What causes scarlet fever?
Scarlet fever is caused by a tiny germ (bacterium) called Streptococcus pyogenes. The germ is sometimes called 'group A Strep'. This germ causes quite a few illnesses, including skin infections, chest infections and infections of the heart.
Sometimes the germs (bacteria) only cause a sore throat, without causing the rash of scarlet fever. This is often called a 'Strep throat' or simple tonsillitis. But in scarlet fever, the streptococcus bacterium releases toxins that spread through the body. The toxins cause the rash and, if untreated, can cause problems in the kidneys and heart even years later, which you can read more about in the treatment and complications section.
How is scarlet fever diagnosed?
In general the diagnosis can be made on the clinical picture: a child with a high temperature (fever), sore throat, a red tongue and a rough-feeling rash on their chest and tummy. No tests are usually necessary.
If there is any doubt as to the diagnosis, a doctor can take a 'throat swab' - using something that looks like a long cotton bud. They will send it to the hospital to be tested for the germ (bacterium) that causes scarlet fever. But the results will take a few days to come back, so if scarlet fever is suspected it's usually best to start the antibiotics first.
There is a blood test which can detect the scarlet fever germ (the anti-streptolysin titre test, or ASO for short). But the blood test is only positive from between one week and one month after the infection. So it won't tell you if someone has scarlet fever right now, only if they had it in the past.
A family doctor, or GP, will be able to recognise scarlet fever and should not need blood tests or throat swabs to make the diagnosis.
Further reading and references
Scarlet fever: guidance and data; Public Health England
Interim guidelines for the public health management of scarlet fever outbreaks in schools, nurseries and other childcare settings; Public Health England, 2014 (archived content)
Wong S et al; Emerging Microbes and Infections, Nature.com, 2012
Feverish illness in children - Assessment and initial management in children younger than 5 years; NICE Guideline (Updated August 2017)
Scarlet fever; NICE CKS, May 2010
No authors listed; Varicella, herpes zoster and nonsteroidal anti-inflammatory drugs: serious Prescrire Int. 2010 Apr19(106):72-3.
No authors listed; Sixty seconds on . . . scarlet fever. BMJ. 2016 Mar 23352:i1658. doi: 10.1136/bmj.i1658.
My son who will be 3 in April has been on antibiotics (not penicillin as his allergic). He also has asthma. His diagnosis GP- after noticing the rash 2 days prior is German measles. He however has...c30262
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