Scarlet Fever - Causes and diagnosis

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.

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Dr Oliver Starr
Peer Reviewer:
Dr Laurence Knott
Document ID:
4533 (v44)
Last Checked:
03 May 2017
Next Review:
02 May 2020

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.