Anal fistula - Causes of an anal fistula

An anal fistula is most commonly caused by an anal abscess. It can also be caused by conditions that affect the intestines (part of your digestive system).

Anal abscess

An abscess is a painful collection of pus. An anal abscess usually develops after a small gland just inside the anus becomes infected with bacteria.

The cause of the abscess is often unknown, although abscesses are more common in people with immune deficiencies, such as HIV and AIDS.

Abscesses are usually treated with a course of antibiotics (medication to treat infections caused by bacteria). In most cases, you will also need to have the infected fluid drained away from the abscess.

If an anal abscess bursts before it has been treated, it can sometimes cause an anal fistula to develop. A fistula may also occur if an abscess has not completely healed, or if the infected fluid has not been entirely drained away.

Approximately 30-50% of people with an anal abscess will develop an anal fistula. Around 80% of all anal fistulas develop from an infection in the anus.

Inflammation of the intestines

An anal fistula can be a complication of conditions that cause the intestines to become inflamed, including:

• Crohn's disease– a chronic condition that causes inflammation of the lining of the digestive system
• diverticulitis– infection of the small pouches that can stick out of the side of the large intestine (colon)

Crohn's disease is a type of inflammatory bowel disease (IBD).

Other causes

Other infections or conditions that can lead to the development of an anal fistula include:

• cancer of the anorectum – the rectum is an area at the end of the colon where faeces are stored
• tuberculosis – a bacterial infection that mainly affects the lungs, but can also spread to many different parts of the body
• HIV and AIDS – a virus that attacks the body's immune system (its defence against disease and infection)
• chlamydia – a sexually transmitted infection that often causes no symptoms
• syphilis – a bacterial infection that is passed on through sexual contact, injecting drugs or blood transfusions

An anal fistula may also develop as a result of:

• a growth or ulcer (painful sore)
• a complication of surgery
• a health problem you were born with

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