Prevotella melaninogenica bloodstream infection in colon cancer combined with appendiceal abscess: A case report

Abstract

BACKGROUND

Colon cancer combined with appendiceal abscess is easily misdiagnosed clinically. Prevotella melaninogenica bloodstream infection is rare due to detection limitations.

CASE SUMMARY

A 60-year-old male was admitted with recurrent abdominal pain. He was initially treated for appendiceal abscess. During the second admission, high fever and severe inflammation occurred, and Prevotella melaninogenica was detected in anaerobic blood culture. He received anti-infection therapy and radical resection of right colon cancer. Postoperative pathology confirmed colon cancer with appendiceal invasion.

CONCLUSION

Refractory appendiceal abscess suggests potential colon cancer. Prevotella melaninogenica can cause severe bloodstream infection in tumor patients.

Key Words: Colon cancer; Appendiceal abscess; Prevotella melaninogenica; Bloodstream infection; Case report

Core Tip: Refractory appendiceal abscess in adult patients should raise high clinical suspicion of underlying colon cancer, especially when conservative treatment fails. Prevotella melaninogenica, an anaerobic opportunistic pathogen, can cause life-threatening bloodstream infection in colon cancer patients. Timely pathogen detection and radical surgery are essential to improve patient prognosis and reduce mortality.

INTRODUCTION

Colon cancer is a common malignant tumor worldwide. Tumor invasion of the appendix can lead to appendiceal abscess, with atypical early symptoms and a high misdiagnosis rate[1-3]. Prevotella melaninogenica is a gram-negative anaerobic opportunistic pathogen, and its bloodstream infection associated with colon cancer is extremely rare[4,5]. Here we report a case of colon cancer combined with appendiceal abscess and Prevotella melaninogenica bacteremia to improve clinical recognition.

CASE PRESENTATION

Chief complaints

A 60-year-old man was admitted on September 9, 2025 due to abdominal pain for 5 months and worsening for half a month.

History of present illness

Five months ago, he had intermittent epigastric pain without specific treatment. Half a month ago, right lower abdominal pain and distension worsened. Abdominal computed tomography (CT) suggested appendiceal abscess and ascending colon wall thickening. Colonoscopy showed an occupying lesion in the ascending colon (Figure 1A and B).

Figure 1 Colonoscopy showed an occupying lesion in the ascending colon. A: Colonoscopy (September 10, 2025): An occupying lesion in the ascending colon causing luminal stenosis; B: Colonoscopic biopsy pathology: Chronic moderate active inflammation of the ascending colon mucosa with focal vascular proliferation and erosion.

History of past illness

No hypertension, diabetes, cardiovascular diseases, surgery, blood transfusion, or drug allergies.

Personal and family history

No smoking or drinking. No family history of malignant tumors.

Physical examination

First admission: Abdomen was soft, right lower quadrant tenderness, no mass.

Second admission: Temperature 40.0 °C, pulse 130 bpm, blood pressure 135/87 mmHg, right lower quadrant mass with tenderness. Quick sequential organ failure assessment score = 2 points.

Laboratory examinations

September 10, 2025: White blood cell 9.49 × 10⁹/L; C-reactive protein 170.00 mg/L; cancer antigen 125 86.94 U/mL.

November 22, 2025: White blood cell 7.11 × 10⁹/L; C-reactive protein 303 mg/L; procalcitonin 10.30 ng/mL; carcinoembryonic antigen 11.93 ng/mL.

Imaging examinations

September 8, 2025 CT: Appendiceal abscess; ascending colon wall thickening (Figure 2).

Figure 2 Abdominal contrast-enhanced computed tomography (September 8, 2025). Appendiceal abscess with peripheral inflammation; thickened intestinal wall of the ascending colon suggesting tumor or inflammatory lesion.

November 22, 2025 CT: Abscess enlargement (Figure 3).

Figure 3 Abdominal computed tomography findings of appendiceal abscess and secondary small bowel obstruction. A: Abdominal contrast-enhanced computed tomography (November 22, 2025): Enlarged appendiceal abscess after puncture; persistent thickening of the ascending colon wall; B: Abdominal computed tomography (November 25, 2025): Small bowel obstruction caused by the inflammatory mass and abscess.

November 25, 2025 CT: Intestinal obstruction (Figure 3).

FINAL DIAGNOSIS

Final diagnosis included: (1) Right colon cancer (pT3N2aMx) with appendiceal invasion; (2) Appendiceal abscess; (3) Prevotella melaninogenica bloodstream infection; and (4) Intestinal obstruction.

TREATMENT

First admission

Ultrasound-guided puncture and drainage; anti-infection without anaerobic coverage.

Second admission

Anti-infection with meropenem; multidisciplinary team recommended surgery.

Operation (November 27, 2025)

Laparoscopic right hemicolectomy + adhesiolysis + ileostomy (Figure 4).

Figure 4 Gross specimen after right hemicolectomy. Ileocecal tumor with appendiceal abscess and local perforation.

OUTCOME AND FOLLOW-UP

Postoperative recovery was smooth. Pathology confirmed moderately-poorly differentiated adenocarcinoma with lymph node metastasis and appendiceal cancer invasion. The patient was discharged after suture removal (Figure 5).

Figure 5 Postoperative histopathology. A: Moderately-poorly differentiated adenocarcinoma of the ascending colon with lymph node metastasis; B: Postoperative pathology: Cancer cell infiltration in the subserosal fibrous tissue of the appendix.

DISCUSSION

Colon cancer combined with appendiceal abscess accounts for 0.3%-0.4% of colon cancer cases[6]. Tumor necrosis, perforation and bacterial translocation lead to abscess formation[7]. Anatomical features of the ascending colon easily lead to misdiagnosis[8,9].

The main mechanism may be that after cancer cells invade the intestinal wall or surrounding adjacent tissues, the cancer cells grow rapidly, resulting in insufficient blood supply within the tumor, resulting in ischemia, necrosis, and local perforation. Bacteria in the intestinal lumen are translocated, stimulating the formation of local abscesses by opportunistic pathogenic bacteria. When the condition is mild, it can be partially wrapped, but when the condition is severe, it can lead to diffuse peritonitis[10]. The early symptoms of abscesses formed around colon cancer are often not typical. There may be no obvious abdominal pain at first. When patients notice physical abnormalities and come to the hospital for treatment, the course of the disease is often longer. Such patients are often accompanied by complications such as malnutrition, water and electrolyte balance disorders, hypoalbuminemia, and anemia[10,11]. Prevotella melaninogenica mainly colonizes the digestive tract, and bacteremia is rare and often related to abdominal tumors[12,13]. Routine culture has a low detection rate[14,15].

This patient had recurrent abscess and obstruction, confirming colon cancer. Blood culture detected Prevotella melaninogenica, which was highly suspected to be the pathogen of bloodstream infection. Meropenem was effective[16,17].

Clinical lessons: (1) Refractory appendiceal abscess requires evaluation for colon cancer; (2) Anaerobic blood culture is necessary in severe infection; and (3) Early surgery improves prognosis[18-20].

CONCLUSION

Colon cancer combined with appendiceal abscess is easily misdiagnosed. Prevotella melaninogenica can cause severe bloodstream infection in tumor patients. Clinicians should strengthen anaerobic pathogen detection and perform timely surgery for refractory cases.