Viral Exanthemas

An “exanthem” is an eruptive widespread rash. Originally there were six “viral exanthemas” known as first, second, third, fourth, fifth and sixth disease. These have since been renamed as we have learned more about their underlying causes:

  • First disease: Measles
  • Second disease: Scarlet Fever
  • Third disease: Rubella (AKA German Measles)
  • Fourth disease: Dukes’ Disease
  • Fifth disease: Parvovirus B19 
  • Sixth disease: Roseola Infantum



Measles is caused by the measles virus. It is highly contagious via respiratory droplets. Symptoms start 10 – 12 days after exposure, with fevercoryzal symptoms and conjunctivitis.

Koplik spots are greyish white spots on the buccal mucosa. They appear 2 days after the fever. They are pathognomonic for measles, meaning if a patient has Koplik spots, you can diagnose measles.

The rash starts on the face, classically behind the ears, 3 – 5 days after the fever. It then spreads to the rest of the body. The rash is an erythematous, macular rash with flat lesions.

Measles is self resolving after 7 – 10 days of symptoms. Children should be isolated until 4 days after their symptoms resolve. Measles is a notifiable disease and all cases need to be reported to public health. 30% of patients with measles develop a complication.

Complications include:

  • Pneumonia
  • Diarrhoea
  • Dehydration
  • Encephalitis
  • Meningitis
  • Hearing loss
  • Vision loss
  • Death


Scarlet Fever

Scarlet fever is associated with group A streptococcus infection, usually tonsillitis. It is not caused by a virus.

Scarlet fever is caused by an exotoxin produced by the streptococcus pyogenes (group A strep) bacteria. It is characterised by a red-pink, blotchy, macular rash with rough “sandpaper” skin that starts on the trunk and spreads outwards. Patients can have red, flushed cheeks.

Other features:

  • Fever
  • Lethargy
  • Flushed face
  • Sore throat
  • Strawberry tongue
  • Cervical lymphadenopathy

Treatment is with antibiotics for the underlying streptococcal bacterial infection. This is with phenoxymethylpenicillin (penicillin V) for 10 days. Scarlet fever is a notifiable disease and all cases need to be reported to public health. Children should be kept off school until 24 hours after starting antibiotics.

Patients can have other conditions associated with group A strep infection:

  • Post-streptococcal glomerulonephritis
  • Acute rheumatic fever



Rubella is caused by the rubella virus. It is highly contagious and spread by respiratory droplets. Symptoms start 2 weeks after exposure.

It presents with a milder erythematous macular rash compared with measles. The rash starts on the face and spreads to the rest of the body. The rash classically lasts 3 days. It can be associated with a mild fever, joint pain and a sore throat. Patients often have enlarged lymph nodes (lymphadenopathy) behind the ears and at the back of the neck.

Management is supportive and the condition is self limiting. Rubella is a notifiable disease and all cases need to be reported to public health. Children should stay off school for at least 5 days after the rash appears. Children should avoid pregnant women.

Complications are rare but include thrombocytopenia and encephalitis. Rubella is dangerous in pregnancy and can lead to congenital rubella syndrome, which is a triad of deafness, blindness and congenital heart disease.


Duke’s Disease

Duke’s disease, also known as fourth disease, has very interestingly been mostly forgotten and is never used in clinical practice. Even when it was first described as a similar disease to rubella, there was disagreement about whether it actually existed or whether doctors were simply misdiagnosing a different illness. Since viral and bacterial testing has become possible, no organism has been found that could explain a specific “fourth disease”.

It is very common for children to get non-specificviral rashes”. It is likely that “fourth disease” was used to describe these non-specific viral rashes that are now understood to be caused by many potential viruses.


Parvovirus B19

Parvovirus B19 is also known as fifth diseaseslapped cheek syndrome and erythema infectiosum. It is caused by the parvovirus B19 virus.

Parvovirus infection starts with mild fever, coryza and non-specific viral symptoms such as muscle aches and lethargy. After 2 – 5 days the rash appears quite rapidly as a diffuse bright red rash on both cheeks, as though they have “slapped cheeks”. A few days later a reticular mildly erythematous rash affecting the trunk and limbs appears that can be raised and itchy. Reticular means net-like.

The illness is self limiting and the rash and symptoms usually fade over 1 – 2 weeks. Healthy children and adults have a low risk of any complications and are managed supportively with plenty of fluids and simple analgesia. It is infectious prior to the rash forming, but once the rash has formed they are no longer infectious and do not need to stay off school.

Patients that are at risk of complications include immunocompromised patients, pregnant women and patients with haematological conditions such as sickle cell anaemia, thalassaemia, hereditary spherocytosis and haemolytic anaemia. These patients require serology testing for parvovirus to confirm the diagnosis and checking of the full blood count and reticulocyte count for aplastic anaemia. People that would be at risk of complications that have come in contact with someone with parvovirus prior to the rash forming, should be informed and may need investigations.


  • Aplastic anaemia
  • Encephalitis or meningitis
  • Pregnancy complications including fetal death
  • Rarely hepatitis, myocarditis or nephritis


Roseola Infantum

Roseola infantum is also known as just roseola or sixth disease. This is caused by human herpesvirus 6 (HHV-6) and less frequently by human herpesvirus 7 (HHV-7).

Roseola has a typical pattern of illness. It presents 1 – 2 weeks after infection with a high fever (up to 40ºC) that comes on suddenly, lasts for 3 – 5 days and then disappears suddenly. There may be coryzal symptoms, sore throat and swollen lymph nodes during the illness. When the fever settles, the rash appears for 1 – 2 days. The rash consists of a mild erythematous macular rash across the arms, legs, trunk and face and is not itchy.

Children make a full recovery within a week and do not generally need to be kept off nursery if they are well enough to attend.

The main complication to be aware of is febrile convulsions due to high temperature. Immunocompromised patients may be at risk of rare complications such as myocarditis, thrombocytopenia and Guillain-Barre syndrome.


Last updated January 2020
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