Strep throat is caused by an infection with Streptococcus pyogenes, commonly known as Group A Streptococcus (GAS). Unlike many viral sore throats, an infection with this bacterium carries a risk of severe consequences if left untreated by antibiotics. The most serious long-term damage is triggered by the body’s immune response to the bacteria, not the bacteria itself, necessitating prompt medical attention. Failure to eradicate the bacteria allows the infection to progress, leading to dangerous localized issues and systemic complications.
Acute Progression of Untreated Strep
When Group A Streptococcus infection is not cleared, the initial symptoms of fever and sore throat can worsen rapidly. The infection can spread beyond the tonsils into the surrounding tissues, leading to a localized collection of pus. This is known as a peritonsillar abscess, or quinsy, which typically forms between the tonsillar capsule and the pharyngeal muscles.
The formation of this abscess causes a significant escalation of symptoms, including severe, unilateral throat pain and difficulty opening the mouth (trismus). Swallowing becomes extremely painful, which can lead to drooling and dehydration. As the swelling and pus pocket grow, the voice often takes on a muffled quality, sometimes described as a “hot potato” voice.
A peritonsillar abscess is considered a medical emergency due to the risk of airway obstruction. The expanding pus pocket can push the soft palate and uvula to the side, narrowing the airway. If the abscess ruptures, the pus can be aspirated into the lungs, potentially causing pneumonia or a lung abscess. In severe cases, the infection can extend deeper into the neck, potentially affecting large blood vessels or causing life-threatening conditions like sepsis.
Risk of Rheumatic Fever
The primary systemic consequence of untreated strep is Acute Rheumatic Fever (ARF), an inflammatory condition that can affect the heart, joints, brain, and skin. ARF is not caused by the bacteria directly infecting these organs, but by a misguided immune response that develops approximately two to four weeks after the initial throat infection. This reaction occurs because the antibodies created by the immune system to fight the strep bacteria mistakenly recognize and attack the body’s own tissues, a process called molecular mimicry.
The most damaging manifestation is carditis, or inflammation of the heart tissue, which occurs in about half of ARF cases. This inflammation most often targets the heart valves, particularly the mitral and aortic valves, leading to scarring and damage that may not become fully apparent for years. Rheumatic Heart Disease (RHD) can cause permanent valve dysfunction, heart failure, and increase the risk of other cardiac issues.
ARF also commonly causes a migratory polyarthritis, which presents as painful, swollen, and red joints that affect the large joints like the knees, ankles, and elbows. The inflammation typically moves from one joint to another over a period of days, but this joint pain usually resolves without any lasting damage. In a smaller number of cases, the central nervous system is affected, resulting in Sydenham chorea, which involves involuntary, purposeless, and erratic movements of the limbs and face. This neurological complication is caused by the autoimmune attack on neurons in the brain’s basal ganglia.
Post-streptococcal Glomerulonephritis
Post-streptococcal Glomerulonephritis (PSGN) is a systemic complication that can arise from an untreated strep infection, involving damage to the kidneys. Similar to Rheumatic Fever, PSGN is an immune-mediated condition that arises after the initial infection, typically developing about 10 days to three weeks following the sore throat. PSGN is caused by specific “nephritogenic” strains of Group A Streptococcus, which are different from those that typically trigger Rheumatic Fever.
The underlying mechanism involves immune complexes—clumps of strep antigens and antibodies—circulating in the bloodstream and becoming trapped in the glomeruli, the delicate filtering units of the kidneys. This deposition of immune complexes causes inflammation and damage, which impairs the kidneys’ ability to filter waste and excess fluid. The resulting symptoms frequently include dark or rust-colored urine and generalized swelling, or edema.
Edema is noticeable around the face and eyes, as well as in the hands and feet. High blood pressure is also a common finding because the compromised kidneys struggle to regulate fluid balance. Although most individuals, particularly children, recover fully from PSGN within a few weeks, the condition can rarely lead to long-term kidney damage, chronic kidney disease, or even acute renal failure.
The Window for Preventing Complications
Preventing the most severe systemic complications, particularly Rheumatic Fever, is the primary reason for treating strep throat with antibiotics. The risk of developing ARF is significantly reduced if the infection is treated promptly. For the antibiotic treatment to be most effective at preventing ARF, it must be initiated within nine days of the onset of symptoms.
This timeline underscores that the goal of treatment is not simply to shorten the duration of the sore throat or provide symptom relief, but to eliminate the bacteria before the autoimmune process can begin. Even if the acute symptoms appear to be improving on their own, the bacteria may still be present and capable of triggering the dangerous immune response. Therefore, completing a full course of antibiotics, such as a 10-day regimen of penicillin, is necessary to fully eradicate the organism.
Unlike Rheumatic Fever, antibiotic treatment of the initial strep throat infection has not been shown to prevent the development of Post-streptococcal Glomerulonephritis. Nonetheless, treating the infection remains a medical necessity to prevent the life-threatening progression of localized infection and the development of the most common long-term complication, Rheumatic Fever.