Anorectal abscesses are common in colorectal surgery and are often attributed to obstruction of the anal glands[1]. Anal fistulas are abnormal channels connecting the anorectal region to the perianal skin, typically resulting from chronic infection and epithelialization of the drainage tract following an abscess[[2], [3], [4]]. Clinically, anal fistulas present with local pain, inflammation, purulent discharge, and incontinence, significantly impacting patients' quality of life[3]. Anorectal abscesses are associated with fistulas in 30 % to 70 % of cases, with an incidence of 1.69 cases per 10,000 individuals, occurring twice as frequently in males as in females[[4], [5], [6]]. Treatment strategies—such as incision and drainage, ligation of the intersphincteric fistula tract, drainage seton placement, fistulotomy, and fistulectomy[4,[7], [8], [9], [10]]—have advanced in recent years. However, research on the etiopathogenesis of these conditions remains limited. In particular, the complex mechanisms involving intestinal, immune, and other interconnected pathways are still not fully understood.

While anal fistulas commonly arise from infections of the anal glands, they can also develop as complications of Crohn's disease. Both anorectal abscesses and anal fistulas are characterized by inflammatory processes as central pathological features[3,11]. Recent research emphasizes the critical role of gut microbiota in the pathogenesis and progression of various intestinal disorders[12]. Microbial dysbiosis has been linked to conditions like colorectal cancer and inflammatory bowel disease (IBD)[[13], [14], [15]]. For instance, patients with IBD often exhibit increased levels of pathogenic Proteobacteria and a reduced presence of beneficial Firmicutes[14,16]. In a study using 16S rRNA gene sequencing, Cai et al. identified 36 taxa associated with anal fistula, observing an enrichment of Ruminococcus and Bacteroides [17]. These findings point to a possible association between rectal microbiota composition and the occurrence of anal fistula. While these studies enhance our understanding of microbial and inflammatory interactions in anal fistulas, the precise mechanisms by which gut microbiota contribute to the onset of anorectal abscess and anal fistula—especially through immune modulation—remain inadequately defined. Increasing evidence points to immune cells as key mediators linking microbial disturbances to chronic inflammatory diseases. Mendelian randomization (MR) studies have successfully utilized genetic instruments for immune cell phenotypes to infer causal relationships in a range of immune-related disorders, including heart failure[18], pancreatitis[19], varicose veins[20], intracerebral hemorrhage[21], and biliary tract cancers[22]. Similarly, gut microbiota has been causally implicated in gastrointestinal diseases such as pancreatitis, biliary tract cancer, and gastroesophageal reflux disease through bidirectional MR frameworks[[23], [24], [25]]. These findings provide a strong rationale for employing MR to unravel the causal pathways linking microbiota, immune phenotypes, and downstream diseases.

MR leverages genetic variants from genome-wide association studies (GWAS) as instrumental variables (IVs), offering a robust framework to investigate causal relationships between genetic predispositions and clinical outcomes[26]. This methodology provides valuable insights into disease mechanisms and aids in the development of targeted therapies while minimizing biases commonly associated with observational studies[27]. The reliability of MR results is heavily influenced by the selection of GWAS datasets and the rigor of IVs selection criteria.

In this study, to the best of our knowledge, the most current and comprehensive GWAS datasets were utilized, incorporating 211 gut microbiota taxa and 731 immune cell phenotypes. This study aimed to identify gut microbiota associated with anorectal abscess and anal fistula and investigate the mediating roles of relevant immune phenotypes. The findings shed light on the etiology and underlying mechanisms of anorectal abscess and anal fistula.