Anal Abscess vs. Anal Fistula: What's the Difference?

An anal abscess and an anal fistula are essentially two different stages of the same disease process. An anal abscess is the acute (sudden) phase—a painful, swollen pocket of pus caused by an infection ^[1]. An anal fistula is the chronic (long-term) phase—a small tunnel or track left behind under the skin after the abscess has drained ^[1].

The “Cryptoglandular” Connection

Doctors often explain this relationship using the cryptoglandular hypothesis ^[1]. Just inside your anus, there are tiny glands that make mucus. If one of these glands gets blocked, bacteria can build up and cause an infection, forming a painful pocket of pus (the abscess) ^[1]. While a blocked gland is the cause for the vast majority of cases, it is important to note that other underlying conditions, such as Crohn’s disease or an elevated body mass index (BMI), can also increase the risk of these issues developing ^[2][3].

Once the abscess drains—whether it bursts on its own or a doctor drains it—the pressure is relieved. However, the path the pus took to get to the surface of the skin can remain open. This hollow tunnel connecting the infected internal gland to the opening on the outside skin is the fistula ^[1].

The Link Between the Two

Hearing both terms from your doctor can be confusing, but they are deeply connected. Research shows that approximately 30% to 50% of people who have an anal abscess drained will eventually develop an anal fistula ^[4][5][6]. Because of this high likelihood, doctors often monitor patients closely after an abscess is treated. Most secondary fistulas develop within the first year after the initial abscess ^[5].

How Symptoms Differ

Because they represent different stages of infection, their symptoms feel different:

• Anal Abscess Symptoms: These tend to appear suddenly. You may feel severe, constant anal pain accompanied by a noticeable lump, swelling, and redness ^[1]. You might also develop a fever or chills due to the active infection ^[1]. When to seek immediate care: If you notice spreading skin redness, an inability to urinate, or a worsening high fever, contact a doctor immediately as the infection may be becoming severe.
• Anal Fistula Symptoms: These tend to be ongoing or intermittent. You might notice continuous or off-and-on drainage of pus, blood, or fluid from a small opening near the anus ^[1]. The pain is usually less intense than an abscess. However, if the outside opening of the tunnel closes, pus can build up again, leading to a flare-up of severe pain ^[1]. If this happens, a new abscess may be forming, and you should contact your doctor.

Different Stages, Different Treatments

Just as their symptoms differ, so does the medical approach to treating them. An active anal abscess typically requires immediate drainage to relieve the painful pressure and clear the acute infection. In contrast, an anal fistula rarely heals on its own and typically requires a separate, planned surgical procedure to permanently close the chronic tunnel.