Endocarditis is an infection of the inner lining of the heart, almost always centered on the heart valves. Bacteria (or rarely fungi) settle on a valve, trigger clot formation around themselves, and build up a mass called a vegetation. Left untreated, the infection destroys valve tissue and can send infected clots to the brain, lungs, or other organs. Stroke occurs in roughly 16% to 25% of patients with infective endocarditis.
How the Infection Takes Hold
The lining of a healthy heart valve is naturally resistant to infection. For bacteria to gain a foothold, something usually has to go wrong first. A small area of damage on the valve surface attracts platelets and a clotting protein called fibrin, which form a tiny, sterile deposit. When bacteria happen to pass through the bloodstream, whether from a skin wound, a dental procedure, or an infection elsewhere in the body, they can land on that deposit and begin multiplying.
Once bacteria embed themselves in this clot-like matrix, they become remarkably hard for the immune system to reach. The growing mass of bacteria, platelets, and fibrin is the vegetation visible on imaging. Some especially aggressive bacteria, particularly Staphylococcus aureus, can skip the “damaged valve” step entirely. S. aureus can bind directly to healthy valve cells, slip inside them, and begin destroying tissue from within while evading immune defenses.
Common Causes
Staphylococcus aureus is the single most frequent cause, responsible for roughly one in five to one in four cases on natural valves and an even larger share among people who inject drugs. Viridans group streptococci, a family of bacteria that normally live in the mouth, account for about 32% of natural valve infections. In people with prosthetic (artificial) valves, coagulase-negative staphylococci take the lead at around 32%, followed closely by S. aureus. Fungal endocarditis is uncommon, representing fewer than 3% of cases in large studies, but it tends to be harder to treat.
Who Is Most at Risk
Certain groups face a substantially higher chance of developing endocarditis. The highest risk belongs to people who have already had the infection once. After that come people with prosthetic heart valves and those born with cyanotic congenital heart disease, a category of defects where oxygen-poor blood mixes with oxygen-rich blood. Other important risk factors include:
- Injection drug use. Injecting with non-sterile needles pushes bacteria directly into the bloodstream. S. aureus causes roughly 85% of right-sided cases in this group.
- Degenerative valve disease. Age-related wear on the aortic or mitral valve creates surface irregularities where bacteria can attach.
- Implanted cardiac devices. Pacemaker and defibrillator leads provide another surface for bacteria to colonize.
- Poor dental health. Gum disease and dental procedures can introduce mouth bacteria into the bloodstream.
Symptoms and Physical Signs
Endocarditis often mimics other illnesses, which is one reason diagnosis is frequently delayed. A persistent fever that doesn’t respond to typical treatments is the most common symptom. Fatigue, night sweats, loss of appetite, and unexplained weight loss often accompany it. Joint and muscle aches are common, as are chills.
Several physical signs are distinctive enough that doctors consider them hallmarks of the disease. Splinter hemorrhages are tiny vertical streaks of blood under the fingernails or toenails, caused by small clots lodging in the nail bed. Osler nodes, first described in 1893, are painful red bumps on the fingers or toes. Janeway lesions look similar but are painless, flat, red or purplish spots on the palms or soles. None of these signs appear in every case, but when present alongside a fever and a new or changing heart murmur, they strongly suggest endocarditis.
Right-Sided vs. Left-Sided Infection
The side of the heart affected changes both the symptoms and the outlook. Left-sided endocarditis, involving the mitral or aortic valve, is more common overall and more dangerous. Vegetations on these valves can break off and travel to the brain (causing stroke), the kidneys, or the spleen. Right-sided endocarditis, involving the tricuspid valve, is the predominant pattern in people who inject drugs. Clots from the right side travel to the lungs, causing repeated lung infections and abscesses rather than stroke. Right-sided disease generally carries a better short-term prognosis, though repeated episodes are common when substance use continues.
How It Is Diagnosed
Diagnosis rests on two pillars: blood cultures and heart imaging. Blood cultures identify the specific bacterium or fungus, which guides the choice of treatment. Multiple sets are drawn before any antibiotics are started.
For imaging, echocardiography (ultrasound of the heart) is the primary tool. Two types exist. A transthoracic echocardiogram (TTE) is the standard, noninvasive version where a probe is placed on the chest wall. A transesophageal echocardiogram (TEE) uses a small probe passed down the throat to get much closer to the heart valves. The difference in accuracy is dramatic: in one comparative study, TEE detected 87% of confirmed cases while TTE detected only 23%. TEE is significantly better at spotting small vegetations, holes in valve tissue, and abscesses. For this reason, guidelines recommend TEE whenever endocarditis is strongly suspected, particularly in patients with prosthetic valves or unclear TTE results.
Treatment: Weeks of IV Antibiotics
Endocarditis requires prolonged intravenous antibiotic therapy, typically lasting four to six weeks. The length depends on the type of bacteria, which valve is affected, and whether the valve is natural or prosthetic. Uncomplicated right-sided infections caused by S. aureus can sometimes be treated with a shorter two-week course. Prosthetic valve infections generally require six weeks. The antibiotics must be given intravenously because oral medications cannot achieve high enough concentrations in the bloodstream to penetrate the vegetations and kill bacteria buried inside them.
Many patients begin treatment in the hospital and may transition to outpatient IV therapy once they are stable, receiving antibiotics through a long-term IV line at home or at an infusion center. Blood cultures are repeated during treatment to confirm the infection is clearing.
When Surgery Becomes Necessary
Roughly a quarter to half of endocarditis patients ultimately need valve surgery during the same hospitalization. The three main reasons for operating are heart failure from a badly damaged valve, infection that antibiotics cannot control, and large vegetations that pose a high risk of embolism. European guidelines use a vegetation size of 10 mm as a key threshold for surgical decision-making, with vegetations over 20 mm on the right side warranting surgery if they keep sending infected clots to the lungs despite appropriate antibiotics.
Surgery can involve repairing the existing valve or replacing it with a mechanical or biological prosthetic. For people who inject drugs, early surgical survival rates are generally good because patients tend to be younger with fewer other health problems. However, recurrent endocarditis and death after surgery are significantly more common in this group when substance use disorder goes untreated, which is why addiction treatment is considered a critical part of the overall plan.
Stroke and Other Complications
The most feared complication of endocarditis is stroke. Pieces of vegetation break away, travel through the bloodstream, and block arteries in the brain. A large study of nearly 18,000 patients with endocarditis found that stroke risk was highest in the month after diagnosis, with an absolute risk increase of 9.1% during that window. About 83% of these strokes were caused by blockages (ischemic strokes), while roughly 14% involved bleeding (hemorrhagic strokes). The risk drops substantially once effective antibiotic therapy is underway and vegetations begin to shrink or stabilize.
Beyond stroke, infected emboli can damage the kidneys, spleen, and other organs. Heart failure from valve destruction is the leading cause of death in endocarditis. Abscesses can form in the heart muscle itself, sometimes disrupting the electrical system and causing dangerous heart rhythm problems.
Antibiotic Prophylaxis Before Dental Work
For a select group of high-risk patients, taking a single dose of antibiotics before certain dental procedures can reduce the chance of developing endocarditis. Current American Heart Association guidelines recommend prophylaxis only for people at the highest risk of a bad outcome: those with prosthetic heart valves, a history of previous endocarditis, certain types of congenital heart disease (particularly cyanotic defects, recent repairs with prosthetic material, or repairs with residual defects). If you fall into one of these categories, your dentist and cardiologist should coordinate before procedures that involve the gums or tooth roots.
Non-Infective Endocarditis
Not all endocarditis is caused by infection. Nonbacterial thrombotic endocarditis (NBTE) produces sterile vegetations made of platelets and fibrin, without any bacteria involved. It is most commonly linked to advanced cancer, appearing in 32% to 80% of cases identified in autopsy studies. Systemic lupus erythematosus accounts for about 10% of cases. Other associated conditions include antiphospholipid syndrome, rheumatoid arthritis, and, more recently, COVID-19 infection. NBTE vegetations tend to be small and fragile, and they can still cause strokes and organ damage through embolism. Treatment focuses on the underlying disease rather than antibiotics.