Diagnosis of Scarlet Fever

Scarlet fever, unlike a century ago, has reduced in its severity and intensity. In the 1800’s for example there were severe epidemics of this disease across Europe and worldwide killing many children. These days with the advent of antibiotics deaths are rare.

Diagnosis is usually made by clinical features of fever and a typical rash. The incubation period is usually 2 to 4 days after the infection is acquired. The condition usually begins suddenly with fever.

The rash follows 12-48 hours after the fever. Scarlet fever may begin with a general feeling of being unwell with sore throat, fever, headache, muscle pain, abdominal pain etc. The rash appears on second day of illness.

Diagnosing Scarlet fever based on the rash

The rash has typical features that the physician uses to diagnose the condition. It begins at the neck and spreads to the arms, legs, chest and groin.

The rash appears like a fine pink layer over the skin usually sparing the face. It is punctuate - pin-point dark red spots over a reddish discoloration of skin.

The face appears red and flushed with a pale area around the mouth called “circumoral pallor”. This effect is more prominent than in other fevers.

The rash remains for several days but after a few days it gains prominence in the skin folds due to breakage of the capillary or small skin blood vessels. This is seen in the armpits and groin and is called Pastia's sign and the lines known as “Pastia's lines”.

The skin begins to peel after the fever reduces and this may last for several weeks. This commonly affects the soles and palms and tips of fingers, toes, ears etc.

Throat examination

The physician examines the throat of the patient. There may be small red blotchy spots over the roof of the mouth (palate). These are usually bleeding spots rather than rash.

The tonsils along with neck lymph nodes are swollen. The tonsils appear red and may be covered with a pale whitish discharge or layer. The neck glands are usually swollen and may be painful to touch.

Examination of the tongue

The tongue for the first 2 days has a white coating and is called the “white strawberry tongue”. There are prominent red papillae underneath this “white fur”.

After the initial 2 days the white fur sloughs off. This is called desquamation. The tongue appears raw and red and has prominent papillae. This is called the “raspberry tongue” or “red strawberry tongue”.

Confirming the diagnosis of Scarlet fever

For confirmation of diagnosis a sample of saliva or a throat swab is taken from the back of the throat. The secretions on the cotton tipped swab is then wiped onto a sterile, dry glass slide. The slide is stained with appropriate dyes and stains and examined under the microscope.

Sometimes the sample is incubated at favourable temperatures, humidity and nutrient medium in a petri dish. This allows selective growth of the infecting bacteria. This is called a culture study.

The bacteria grows selectively as colonies on the nutrient media. They are then stained on glass slides and examined under the microscope. These bacteria are also tested with a wide range of antibiotics to see which ones they are susceptible too. This is called a culture and sensitivity test.

Ruling out alternative diagnoses

Other viral fevers must be ruled out. These include:

  • Measles

  • fifth disease

  • rubella (German measles)

  • Infectious mononucleosis

  • Toxic shock syndrome

  • Kawasaki disease

  • Staphylococcal scalded skin syndrome

  • drug and plant allergies

  • Toxic epidermal necrolysis

  • severe sunburn etc.

Rapid antigen tests for Scarlet fever

At present there are detection kits that can diagnose a Streptococcal throat infection rapidly. These are called Rapid antigen tests (RATs).

Blood test for Scarlet fever

A full blood count is advised in patients with Scarlet fever. There may be rise in lymphocyte (type of white blood cells) counts and in the second week eosinophils (another type of white blood cells) may rise in numbers.

Further Reading

Last Updated: Jun 11, 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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