Strep throat is caused by the bacterium Group A Streptococcus (GAS). This common bacterial infection typically presents as a painful inflammation of the throat and tonsils, often accompanied by fever and difficulty swallowing. While many sore throats are caused by viruses and resolve on their own, a GAS infection requires prompt antibiotic treatment. Failure to treat strep throat allows the bacteria to persist, significantly increasing the risk of both local complications and severe, systemic diseases that can cause permanent damage to various organs.
Prolonged Symptoms and Local Complications
When a strep infection is left untreated, the immediate acute symptoms of fever, severe sore throat, and inflamed tonsils will continue beyond the typical duration of a viral illness. The bacteria remain active in the pharynx, which can lead to the infection spreading to adjacent tissues and forming pockets of pus, known as suppurative complications.
One common localized complication is a peritonsillar abscess, often called quinsy, which is a collection of pus that develops between one of the tonsils and the wall of the throat. Symptoms include intense pain on one side, a muffled or “hot potato” voice, and difficulty opening the mouth fully (trismus). The infection can also spread deeper into the tissues at the back of the throat, forming a retropharyngeal abscess, which can threaten the airway. Both complications require urgent medical intervention, often involving surgical drainage in addition to antibiotics.
The Risk of Rheumatic Fever
The most widely known systemic complication of untreated strep throat is Acute Rheumatic Fever (ARF), a condition that develops in susceptible individuals two to four weeks following the initial infection. ARF is not caused by the bacteria themselves but by a misguided immune response known as molecular mimicry. The immune system creates antibodies to fight the GAS bacteria, but certain bacterial proteins share structural similarities with proteins found in human tissues.
The body’s protective antibodies mistakenly attack its own cells, primarily targeting the heart, joints, brain, and skin. Inflammation of the heart, or carditis, is the most serious manifestation, potentially damaging the heart valves and leading to Rheumatic Heart Disease (RHD), a permanent condition that can cause lifelong cardiac issues. The large joints, such as the knees and ankles, may experience migratory polyarthritis, which is painful inflammation that moves from joint to joint.
The brain can also be affected, leading to Sydenham chorea, a neurological disorder characterized by involuntary, jerky movements. Other symptoms can include a distinctive, non-itchy rash called erythema marginatum and small lumps under the skin called subcutaneous nodules. The primary reason for universally treating strep throat with antibiotics is the effective prevention of ARF, as eliminating the bacterial trigger prevents the development of this autoimmune process.
Post-Streptococcal Glomerulonephritis (PSGN)
A separate, non-suppurative complication that affects the kidneys is Post-Streptococcal Glomerulonephritis (PSGN). This condition occurs when immune complexes—clumps of antibodies and bacterial antigens—become trapped in the glomeruli, the microscopic filtering units of the kidneys. This deposition triggers an inflammatory response that impairs the kidneys’ ability to filter waste and excess fluid from the blood.
PSGN typically appears one to two weeks after a strep throat infection or up to six weeks after a streptococcal skin infection, such as impetigo. The resulting inflammation often leads to symptoms like hematuria (blood in the urine) and edema, or swelling, particularly around the face and eyes. High blood pressure (hypertension) is also a common finding due to fluid retention. Unlike ARF, antibiotic treatment of strep throat has not been reliably shown to prevent PSGN, but it is still given to eradicate the organism.
Other Invasive and Suppurative Infections
Uncontrolled GAS infection can lead to the bacteria spreading beyond the throat to normally sterile sites in the body, resulting in life-threatening conditions collectively known as invasive Group A Streptococcal disease (iGAS). These conditions are rare but represent the most acute and severe consequences of the bacteria’s spread.
One such severe infection is necrotizing fasciitis, sometimes mislabeled as a “flesh-eating disease,” which is a rapid and destructive infection of the deep soft tissue. The bacteria can also cause Streptococcal Toxic Shock Syndrome (STSS), a rapidly progressing illness marked by low blood pressure, shock, and multi-organ failure. Mortality rates for STSS are high. Other invasive infections include cellulitis, pneumonia, and sepsis, a widespread bloodstream infection.