Strep throat is an infection caused by the bacterium Streptococcus pyogenes, also known as Group A Streptococcus (GAS). While symptoms like a sudden sore throat, fever, and pain when swallowing may eventually resolve, leaving the infection untreated carries a serious risk of developing secondary, more severe conditions. The primary concern is that the body’s response to the persistent bacteria will trigger dangerous complications in other organ systems. These complications range from localized abscesses near the infection site to systemic, autoimmune diseases affecting the heart and kidneys.
Local Persistence and Direct Spread
Failing to eliminate the GAS bacteria allows the infection to linger and spread to adjacent tissues. Symptoms like fever and throat pain persist longer than the quick resolution seen with antibiotic treatment, creating an opportunity for the bacteria to invade surrounding structures.
One common localized complication is a peritonsillar abscess, often called quinsy. This involves pus forming behind a tonsil, causing severe throat pain, difficulty opening the mouth, and a muffled voice. The infection can also spread directly to the middle ear (otitis media) or into the sinuses (sinusitis).
In more severe instances, the infection can extend deeper into the neck, forming retropharyngeal or parapharyngeal abscesses. These deep neck space infections can pose a threat to the airway or potentially erode into nearby blood vessels. The risk of these localized complications increases significantly when the primary infection is not quickly eradicated.
The Risk of Acute Rheumatic Fever
The most significant consequence of untreated strep throat is Acute Rheumatic Fever (ARF), a delayed, non-suppurative complication. ARF is an autoimmune reaction where the body’s immune system mistakenly attacks its own tissues, triggered by a phenomenon called molecular mimicry.
Molecular mimicry occurs because certain proteins on the surface of S. pyogenes, specifically the M protein, closely resemble proteins found in human connective tissues. Antibodies created to fight the bacteria cross-react with tissues in the heart, joints, skin, and brain. ARF symptoms typically appear two to four weeks after the initial, untreated strep infection.
Cardiac and Joint Effects
When the heart is affected, the condition is known as rheumatic carditis, which can lead to permanent scarring and damage of the heart valves, resulting in Rheumatic Heart Disease (RHD). This valve damage can cause lifelong problems, including heart failure. ARF can also manifest as polyarthritis, presenting as painful, migratory inflammation affecting large joints like the knees, ankles, elbows, and wrists.
Neurological involvement can occur, leading to Sydenham chorea, which causes involuntary, jerky movements of the face and hands. Although rare in developed nations, ARF remains a serious global public health concern. The potential for irreversible heart damage is the main driver for mandatory antibiotic treatment.
Potential for Post-Streptococcal Glomerulonephritis
Post-Streptococcal Glomerulonephritis (PSGN) is another serious, delayed autoimmune complication affecting the kidneys. PSGN is an inflammatory condition of the glomeruli, the tiny filtering units within the kidneys. Like ARF, this condition is an immune response that follows an untreated strep infection, typically appearing one to two weeks later.
The mechanism involves the formation of immune complexes—clumps of strep antigens and antibodies—which are deposited in the glomeruli. These complexes trigger an inflammatory response that damages the filtering structures. This inflammation impairs the kidney’s ability to filter waste products and excess fluid from the blood.
Symptoms of PSGN include hematuria (blood in the urine, often dark or rusty) and edema (swelling), particularly around the eyes and face, due to fluid retention. High blood pressure is also common, resulting from impaired kidney function.
While most cases of PSGN in children resolve completely, some adults may face a poor prognosis, potentially leading to chronic kidney disease or end-stage renal disease.
Preventing Complications Through Antibiotic Use
The primary goal of treating strep throat with antibiotics is to rapidly eliminate the S. pyogenes bacteria, preventing the onset of serious complications like ARF and PSGN. Antibiotic administration, typically penicillin or amoxicillin, within the first nine days of illness onset is highly effective at neutralizing this risk. This treatment window interrupts the chain of events that leads to the autoimmune reaction.
Antibiotics also shorten the duration of symptoms and reduce the likelihood of spreading the infection to others. A person is considered non-contagious after approximately 24 hours of starting the correct medication. Crucially, the entire prescribed course of antibiotics must be completed, even if symptoms improve quickly.
Stopping the medication prematurely risks allowing residual bacteria to survive, which can lead to a relapse or still trigger delayed autoimmune responses. The full course ensures complete bacterial eradication, closing the window of vulnerability for developing long-term heart or kidney damage.