Bacterial endocarditis is an infection of the inner lining of the heart, almost always centered on one or more heart valves. Bacteria enter the bloodstream, attach to damaged or abnormal valve tissue, and form clumps of bacteria, blood cells, and debris called vegetations. These vegetations can destroy valve tissue, block blood flow, and send infected fragments to other organs. Even with treatment, the condition carries a 30-day mortality rate around 20%, making early detection critical.
How the Infection Takes Hold
Your heart valves open and close with every heartbeat, directing blood through four chambers. Healthy valve surfaces are smooth and resistant to bacterial attachment. But when a valve is scarred, roughened, or replaced with an artificial one, bacteria circulating in the blood can latch on and begin multiplying.
Once bacteria colonize the valve surface, the body’s clotting system responds by layering platelets and a protein called fibrin over the site. Instead of clearing the infection, this creates a sheltered environment where bacteria thrive. The result is a vegetation: a mass that can range from a few millimeters to over a centimeter in size. As the vegetation grows, it erodes the valve leaflets, causes leaking (regurgitation), and can break apart, sending fragments into the bloodstream that lodge in the brain, lungs, kidneys, or spleen.
Which Bacteria Cause It
Three families of bacteria account for 80% to 90% of all cases: staphylococci, streptococci, and enterococci. Staphylococcus aureus alone is responsible for roughly 30% of cases overall and around 50% of infections acquired in hospitals or healthcare settings. It tends to cause a more aggressive, rapidly destructive form of the disease.
Streptococci from the viridans group, bacteria that normally live in the mouth, cause about 20% of infections picked up in everyday life. Enterococci, commonly found in the gut, cause 15% to 18% of cases regardless of where the infection originates. Fungi account for only about 1% of cases but are notoriously difficult to treat.
Who Is Most at Risk
The single biggest risk factor is having something abnormal about your heart valves. Artificial (prosthetic) heart valves provide a surface that bacteria attach to more readily than natural tissue. Valves scarred by rheumatic fever, prior infection, or congenital heart defects are also vulnerable. A previous episode of endocarditis significantly raises the chance of a second one.
Beyond heart-specific risks, several other factors increase vulnerability:
- Age over 60, which is when most cases occur
- Long-term intravenous catheters, which give bacteria a direct route into the bloodstream
- Intravenous drug use, which repeatedly introduces bacteria and often damages the tricuspid valve on the right side of the heart
- Poor dental health, since bacteria from infected gums can enter the bloodstream through small cuts in the gum tissue
Symptoms and Physical Signs
Endocarditis can develop quickly over days (acute) or slowly over weeks to months (subacute). The acute form, often caused by Staphylococcus aureus, produces high fevers, rapid heart rate, and can deteriorate into heart failure within days. The subacute form is sneakier: low-grade fevers, night sweats, fatigue, unexplained weight loss, and joint pain that may be mistaken for the flu or other common illnesses.
About half of patients develop at least one of the classic physical signs that point toward endocarditis. These include splinter hemorrhages, which look like thin dark red lines under the fingernails or toenails. Osler nodes are small, painful red-to-purple bumps on the fingertips or toe pads that last hours to days. Janeway lesions are painless flat spots on the palms or soles. Roth spots, visible only during an eye exam, are tiny retinal hemorrhages with pale centers, found in roughly 5% of patients.
A new or changing heart murmur is one of the most important clinical clues. Because vegetations distort valve function, they create turbulent blood flow that produces sounds a doctor can hear with a stethoscope.
How It Is Diagnosed
Diagnosis relies on a combination of blood cultures and heart imaging, organized through a standardized set of criteria originally developed at Duke University and updated most recently in 2023. A definitive diagnosis requires either two major criteria, one major criterion plus three minor criteria, or five minor criteria. Major criteria include positive blood cultures with organisms known to cause endocarditis and imaging evidence of valve involvement.
Two types of ultrasound imaging are used to look at the heart valves. A standard echocardiogram (performed by pressing a probe against the chest wall) detects vegetations in only about 21% of confirmed cases. A transesophageal echocardiogram, where a small probe is guided down the throat to sit just behind the heart, picks up vegetations in about 86% of confirmed cases. Because of this dramatic difference in sensitivity, the throat-based approach is considered essential when endocarditis is strongly suspected but the chest-wall scan looks normal.
Treatment: Weeks of IV Antibiotics
Treating endocarditis requires prolonged intravenous antibiotics, typically lasting two to six weeks depending on the type of bacteria and whether a natural or prosthetic valve is infected. Prosthetic valve infections generally require the longer end of that range. Treatment usually begins in the hospital, where doctors can monitor response and adjust medications based on lab results identifying the exact bacterium and its drug sensitivities.
Infections on natural valves caused by susceptible streptococci can sometimes be treated in as little as two weeks with a combination approach. Staphylococcus aureus infections on natural valves typically need four to six weeks. Prosthetic valve infections are the most complex, often requiring six weeks of antibiotics combined with additional drugs to penetrate the biofilm that forms on artificial materials.
When Surgery Becomes Necessary
Antibiotics alone aren’t always enough. Surgery during the initial hospitalization, before finishing the full antibiotic course, is indicated in several situations: when the infected valve is leaking badly enough to cause heart failure symptoms, when the infection is caused by fungi or highly resistant bacteria, or when fevers and positive blood cultures persist more than five to seven days despite appropriate antibiotics.
Vegetation size also matters. On the left side of the heart, vegetations larger than 10 mm that have already caused embolic events (fragments traveling to other organs) are considered high risk and typically prompt surgical intervention. On the right side, the threshold is higher, around 20 mm, partly because right-sided fragments travel to the lungs rather than the brain. Surgery usually involves repairing or replacing the damaged valve and cleaning out infected tissue.
Complications and Outlook
Embolic complications, where fragments of the vegetation break off and travel through the bloodstream, occur in roughly one-third of patients. The brain is the most dangerous destination: embolic strokes can cause permanent neurological damage. Fragments can also lodge in the spleen, kidneys, or lungs, causing abscesses or infarctions in those organs.
Overall, about 20% of patients die within 30 days, and roughly 30% die within one year. Outcomes are substantially worse for patients who require intensive care, with one-year mortality reaching as high as 67% in that group. Staphylococcus aureus infections, prosthetic valve involvement, and delayed diagnosis all push mortality higher.
Preventing Endocarditis
For people at the highest risk, preventive antibiotics before certain dental procedures can reduce the chance of infection. Current guidelines from the American Heart Association and European Society of Cardiology reserve this precaution for a specific group: people with prosthetic or repaired heart valves, a history of prior endocarditis, or certain congenital heart conditions. For everyone else, the evidence shows no meaningful benefit from preventive antibiotics.
Good oral hygiene is a straightforward and effective prevention strategy for anyone at risk. Brushing, flossing, and regular dental care reduce the load of bacteria in the mouth that could potentially enter the bloodstream. For people with long-term intravenous catheters, careful sterile technique and removing the catheter as soon as it’s no longer needed are important protective steps.