Rheumatic heart disease is caused by the body’s own immune system attacking heart tissue after a throat infection with group A Streptococcus bacteria. It doesn’t happen during the infection itself. Instead, the damage unfolds over weeks and sometimes years, as the immune response triggered by the bacteria mistakenly targets proteins in the heart that resemble proteins on the bacterial surface. Around 55 million people worldwide live with rheumatic heart disease, and it kills roughly 360,000 each year.
It Starts With Strep Throat
The chain of events begins with an ordinary strep throat, the kind millions of people get every year. When group A Streptococcus bacteria infect the throat, the immune system mounts a defense, producing antibodies and activating immune cells to clear the infection. In most cases, a course of antibiotics wipes out the bacteria before anything else happens. The problem arises when strep throat goes untreated or undertreated.
About one to five weeks after an untreated strep infection, some people develop acute rheumatic fever. This is the inflammatory condition that sits between the original infection and long-term heart damage. Not everyone with untreated strep throat gets rheumatic fever, and not everyone with rheumatic fever develops heart disease. But rheumatic fever is the only path to rheumatic heart disease, which makes that initial strep infection the true starting point.
Molecular Mimicry: Why the Immune System Attacks the Heart
The core mechanism behind rheumatic heart disease is a case of mistaken identity at the molecular level. The group A Streptococcus bacterium carries a surface protein called M protein. Parts of this protein are structurally similar to proteins found in human heart tissue, particularly a protein in heart muscle called cardiac myosin. When the immune system builds antibodies to fight the M protein, those same antibodies can latch onto heart tissue because the two proteins look alike.
This phenomenon is called molecular mimicry. The immune system essentially can’t tell the difference between the invader and the body’s own cells. Antibodies originally aimed at the bacteria bind to heart valve tissue instead. At the same time, immune cells called T lymphocytes infiltrate the heart valves, causing inflammation, swelling, and progressive damage. Researchers have confirmed that patients with rheumatic fever carry antibodies that react to both streptococcal M protein and human cardiac myosin, providing direct evidence of this cross-reaction.
What Rheumatic Fever Does to the Body
Rheumatic fever produces several distinct symptoms, but heart inflammation is the one with lasting consequences. More than half of people who develop rheumatic fever experience some degree of carditis, meaning inflammation of the heart. The other hallmarks of rheumatic fever, including joint pain, involuntary movements (caused by antibodies binding to structures deep in the brain), a distinctive skin rash, and small nodules under the skin, all resolve completely within weeks. Heart damage does not.
Doctors identify rheumatic fever using a set of clinical criteria. A diagnosis typically requires either two major signs (such as carditis and arthritis together) or one major sign plus two minor ones (such as fever and elevated markers of inflammation). In regions where strep is common, the diagnostic criteria are slightly broader, allowing a single swollen joint to count as a major sign rather than requiring multiple joints to be affected. This recognizes that the disease can look different in populations with higher rates of strep exposure.
How Heart Valves Become Permanently Damaged
The mitral valve, which controls blood flow between the left upper and lower chambers of the heart, is affected in nearly all cases of rheumatic heart disease. In the early stages, the inflammation causes the valve to leak, allowing blood to flow backward. Over time, repeated bouts of inflammation lead to scarring, thickening, and stiffening of the valve tissue. In later stages, the valve can narrow so severely that blood struggles to pass through at all.
The structural changes are progressive and cumulative. The thin cords that anchor the valve leaflets (called chordae tendineae) fuse together and shorten. In 90% of patients with significant valve narrowing, this cord shortening is a dominant feature. The edges of the valve leaflets themselves can fuse at their seams, creating a rigid, funnel-shaped opening instead of a flexible, two-leafed gate. Patients under 30 often still have relatively pliable valve tissue, but by age 40 and beyond, two-thirds have scarred, rigid valves.
The aortic valve, which sits between the left ventricle and the main artery leaving the heart, can also be affected, though it more commonly develops calcium deposits as the damage progresses. Each new episode of rheumatic fever layers additional scarring onto already-damaged valves, which is why preventing repeat infections is so critical.
Why Some People Are More Vulnerable
Only a fraction of people with untreated strep throat ever develop rheumatic fever, and genetics play a role in determining who is susceptible. Specific variations in genes that govern how the immune system recognizes threats appear to raise or lower risk. One study found that people carrying a particular immune gene variant (HLA-DRB1*07) were nearly three times more likely to develop rheumatic heart disease compared to people without it. Conversely, another variant (HLA-DRB1*11) appeared protective, cutting the risk roughly in half. These genetic differences help explain why rheumatic fever can cluster in families even when everyone in a household is exposed to the same strep bacteria.
Genetics alone don’t tell the whole story. Living conditions matter enormously. Overcrowding makes strep throat spread faster and recur more often. Limited access to healthcare means infections go untreated. Low income, rural living, and lack of education about symptoms all contribute to delayed treatment. This is why rheumatic heart disease has largely disappeared from wealthy countries but remains a major killer in sub-Saharan Africa, South Asia, the Middle East, and Pacific Island nations. It also persists among Indigenous populations in wealthier countries, where healthcare access gaps mirror conditions in lower-income settings.
Single Episode vs. Repeated Damage
Rheumatic heart disease can result from a single severe episode of rheumatic fever, but it more commonly develops after multiple episodes. Each time strep throat recurs and triggers another round of rheumatic fever, the immune system attacks the heart valves again. Scar tissue builds on scar tissue. Valves that leaked slightly after the first episode may narrow significantly after the second or third. This is why people who have had one episode of rheumatic fever face a critical window of ongoing risk. Without preventive measures, the odds of another strep infection triggering another immune attack remain high, especially in environments where strep circulates freely.
How Prevention Breaks the Chain
Because the entire disease process depends on that initial strep throat going untreated, the most effective form of prevention is straightforward: treating strep throat with antibiotics before the immune system has time to develop a cross-reactive response. A standard course of penicillin or amoxicillin, started within the first week or so of symptoms, can prevent rheumatic fever from ever developing.
For people who have already had rheumatic fever, the priority shifts to preventing future strep infections from triggering additional heart damage. Since 1955, the standard approach has been regular penicillin injections given every four weeks, often continued for years or even decades depending on the severity of heart involvement. This ongoing prevention strategy is one of the most effective interventions in global health, yet its reach remains limited in the regions that need it most. Newer research is exploring longer-acting formulations that could be given every three months instead of monthly, which could improve access in remote areas where regular clinic visits are difficult.
The fundamental challenge of rheumatic heart disease is not medical complexity. The bacteria that start the process are common and treatable. The real barrier is ensuring that every child with a sore throat in a high-risk community gets a simple course of antibiotics before an avoidable autoimmune cascade permanently reshapes their heart.