Scarlet fever infection typically results in a rash (exanthema) characteristic of the disease. It usually takes about 48 hours after the onset of the disease for the rash to appear. These are small, pinhead-sized, “nodular-stained” red spots that stand out slightly from the skin surface. They are found mainly on the face, trunk, arms and legs, and especially on the groin.
What does the rash look like?
If one strokes over the exanthema, a white stripe (demographism albus) forms for a short time. Also typical are a reddening of the cheeks with paleness in the mouth area and an extreme red colouring of the tongue, also called raspberry or strawberry tongue. Smallest bleedings into the uppermost skin layers can also occur, they are called petechiae, but are not specific for scarlet fever. The rash typically does not itch and usually fades after 4-7 days. After a few weeks, a characteristic scaling of the skin occurs in the affected areas.
The characteristic rash must be associated with scarlet fever. The infection with certain bacteria, so-called beta-hemolytic streptococci, is the cause of the development of the rash. The bacteria are transmitted by so-called droplet infections.
This means that coughing or sneezing of an infected person easily leads to infection of the contact persons. Sometimes an infection can also occur through contact with infected objects (smear infection). The toxins that the bacteria produce in the body (bacterial exotoxins or superantigens) are responsible for the symptoms.
These toxins are recognized by the immune system and the rash appears as an external sign of activation of the immune system. As there are many subgroups of these toxins, a new outbreak of the disease with a rash can occur even after an infection has occurred. This toxin stimulates the body’s own defence cells and thus leads to an increased release of cytokines. The so-called cytokines have an effect on vessel walls, among other things, and lead to an increase in permeability there. As a result, erythrocytes can escape from the vessels and cause the visible reddish rash of the skin.
Scarlet fever is a so-called gaze diagnosis. The typical symptoms, including in particular the rash, point the way to the diagnosis of the infection. If the diagnosis is uncertain, if the symptoms are not particularly typical, an antigen detection can be performed.
If the result is positive, the diagnosis can be confirmed by a rapid test. Bacterial cultures can also be prepared, but these take some time to establish the diagnosis. The rumble-feed test is another diagnostic procedure used to detect scarlet fever.
In this test, blood is stored in the arm with the help of an inflated blood pressure cuff. In the case of an existing infection with the toxin-producing pathogen, the increased blood pressure in the arm, under the condition of increased permeability of the vessels, leads to small bleedings in the skin. These can be indicative for the diagnosis.