An anal fistula refers to a small infected tunnel connecting the anus to the outer skin. Your body eliminates solid waste products or feces through the anus. When any of the glands inside the anus becomes infected or blocked, an abscess may form, which in turn results in formation of anal fistula. In about 50% of the cases, abscesses develop into fistulae. What are the causes for anal fistula? Anal fistulae occur primarily due to the formation of an abscess or infection in the anus.