Scarlet fever is a bacterial infection that produces a toxin causing a distinctive red, sandpaper-textured rash across the body, along with high fever, sore throat, and a swollen tongue. It’s caused by group A streptococcus, the same bacteria behind strep throat, but scarlet fever develops when the strain produces a specific toxin that triggers widespread skin inflammation. Most cases resolve fully with antibiotics, but untreated infections can lead to serious complications affecting the heart and kidneys.
How the Infection Causes the Rash
Scarlet fever starts as a strep throat infection. What sets it apart is that the particular strain of bacteria releases what’s called an erythrogenic toxin into the bloodstream. This toxin triggers a chain reaction in the skin: it causes local inflammation, dilates blood vessels, and shifts the immune response in the skin itself. The result is the characteristic scarlet color that gives the disease its name.
The rash isn’t the bacteria spreading across the skin. It’s the body’s inflammatory response to a toxin circulating through the blood. That’s why the rash appears over large areas of the body rather than just at the site of infection in the throat.
What Scarlet Fever Feels Like
The illness typically begins with a sudden sore throat and fever, often above 101°F. Within a day or two, the rash appears. It usually starts on the neck and chest before spreading to the arms, legs, and trunk. The texture is distinctive: it feels like fine sandpaper when you run your hand across it. The skin looks flushed and red, and pressing on it briefly turns it white before the color returns.
In skin folds like the armpits, elbows, and groin, the rash can concentrate into deeper red lines called Pastia’s lines. These streaks form because the thin, creased skin in those areas traps more of the inflammatory response.
The tongue goes through its own progression. Early on, it may develop a white coating with red, swollen bumps poking through, sometimes called “white strawberry tongue.” Within a few days, the white coating peels away, leaving the tongue bright red and bumpy. Other common symptoms include headache, body aches, nausea, and swollen glands in the neck.
How Long It Lasts
The rash typically persists for about one week. After it fades, the skin often peels, particularly on the fingertips, toes, and groin. This peeling phase, called desquamation, can last for several weeks and is a normal part of recovery, not a sign that the infection is worsening. It happens because the inflammatory process damaged the outer layer of skin, which then sheds as new skin grows in underneath.
With antibiotic treatment, fever usually breaks within 24 to 48 hours, and most children feel significantly better within a few days. The full course of antibiotics runs 10 days regardless of how quickly symptoms improve.
How It’s Diagnosed
Doctors diagnose scarlet fever based on the combination of the rash and a confirmed strep infection. A rapid strep test can return results in minutes, but its sensitivity varies widely, catching between 58% and 96% of true infections depending on the test and technique used. Its specificity is high, around 97%, meaning a positive result is very reliable. When the rapid test comes back negative but the doctor still suspects strep, a throat culture provides a definitive answer, though results take one to two days.
Why Treatment Matters
The standard treatment is a 10-day course of penicillin or amoxicillin. These antibiotics are highly effective against group A strep and remain the first-line choice. For people with penicillin allergies, alternatives are available, though it’s worth noting that about one in three invasive group A strep infections are now caused by bacteria resistant to erythromycin and clindamycin, two common backup antibiotics.
Completing the full antibiotic course does more than clear the immediate infection. It significantly reduces the risk of rheumatic fever, a serious inflammatory condition that can develop weeks after a strep infection and permanently damage the heart valves. This is the main reason doctors emphasize finishing all 10 days of medication even after symptoms resolve.
Complications of Untreated Scarlet Fever
Left untreated, scarlet fever can trigger two major delayed complications. Rheumatic fever develops when the immune system, still activated from fighting the strep bacteria, mistakenly attacks the body’s own tissues, particularly the heart. This can cause lasting heart valve damage that may require lifelong management or surgery.
The second major risk is post-streptococcal glomerulonephritis, a condition where the tiny blood vessels in the kidneys’ filtering units become inflamed. This makes the kidneys less able to filter blood properly. Symptoms include dark or bloody urine, swelling in the face and ankles, and reduced urine output. Most children recover fully from this kidney inflammation, but in some cases it can progress to chronic kidney disease, kidney failure, heart failure from fluid overload, or dangerously high blood pressure.
These complications are uncommon with proper antibiotic treatment, which is why prompt diagnosis matters.
Who Gets Scarlet Fever Now
Scarlet fever primarily affects children between ages 5 and 15, though adults can get it too. It spreads through respiratory droplets from coughing and sneezing, and through direct contact with an infected person’s saliva or nasal secretions.
Rates of serious group A strep disease have been climbing since 2014, and preliminary 2023 data from the CDC show that serious infections reached a 20-year high. The increase has been largest among adults aged 18 through 64, and the number of both minor and serious group A strep infections during peak season now exceeds pre-pandemic levels. While scarlet fever itself remains a milder form of group A strep disease, the rising prevalence of the bacteria means more potential exposures, particularly during winter and early spring when strep throat circulates most actively.