emergency with abdominal distension and hypogastric pain associated with emesis, without fever.

On admission, vital signs were stable (blood pressure 125/84 mmHg, heart rate 78 bpm, respiratory rate 18 breaths/min, temperature 37°C). Physical examination revealed no peritoneal signs.

Laboratory studies showed neutrophilia with preserved renal function and normal electrolytes.

She improved clinically and was discharged with analgesia. Four months later, she presented again with a 20-day history of progressive lower abdominal pain, subjective weight loss, and recurrent emesis. Physical examination revealed a palpable pelvic mass with inflammatory skin changes.

Laboratory tests were unremarkable. Contrast-enhanced abdominal CT scan showed parietal thickening of the transverse colon with adjacent fat stranding and a prevesical soft-tissue mass measuring 60 × 105 × 41 mm with diffuse enhancement, raising suspicion of a neoplastic process (Fig. 1).

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Figure 1.

Contrast-enhanced abdominal CT scan. Panels A–C correspond to the initial study, showing axial (A), sagittal (B), and coronal (C) views of a soft-tissue density lesion with diffuse enhancement, measuring 60 × 41 × 103 mm, located in the prevesical region with extension to the abdominal wall. Panel D shows a sagittal view from follow-up CT performed several months later, demonstrating complete resolution of the previously described lesion.

Tumour markers CA-125 and CA 19-9 were within normal limits.

Given the history of ovarian tumour, differential diagnoses included recurrence or colonic neoplasm with extrinsic compression.

Colonoscopy however revealed only grade I internal haemorrhoids, rectal varices, and diverticulosis. Empiric antibiotics were initiated for suspected abdominal wall cellulitis. Ultrasound-guided drainage of the pelvic mass yielded 3 mL of haemopurulent fluid, but symptoms persisted, prompting exploratory laparoscopy, which revealed a large, firm, friable mass with marked inflammation and pearlescent ‘fish flesh’ appearance, leading to conversion to open laparotomy.

The 10 × 12 cm mass was densely adherent to the hypogastric abdominal wall, transverse colon serosa, omentum, and small bowel loops, with significant distortion of normal anatomy. The appendix was also adherent. Due to extensive involvement and high haemorrhagic risk, complete resection was not attempted, and only an appendectomy was performed. Tissue samples were however obtained, which, on histopathological examination, revealed an appendix without acute inflammation. The intra-abdominal mass showed a mixed inflammatory infiltrate composed of xanthomatous histiocytes, lymphocytes, plasma cells, and neutrophils, with filamentous bacterial colonies arranged in radiating patterns, consistent with Actinomyces spp., without evidence of malignancy (Fig. 2).

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Figure 2.

In the cross-section of an intra-abdominal mass (A-4x, B-10x, C-40x), an aggregation of mixed inflammatory cells composed of foamy histiocytes, lymphocytes, plasma cells, and neutrophils is observed. Filamentous structures forming radiating conglomerates (Splendore-Hoeppli phenomenon) can be seen, consistent with Actinomycosis.

Intravenous penicillin G followed by oral amoxicillin for six months was administered. At outpatient follow-up, her abdominal pain had resolved and follow-up imaging demonstrated complete resolution of the inflammatory mass (Fig. 1), with no evidence of residual disease or progression.

Discussion

Abdominal actinomycosis is an uncommon chronic infection that can present as an infiltrative mass and closely mimic malignant disease, representing a diagnostic challenge, particularly in older adults.^1,2 In this case, the subacute course, weight loss, and extensive involvement of adjacent structures initially favoured a neoplastic aetiology.

Abdominal involvement accounts for up to 20% of actinomycosis cases, most commonly affecting the ileocecal region and appendix.^1,3 Although Actinomyces spp. are commensals of the gastrointestinal tract, disruption of mucosal integrity may facilitate invasive disease. ⁴ Notably, no clear predisposing factors were identified in our patient.

Radiological findings are nonspecific and often indistinguishable from malignancy or other chronic inflammatory conditions.^2,5 Consequently, definitive diagnosis usually relies on histopathological identification of sulfur granules composed of filamentous bacteria arranged in radiating patterns. ⁶

Prolonged beta-lactam therapy remains the cornerstone of treatment, as dense fibrotic tissue limits antibiotic penetration. Current recommendations include intravenous penicillin at doses of 18–24 million units daily for 2–6 weeks, followed by prolonged oral therapy with penicillin V or amoxicillin for a total duration of 6–12 months. In patients with penicillin allergy, ceftriaxone may be considered as an alternative.^1,7 Surgical intervention plays a complementary role, particularly for the diagnosis and management of complications, but should be individualised. ⁸

In our case, limited surgical exploration to establish a diagnosis, combined with prolonged antimicrobial therapy, resulted in a favorable outcome.