Rheumatic heart disease

“Rheumatism” is used to describe inflammation in the joints, muscles, and the fibrous tissue, so rheumatic fever is a type of inflammatory disease that can damage the heart tissue, and lead to rheumatic heart disease.

Rheumatic fever develops after streptococcal pharyngitis, inflammation of the throat due to Streptococcus pyogenes where pyogenes literally means “makes pus”. The bacteria is sometimes referred to as “Group A beta hemolytic” streptococcus, and the infection itself is most often just called Strep throat. This particular group of streptococcus has an antigen that lumps it into a group called “group A”, and it also produces an enzyme called streptolysin, that completely lyses nearby red blood cells, or causes them to rupture—rupturing red blood cells is called hemolysis, right? And when those red blood cells rupture and are destroyed, it’s called beta-hemolysis—as opposed to alpha-hemolysis, where cells aren’t actually destroyed, they’re just damaged or bruised.

Some of these strep bacteria have a protein on their cell wall called “M protein”, and this particular protein is highly antigenic, meaning that the immune system sees it and recognizes it as a foreign molecule, and mounts an immune response, which rightfully so, produces antibodies against these proteins. Those antibodies, though, are thought to cross-react with proteins on some of our body’s own cells, like cells in the myocardium (or heart muscle) and heart valves, but also cells in the joints, the skin and the brain.

This phenomenon, where antibodies accidentally target proteins on our own cells because they look like the proteins on foreign cells, is called molecular mimicry, and is an example of what’s called a type 2 hypersensitivity reaction. Once bound to cardiac tissue, the antibodies activate nearby immune cells, which causes a cytokine-mediated inflammatory response and tissue destruction.

Obviously though, not everyone that gets strep throat gets rheumatic fever, right? And it’s actually only a small minority that get it, estimated around 3%, and it’s more likely to happen in children or people in areas of poverty and crowding.