Causes of Scarlet Fever

Scarlet fever is extremely contagious. It is caused by a specific strain of bacteria Group A beta hemolytic Streptococcus.

How is Scarlet fever spread?

The organism from the infected person spreads by travelling in tiny water droplets in air. These are expelled by the infected person to his or her environment when he or she coughs or sneezes. This may be breathed in by healthy persons.

Streptococcal infection also spreads by touch from skin infections. Sharing utensils, towels, baths, clothes and bed linen all increase the risk of contracting the disease from an ill person. The incubation is typically 2 to 4 days but ranges from 12 hours to 7 days.

Carriers of Scarlet fever

Some persons are termed carriers as they have the bacteria in their throat but do not show any symptoms. In fact the symptoms of Scarlet fever develop in most persons who are susceptible to the toxins released by streptococcal bacteria.

For children over 10 years there is development of immunity against such toxins and the risk of the infection is low. In children below two years the maternal immune cells are still present in blood circulation and this serves to protect against the infection.

Patients are contagious both during the acute illness and the stage before appearance of symptoms. If left untreated, people can spread the bacteria for 10 to 21 days or even for weeks or months. If treated, the ability to spread the illness to others usually ends after 24 hours.

Group A beta hemolytic Streptococcus

Group A beta haemolytic streptococcus are normally found in the throat but do not cause illness. They may lead to Scarlet fever and infection in presence of sore throat (Strep throat), skin infections (Impetigo) and pneumonia.

Scarlet fever is associated with sore throat only rarely. Around 10% of Streptococcal sore throats with Group A beta haemolytic streptococcal infections may go on to cause Scarlet fever.

The strain of Streptococci releases a variety of enzymes and toxins including the erythrogenic toxins. These serve to break down blood cells (hemolysing) and cause the characteristic rash of Scarlet fever.

Predisposing factors to Scarlet fever

Predisposing factors that may cause Scarlet fever include: –

  • AgeScarlet fever is essentially a disease of childhood. Children up to two years are not susceptible and those over 10 are mostly protected. The disease commonly affects children between 4 and 8 years of age.

  • Crowded areas - Neither sex nor race influence the risk of Scarlet fever. Socioeconomic status also does not play a role. However, those in crowded areas are more at risk due to raised risk of exposure to the disease. This includes schools and day cares.

  • Time of year - Autumn and winter are seasons when most cases are seen. Spring and Summer show a relatively fewer number of cases.

  • Open wounds - Open wounds, either accidental or surgical lead to an increased susceptibility to the infection with the bacteria that may lead to Scarlet fever.

Further Reading

Last Updated: Jun 5, 2019

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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