Causes of endocarditis 

Your heart is usually well protected against infection, and bacteria usually pass harmlessly by.

However, if your heart valves are damaged or if you have an artificial valve, it will be easier for bacteria to take root and bypass your normal immune response to infection.

The bacteria multiply and spread across the inner lining of your heart (the endocardium). The endocardium becomes inflamed, causing damage to your heart valves.

Once damaged, the valves may no longer be able to supply your heart with the blood it needs, leading to heart failure.

Small clumps of bacteria can develop in the heart at the site of the infection. There is a risk that these clumps will act in a similar way to blood clots, travelling away from the heart and blocking the blood supply to the organs, which can cause organ failure or trigger a stroke.

How bacteria reach the heart

The most common ways that bacteria can enter your blood are explained below.


Everyday activities, such as brushing your teeth or chewing your food, can sometimes allow bacteria to enter the bloodstream. The risk is increased if your teeth and gums are in bad condition, because it makes it easier for bacteria to enter.


Bacteria can spread from the site of a pre-existing infection, such as a skin infection or a gum infection.

Bacteria can also enter your body as a result of a sexually transmitted infection (STI), such as chlamydia or gonorrhoea.

Needles and tubes

Any medical procedure that involves placing a medical instrument inside the body carries a small associated risk of introducing bacteria into your bloodstream.

Instruments that have been linked to endocarditis include:

  • syringes
  • catheters (a tube that is used to drain the bladder)
  • the tubes that are used during dialysis (a treatment that involves replicating the functions of the kidneys)
  • laparoscopes (a small, flexible tube that has a light source and a camera at one end and is used to help diagnose a wide range of conditions)

Who’s at risk

There are a number of things that can make your heart more vulnerable to infection and increase your chances of developing endocarditis. These are discussed below.

Heart valve disease

Heart valve disease is a general term describing conditions that damage the valves of the heart. Two types of heart valve disease known to increase your risk of endocarditis are:

  • valvular stenosis: where the valve(s) of the heart become narrowed, disrupting the blood flow through the heart
  • valvular regurgitation: where the valve(s) of the heart do not close properly, causing blood to leak back in the wrong direction

Heart valve disease can be congenital (where you are born with the condition), or acquired (where you develop the condition in later life).

Causes of acquired heart valve disease include:

  • Having a previous heart attack. A heart attack can damage the muscles that surround and support the valve, preventing the valves from functioning properly.
  • High blood pressure. Left untreated, high blood pressure (hypertension) can weaken the tissue around the valves.
  • Rheumatic fever. Rheumatic fever is a type of bacterial infection that can damage the heart.

Rheumatic fever is rare since the introduction of antibiotics. However, older people who have had rheumatic fever during childhood may go on to develop heart valve disease.

Prosthetic valves

Prosthetic (artificial) valves are used to replace heart valves that have been damaged by heart valve disease.

However, bacteria can also take root around prosthetic valves, which can occasionally trigger endocarditis. This risk is relatively low, estimated to be less than one in 100.

Intravenous drug use

People who habitually inject illegal drugs such as heroin or methamphetamine (crystal meth) into their veins have an increased risk of developing endocarditis. This is because of unsterilised needles allowing bacteria to enter the bloodstream and repeated injections making the skin more vulnerable to infection.

Fungal endocarditis

Endocarditis caused by a fungal infection is rarer than bacterial endocarditis, and usually more serious. You are more at risk of fungal endocarditis if you:

  • inject drugs
  • have a history of heart surgery
  • have a central venous catheter (a tube that is connected to a vein in the neck, groin or chest, and is used to deliver medicines and/or fluids to people who are seriously ill)
  • have a weakened immune system, either as a result of a condition affecting the immune system, such as HIV, or as a side effect of certain types of treatments, such as chemotherapy

Page last reviewed: 04/04/2014

Next review due: 04/04/2016