Flushing Out the Culprit: Mycobacterium abscessus as a Cause of Recurrent Fistula-in-ano

Abstract

Complex fistula-in-ano (FIA) presents significant surgical challenges, with recurrence often driven by inflammatory, anatomical, and microbiological factors. Mycobacterium abscessus, a multidrug-resistant nontuberculous mycobacterium (NTM), is rarely implicated in FIA and is more commonly associated with respiratory and skin infections. A man in his mid 30s with recurrent complex FIA underwent multiple surgeries over four years. Imaging and surgical exploration revealed a left ischiorectal collection and a sinus tract extending to the supralevator space. Tissue cultures grew M. abscessus subspecies massiliense, and histology showed non-necrotising granulomatous inflammation. The patient was a regular bidet user; this was considered the plausible source of inoculation given the organism's presence in water. He was treated with combination antimicrobial therapy comprising concurrent long-term oral azithromycin and clofazimine with an overlapping intravenous regimen including amikacin and imipenem resulting in clinical and radiological resolution. Environmental NTM exposure may underlie refractory FIA. Early recognition and targeted antimicrobial therapy are essential to avoid repeated surgery and prolonged morbidity.

Keywords
rectal fistula; colorectal surgery; abscess; anal fistula; mycobacteroides abscessus; non-tuberculous mycobacteria

Introduction

Fistula-in-ano (FIA) is a chronic pathological connection between the anal canal and the perianal skin, frequently arising as a sequela of anorectal abscesses. These abscesses result from obstruction and subsequent infection of anal glands located near the sphincter complex. When untreated or following spontaneous or surgical drainage, persistent inflammation leads to fistula formation. Both FIA and anorectal abscesses are common and make up about 70% of perianal diseases.¹

Approximately ⅓ of patients who undergo drainage of an anorectal abscess will go on to develop a fistula, and up to 70% may already have a fistula present at the time of diagnosis.² The incidence of FIA is estimated at 9 per 100 thousand individuals, making it more common than Crohn's disease or ulcerative colitis.³ The anatomy of the fistula tract often involves the anal sphincter complex and can vary significantly, which is crucial for both classification and management. Simple fistulas are typically confined to the lower third of the sphincter, whereas complex ones may traverse a significant portion of the sphincter, present anteriorly in women, or occur in the context of conditions such as Crohn's disease or prior pelvic irradiation.⁴

Despite its long-standing recognition in clinical practice, its management is often nuanced and complex due to the variable anatomy of the fistulous tract and the need to balance definitive cure with sphincter preservation. While most cases are attributed to cryptoglandular infections, rare and atypical pathogens may underlie in recurrent or nonhealing disease.⁵ Among these, nontuberculous mycobacteria (NTM) are exceedingly rare pathogens in perianal pathology. To date, only one prospective cohort study investigating this entity has been reported, conducted in India.⁶ To the best of our knowledge the present study represents the only published case report of recurrent FIA caused by NTM, highlighting the rarity of this presentation and the importance of considering atypical infections in patients with persistent or recurrent disease.

Mycobacterium abscessus, a rapidly growing NTM, is primarily associated with pulmonary disease, postoperative wound infections, skin and soft tissue involvement, both in immunocompetent and immunocompromised individuals.⁷ Its role in gastrointestinal or perianal infections is poorly defined, with limited reporting in the surgical literature. Due to its inherent resistance to conventional antimicrobials and its biofilm-forming capacity, infections caused by M. abscessus are often difficult to treat and can complicate standard surgical pathways.⁷

In this report, we present a rare case of recurrent FIA caused by M. abscessus, highlighting the diagnostic challenges, surgical implications, and therapeutic considerations associated with this unusual etiology. This case underscores the importance of maintaining a high index of suspicion for atypical infections in refractory or recurrent fistulous disease and prompts further discussion on the need for microbiological evaluation in select surgical patients.

Case Presentation

A male patient in his mid 30s had been under follow-up for a period of three years due to a symptomatic, recurrent complex FIA. In addition to his perianal pain, he experienced extreme fatigue, subjective fevers and night sweats. Despite undergoing multiple surgical interventions during this time, including several seton placements and fistulotomies, he experienced persistent recurrence of the fistula, with only transient symptomatic relief. The most prolonged asymptomatic interval lasted for approximately 12 months.

Serial pelvic magnetic resonance imaging (MRI) scans consistently demonstrated an internal opening at the 6 o'clock position, with external openings located at the left posterior and medial gluteal clefts. Additionally, computed tomography (CT) scans also revealed inflammatory changes in the perianal region.

During a recent surgical procedure, a sinus tract measuring approximately 15 cm in length was identified, extending from a point 2 cm lateral to the external opening at the left medial gluteal cleft toward the left lateral apex of the ischiorectal fossa. The tract appeared to terminate in a small collection containing minimal granulation tissue. These findings were later confirmed on MRI. The tract and associated collection were curetted, and tissue samples were sent for histopathological examination and bacterial and mycobacterial culture. Several operative specimens cultured M. abscessus subspecies massiliense. The patient subsequently underwent two additional operative procedures, with intraoperative cultures on both occasions again yielding M. abscessus.

Following isolation of Mycobacterium abscessus, the patient commenced a concurrent oral and intravenous (IV) combination antimicrobial therapy, incorporating an intravenous intensive phase. Oral azithromycin (250 mg daily) and clofazimine (100 mg daily) were initiated at the outset and continued for a total duration of 6 months. Furthermore, IV amikacin (1.3 g three times weekly) was administered for approximately 8 weeks; and IV cefoxitin (2 g every 6 hours) was commenced but discontinued after 15 days due to elevated liver function tests. Subsequently, IV imipenem (1 g twice daily) was introduced and continued for 6 weeks. Finally, IV Tigecycline was initiated at 25 mg twice daily and gradually escalated in 5 mg increments to 40 mg but was discontinued after 9 days due to severe gastrointestinal intolerance and pancreatitis (Fig. 1).

Fig. 1
Antimicrobial treatment.

The remainder of the regimen was well tolerated, and the patient continued long-term oral therapy under the supervision of an infectious disease specialist. On further history, the patient reported frequent use of a bidet to relieve chronic perianal discomfort. Given the known presence of M. abscessus in water sources and its biofilm-forming capacity, this was considered a plausible route of inoculation and the likely source of infection.

The patient reported a marked resolution of symptoms over a year of follow-up with progress MRI scans demonstrating a significant decrease in inflammatory changes.

Discussion

The NTM comprise a diverse and expanding group of over 190 Mycobacterium species, excluding M. tuberculosis and Mycobacterium leprae.⁸ These organisms are widely distributed in natural and domestic environments, particularly in soil and water, and their ecological resilience is largely attributed to their capacity for biofilm formation.⁹ Once thought to be transient human colonizers, NTMs are now increasingly recognized as true pathogens capable of causing both pulmonary and extrapulmonary infections. While immunocompromised individuals, such as those with human immunodeficiency virus (HIV), cystic fibrosis, or structural lung disease are at greater risk, NTM infections can also affect otherwise healthy hosts.¹⁰ Transmission typically occurs via inhalation of aerosolized particles (e.g., from showers) or direct inoculation through trauma or invasive procedures.¹¹

The rise in NTM-related disease appears multifactorial, driven by a growing at-risk population and improved diagnostic capabilities. Clinically, NTM have been implicated in pulmonary (nodular, fibrocavitary) and extrapulmonary presentations, including cutaneous, skeletal, and disseminated disease.¹⁰ Notably, different NTM species demonstrate variable geographic prevalence and pathogenic potential, adding complexity to their taxonomy and diagnosis. Management remains challenging due to species-specific antimicrobial resistance patterns and the often indolent, recurrent nature of infection. As such, unusual or refractory clinical presentations, particularly in surgical pathology involving recurrent or nonhealing wounds, should raise suspicion for mycobacterial involvement.

In this case, the patient was an immunocompetent male with no identifiable comorbidities. The presumed source of infection was through frequent bidet use for perianal hygiene and considered a plausible route of environmental inoculation. This highlights the potential for seemingly benign hygiene practices to serve as a vector for rare pathogens in the setting of chronic perianal disease.

To date, the presence of NTM as a causative agent in FIA remains exceedingly rare. Only a single prospective cohort study from India has evaluated the prevalence of NTM in FIA patients.⁶ The investigators only utilized RT-PCR for diagnosis, which carries limitations as it is difficult to discern between colonization and true infection. Importantly, extrapulmonary NTM infections almost always require mycobacterial-specific culture media for definitive diagnosis, as routine tests may fail to isolate these organisms.¹² Additionally, sending separate tissue specimens for histological evaluation can be critical, as characteristic granulomatous inflammation may provide supportive diagnostic evidence when interpreted alongside culture results.¹² Our case is complete with clinical risk factors of bidet use and preexisting FIA, systemic symptoms of NTM infection, consistent histological changes, and culture positivity for M. abscessus. This highlights the paucity of reported cases and the need for greater awareness of NTM-related FIA infection among colorectal surgeons and physicians.

• Funding
  The authors declare that they did not receive funding from agencies in the public, private or non-profit sectors to conduct the present study.

Data Availability

Data will be available upon request to the corresponding author.

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Edited by

• Editor-in-Chief:
  Henrique Sarubbi Fillmann.

Publication Dates

• Publication in this collection
  20 July 2026
• Date of issue
  2026

History

• Received
  22 Dec 2025
• Accepted
  04 Mar 2026