Pathogenic organisms(usually bacteria or fungi) in the blood may colonise any part of the endocardium,but the most common sites are on near the heart valves and round the margins of congenital heart defects. These areas are susceptible to infection because they are exposed to fast-flowing blood that may cause mild trauma. This is a serious illness and often fatal unless adequately treated.
The main predisposing factors are bacteraemia,depressed immune response and heart abnormalities.
BACTERAEMIA.
Microbes in the bloodstream,if not destroyed by phagocytes or antibodies,tend to adhere to platelets and form tiny infected emboli. Inside the heart,the emboli are most likely to settle on already damaged endocardium. Vegetations consisting of platelets and fibrin surround the microbes and seem to protect them from normal body defects and antibodies. Because of this,infection may be caused by a wide range of bacteria,including some that do not normally cause clinical infection. They normally originate from the skin or the mouth.
DEPRESSED IMMUNE RESPONSE.
This enables low-virulence bacteria,viruses,yeasts and fungi to become established and cause infection. These are organisms always present in the body and the environment. Depression of the immune systems may be caused by HIV infection,malignant disease,cytotoxic drugs,radiotherapy or steroid therapy.
HEART ABNORMALITIES.
The sites most commonly infected are already abnormal in some way. Pathogenic organisms present in the bloodstream cannot adhere to healthy endothelium,but if the endothelial lining of the cardiovascular system is damaged,infection is more likely. Often,the cardiac valves are involved,especially if damaged by rheumatic disease or congenital malformation.
