A fistula is an abnormal connection between an organ, vessel, or intestine and another structure. As we know that the last stage of digestion is the collection of and passing from the body of solid wastes. These wastes (what is left of what we eat after the water and nutrients have been taken by our bodies) collect in the rectum and then are expelled through the anus. The inside of the anus is lined with glands and four to six crypts, or pockets. Sometimes one of these pockets gets filled with stool. This can cause the gland to become infected and develop an anorectal abscess.

Types of fistulas include :-

Blind (open on one end only, but connects to two structures)
Complete (has both external and internal openings)
Horseshoe (connecting the anus to the surface of the skin after going around the rectum)
Incomplete (a tube from the skin that is closed on the inside and does not connect to any internal structure)

Causes

Fistula usually results from an injury to the tissue lining the anal canal or an infection in that area. It may occur as a result of :-
Constipation causing injury
Infection in the crypts (crevices) glands lining the anal canal
Cohns disease, a chronic inflammation of the intestines
Abscess of the large intestine (very rare).

As we know that, the immune system is activated only when the body is exposed to harmful invaders. In patients with IBD, however, the immune system is abnormally and chronically activated in the absence of any known invader. The continued abnormal activation of the immune system results in chronic inflammation and ulceration. The susceptibility to abnormal activation of the immune system is genetically inherited.

Symptoms

The common symptoms of fistula which occur in most of the patients are :-

Fever
A vague feeling of being unwell or uncomfortable
Swelling around or near the anus
Redness around the area
Drainage of pus from the area
Regular discharge of pus
Anal pain and swelling

Treatment

More than half of all fissures are either by themselves or with non-surgical treatment. Stool softeners can help reduce pain during bowel movements. Antibiotics can already be used but only for very short time. Special medicated creams may also be used, especially if the fissure has become ulcerated or infected.
To treat a more complicated fistula, such as a horseshoe fistula, the surgeon may lay open only the segment where the tracts join and remove the remainder of the tracts.
If a significant amount of the sphincter muscle must be cut, the surgery may be performed in more than one stage. It may also need to be repeated if the entire tract can't be found.