A perianal fistula typically appears as a small, red, inflamed opening on the skin near the anus. The external opening often oozes pus, blood, or a mix of both, and the surrounding skin may look irritated or raw. In some cases the opening is obvious, but in others it can be subtle, showing up as little more than a slightly raised or hardened spot that only becomes noticeable on close inspection.
What the External Opening Looks Like
The most visible sign of a perianal fistula is its external opening, a small hole in the skin somewhere around the anus. The skin around this opening is usually red and inflamed, and you may notice fluid leaking from it. That fluid can be yellowish pus, blood-tinged drainage, or occasionally fecal matter. It often has a noticeable smell.
The opening itself can range from a tiny pinpoint to a slightly larger hole, sometimes surrounded by a small mound of pink or red granulation tissue, which is the body’s healing tissue that looks raw and bumpy. In longer-standing fistulas, you might also feel a firm, cord-like ridge under the skin extending from the opening toward the anus. That cord is the fistula tract itself, a tunnel of scar tissue connecting the external opening to an internal opening inside the anal canal.
Some people have more than one external opening, which can indicate a branching or complex fistula. The skin between and around the openings may be excoriated, meaning it looks red, peeling, or irritated from constant moisture and drainage.
How It Differs From an Abscess
Because most perianal fistulas develop after a perianal abscess, it helps to understand how the two look different. An abscess is the acute phase: a swollen, tender, warm lump near the anus that feels like it’s throbbing. The skin over it is tight and red, and the area may feel soft or “boggy” when you press on it because it’s filled with pus.
A fistula, by contrast, is the chronic aftermath. Once an abscess drains (either on its own or through surgery), a tunnel can persist between the inside of the anal canal and the skin surface. Instead of one painful swollen lump, you see one or more small openings that drain intermittently. Many people with fistulas describe a cyclical pattern: the area swells and becomes painful for a few days, then drains and feels better, then swells again. That cycle of buildup and release is a hallmark of a fistula rather than a one-time abscess.
What You Can’t See From the Outside
The external opening is only one end of the tunnel. The other end, the internal opening, sits inside the anal canal and is not visible without a medical exam. A fistula tract can take several different paths through the surrounding muscle. The most common type, making up about 45% of cases, runs between the two rings of muscle that control the anus (the internal and external sphincters) without crossing through the outer ring. About 30% of fistulas pass through both muscle rings. Rarer types loop above or completely around the sphincter muscles.
These differences matter because they determine how complex the fistula is to treat, but from the outside, a simple and a complex fistula can look identical. That’s one reason imaging is so important. MRI is the gold standard for mapping fistula tracts before surgery, with sensitivity reaching 100% when certain scan sequences are combined.
Conditions That Look Similar
Not every draining bump near the anus is a fistula. A few other conditions can mimic the appearance.
- Hidradenitis suppurativa (HS) causes recurring abscesses and draining tracts in skin folds, including the groin and buttocks. HS lesions tend to be bilateral (appearing on both sides), are more superficial, and often show up on the buttocks and gluteal creases rather than right next to the anus. You may also see blackhead-like comedones and scarring in the armpits, groin, or under the breasts, which points toward HS rather than a fistula.
- Pilonidal cyst forms in the cleft between the buttocks, usually higher up near the tailbone, rather than close to the anal opening. It can drain pus and look inflamed, but its location is the key distinction.
- Skin tags or hemorrhoids can cause lumps near the anus, but they don’t produce a draining tunnel or ooze pus.
The Crohn’s Disease Connection
Perianal fistulas affect roughly 1 to 3 out of every 10,000 people in the general population, with a European prevalence of about 18 per 100,000. But they are far more common in people with Crohn’s disease, where the lifetime risk of developing a perianal fistula ranges from 14% to 38%. In Crohn’s patients, fistulas tend to be more complex, with multiple openings and branching tracts. If you’ve been diagnosed with a perianal fistula and also have chronic diarrhea, abdominal pain, or unexplained weight loss, Crohn’s disease may be the underlying cause.
What to Watch For Over Time
A fistula that has been quietly draining can flare into something more serious if the external opening seals over and traps infection inside. Signs that this is happening include increasing pain, a growing firm or fluctuant swelling near the anus, fever, and feeling generally unwell. These symptoms suggest a new abscess is forming and needs prompt attention.
Persistent drainage that soaks through a pad or underwear liner, skin breakdown around the opening, or the appearance of new openings near the original one all suggest the fistula is becoming more complex rather than healing on its own. Fistulas rarely close permanently without treatment, so a draining opening that keeps cycling through flare-ups and quiet periods is the typical natural course.