An abscess is a localized spot of pus-filled, infected tissue encircled by inflammation. A fistula is an abnormal tunnel linking a vessel or organ to another vessel or organ, including the surface of the skin. An abscess and a fistula are separate medical conditions, but they share some causes and treatments.
One of the major differences between an abscess and a fistula is that although both can result from underlying disease, injury, infection or the presence of a foreign substance in the body, surgeons will sometimes purposefully create fistulas to treat underlying medical conditions. For example, doctors sometimes artificially connect an artery and a vein in the forearm to produce stable vascular access for hemodialysis, a process by which waste products are artificially removed from kidney patients' blood. This connection is technically a fistula.
The symptoms of an abscess and a fistula can also differ. Abscesses generally cause pain, fever and an overall feeling of malaise. Fistulas, depending on their location and whether they are infected, might not cause any symptoms at all.
Another difference between an abscess and a fistula is that although fistulas often can be treated successfully with antibiotics, it is always necessary to drain abscesses surgically. This is because antibiotics travel through the bloodstream, but there is no vascular access to the infection at the center of an abscess. The pus, which is composed of lymph fluid, white blood cells, dead tissue and bacteria, must be drained surgically. The surgery usually can be done on an outpatient basis, but if the patient has an underlying condition that can alter the immune system, such as diabetes, inpatient treatment might be necessary. Sometimes, draining an abscess surgically can actually create a fistula, which can appear anywhere from two weeks to several months after the abscess surgery.
Depending on their cause and location in the body, fistulas often can be treated with a course of antibiotics. This allows the infection to clear while the tissue reforms its natural barriers. Sometimes during this process, a pocket of infection is trapped between tissue walls, creating an abscess, which must then be treated.