Anal Abscesses and Fistulas

June 5th, 2013 by Eduardo Krajewski, MD, FACS, FASCRS

In cats and dogs, the anus contains glands that secrete fluid that helps animals identify each other by smell. Humans also have these glands, but thankfully we do not use them for identification. Usually we never even notice we have anal glands (they are sometimes called anal crypt glands as well). If these glands get clogged up, however, we notice them very much.

What is an Anal Abscess?

The anal glands have tiny tubes that drain the content of the glands into the anal canal. Sometimes these tubes get blocked, and the glands become infected. Pus builds up, and the gland begins to swell under pressure. At this point an abscess has developed. You might feel a lump near the anus, and it is usually very painful.

What Causes Anal Abscesses?

Many times the cause is not known. In other cases the abscess can be caused by or related to:

  • Infection of the anal fissure
  • Sexually transmitted infection
  • Crohn’s disease or ulcerative colitis
  • Diverticulitis
  • Chemotherapy use, steroid use
  • Diabetes
  • Weak immune system (such as from HIV infection)

What are the Symptoms of Anal Abscesses?

The abscess typically causes a swelling or lump near the anus. Anal abscesses can also be very painful and may or may not be made worse with bowel movements. Other symptoms can include:

  • Fever, general malaise (feeling bad in general)
  • Pus discharge from rectum
  • Constipation

How is an Anal Abscess Diagnosed?

Your doctor will do a rectal exam looking on the outside and feeling on the inside with a gloved and lubricated finger. If the diagnosis is in doubt other tests, like a proctosigmoidoscopy, might have to be done. If nothing can be felt or seen, scanning tests like a CT scan or MRI might have to be ordered to make a definitive diagnosis.

How are Anal Abscesses Treated?

The treatment involves surgery to open and drain the abscess. The complexity of the surgery depends on the size and location of the abscess. Simple abscesses can be drained in the physician’s office or the emergency room. More complex and deep abscesses should be taken care of in an operating room under general anesthesia.

After the abscess is drained your doctor may or may not recommend antibiotics. Indications for post-drainage antibiotics are:

  • Heart valve disease
  • Immunosuppression
  • Extensive cellulitis (infection of skin and tissue surrounding abscess)
  • Diabetes

Pain medications, stool softeners and sitz baths are all frequently used in post-operative care.

What is an Anal Fistula?

Anal fistulas are closely related to anal abscesses. In anal fistulas, the anal crypt gland gets infected forming an abscess. A very tiny tract or tunnel then begins to extend out from the abscess. The tract eventually extends and empties either into the rectum or out to the surface of the skin near the anus. Since the tract communicates with the abscess there is a constant drainage of pus. Like anal abscesses, anal fistulas can be quite painful.

The diagnosis depends on a physical exam and sometimes imaging studies as well. The tracts of anal fistulas can be numerous or complicated so a complete exploration should be done by your doctor. This can be painful so exploration is usually done under anesthesia. The standard treatment is to remove the fistula surgically.


Anal abscesses and anal fistulas are problems that arise from infected anal crypt glands. A physician’s evaluation is critical in making the right diagnosis. In both cases, the best treatment is surgical excision.

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