Conditions: Strep Throat (Group A Streptococcal Pharyngitis)

Strep throat is a common bacterial infection of the throat and tonsils caused by group A Streptococcus. It accounts for a minority of sore throats, since most are caused by viruses, but it matters because, if untreated, it can occasionally lead to more serious problems.

What Strep Throat Is and the Bacterium Behind It

Strep throat is an acute infection of the pharynx (the back of the throat) and the tonsils. It is caused by a bacterium called Streptococcus pyogenes, also known as group A Streptococcus or "group A strep." Worldwide, this organism causes an estimated 600 million or more cases of bacterial sore throat each year.

Although sore throats are extremely common, group A strep is responsible for only a portion of them: roughly 20% to 40% of sore throats in children and about 5% to 15% in adults. The great majority of the remainder are caused by viruses, such as those behind the common cold and influenza. Other causes of sore throat are described on the throat infection (pharyngitis) page.

How It Spreads

Group A strep spreads from person to person mainly through respiratory droplets produced by coughing and sneezing, and through contact with respiratory secretions. It can also spread by sharing items such as cups, utensils, plates, and food with someone who is infected. Less commonly, contaminated surfaces and, rarely, improperly handled food have been linked to outbreaks.

Because transmission depends on close contact, outbreaks tend to occur in households, schools, child care centers, boarding schools, military settings, and contact sports such as wrestling. A person is most contagious during the acute illness; when the infection is untreated, contagiousness gradually fades over several weeks.

Who Gets It: Ages, Seasons, and Carriers

Strep throat can occur at any age but is most common in school-aged children and adolescents, classically between ages 5 and 15, with a peak around 7 to 8 years. It is uncommon before age 3, and true strep throat and its complications are rare in that youngest group. In temperate climates, cases cluster in late fall, winter, and early spring, largely reflecting close indoor contact during the school year.

An important wrinkle is the "carrier" state. A substantial number of healthy children carry group A strep in the throat without being sick; carriage rates can reach as high as about a quarter of children during the strep season or school outbreaks. Carriers generally do not spread the bacteria efficiently and are at low risk of complications, but they can cause confusion because a throat test may come back positive even when a virus is actually causing the current sore throat.

Typical Symptoms and How They Differ from Viral Sore Throats

Strep throat characteristically begins suddenly with a sore throat and pain on swallowing, along with fever. Common findings include red and swollen tonsils, white or yellow patches of pus on the tonsils, tiny red spots on the roof of the mouth (palatal petechiae), and tender, enlarged lymph nodes in the front of the neck. Headache, stomachache, nausea, and vomiting can also occur, particularly in children. A "strawberry tongue" may be seen.

A useful distinction is what strep throat usually lacks. Symptoms that point toward a viral cause rather than strep include cough, runny nose and congestion, hoarseness, conjunctivitis (pink eye), mouth ulcers, and diarrhea. That said, no single symptom reliably separates the two, and there is real overlap, which is why testing is often used to confirm the diagnosis rather than relying on appearance alone.

How It Is Diagnosed

Because judgment based on symptoms alone is unreliable and testing everyone is wasteful, clinicians often start with a clinical scoring system. Tools such as the Centor criteria tally features like fever, tonsillar swelling or exudate, tender neck lymph nodes, and the absence of cough. Their main value is identifying people at low risk who do not need testing at all.

For those with a higher likelihood of strep, laboratory testing confirms the diagnosis. A rapid antigen detection test can give a result within minutes in the office and is quite specific. Because a rapid test can miss some true infections, particularly in children, a negative result is often confirmed with a throat culture, which is more sensitive but takes 24 to 48 hours. Newer molecular (PCR-based) tests are highly sensitive and may not require culture backup. Because these tests cannot distinguish an active infection from harmless carriage, testing is generally avoided in people whose symptoms strongly suggest a virus. Strep throat is treated with antibiotics, which shorten symptoms, reduce spread, and lower the risk of certain complications.

Possible Complications

Most episodes of strep throat are mild and resolve within about a week, but complications can occur. "Suppurative" (pus-forming) complications arise when the infection spreads to nearby tissues and include peritonsillar abscess (a collection of pus beside the tonsil, sometimes called quinsy), retropharyngeal abscess, ear and sinus infections, and infected neck lymph nodes. A peritonsillar abscess typically causes worsening one-sided throat pain, difficulty swallowing, drooling, and a muffled voice.

Group A strep can also trigger "non-suppurative," immune-mediated complications that appear after the throat infection has resolved. Acute rheumatic fever is an inflammatory illness that can cause arthritis, carditis (heart inflammation), and neurologic effects, and over time can lead to permanent heart valve damage (rheumatic heart disease); it is now rare in high-income countries but remains a major problem in many parts of the world. Post-streptococcal glomerulonephritis is a kidney condition that can follow a throat or skin infection and may cause blood in the urine, swelling, and high blood pressure. Scarlet fever is not a separate disease but strep throat accompanied by a characteristic fine, red, sandpaper-like rash produced by a toxin the bacteria make.

PANDAS (pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections) refers to a proposed condition in which strep infection triggers the sudden onset of obsessive-compulsive symptoms or tics in children, thought to involve the immune system affecting a part of the brain called the basal ganglia. PANDAS remains scientifically controversial: experts continue to debate whether and how group A strep causes these neuropsychiatric symptoms, and it is not as firmly established as the other complications described above.