Erythema Infectiosum
On this page
Direct answer
Erythema infectiosum, or fifth disease, is a mild childhood rash caused by human parvovirus B19 and recognised by the slapped cheek appearance on the face. A lacy, reticular rash on the limbs and trunk follows, and the child is usually well apart from the rash. The important exceptions are aplastic crisis in haemolytic anaemia and hydrops fetalis after infection in pregnancy.
What you must remember
- Causative virus: human parvovirus B19, which infects erythroid precursor cells in the bone marrow using the P antigen as its receptor.
- Slapped cheek: bright red cheeks with the area around the mouth spared, the characteristic first rash.
- Lacy rash: a reticular rash on the limbs and trunk that comes and goes over weeks, often triggered by heat, sunlight or exercise.
- Age: school-age children, usually 5 to 15 years, with outbreaks in late winter and spring.
- Aplastic crisis: transient suppression of erythropoiesis in haemolytic anaemia such as sickle cell disease or hereditary spherocytosis.
- Fetal risk: infection in pregnancy can cause fetal anaemia, hydrops fetalis and rarely fetal loss.
- Adults: a symmetrical arthropathy of the small joints, particularly in women, is the usual adult presentation.
- Diagnosis: parvovirus B19 IgM serology, with PCR used where serology is unhelpful.
From a pale child with sickle cell disease to a crisis
A 9-year-old with sickle cell disease develops pallor and fatigue, and the reticulocyte count has fallen sharply from her usual level. The steps below show how the picture of an aplastic crisis from parvovirus B19 is recognised and managed.
- Recognise the drop: a sudden fall in haemoglobin with a low reticulocyte count in a child with haemolytic disease points to aplastic crisis.
- Test for parvovirus: send parvovirus B19 IgM serology, or PCR where serology is unhelpful.
- Assess severity: check for heart failure, marked pallor and fatigue, since severe anaemia may need urgent treatment.
- Transfuse as needed: packed red cells are given for severe anaemia, with close monitoring of the haemoglobin.
- Manage immunocompromised cases: a prolonged infection in an immunodeficient patient is treated with intravenous immunoglobulin, which clears the virus.
- Counsel pregnant contacts: an exposed pregnant woman needs serology, and fetal ultrasound is used if infection is confirmed.
Where the exam tries to catch you
Questions on this topic test the rash sequence, the crisis and the pregnancy risk.
- Trap: treating the slapped cheek rash with antibiotics Correct: the rash is viral and self-limiting, and the child is usually well.
- Trap: reassuring a pregnant woman exposed to a child with the rash without testing Correct: she needs serology, since fetal infection can cause hydrops and anaemia.
- Trap: expecting the reticulocyte count to rise in aplastic crisis Correct: reticulocytes fall sharply because the marrow fails to respond.
- Trap: assuming the lacy rash is still contagious Correct: by the time the lacy rash appears, the child is usually no longer infectious.
Frequently asked questions
Why is erythema infectiosum called slapped cheek disease?
Erythema infectiosum is called slapped cheek disease because of the bright red cheeks that appear as the first rash. The area around the mouth is typically spared, which gives a slapped look. A lacy rash on the limbs and trunk follows in the next few days.
What causes erythema infectiosum?
Erythema infectiosum is caused by human parvovirus B19, which infects erythroid precursor cells in the bone marrow. The virus uses the P antigen as its cellular receptor. The infection is common in school-age children and spreads by respiratory droplets.
Which children are at risk of aplastic crisis with parvovirus B19?
Children with haemolytic anaemia, such as sickle cell disease or hereditary spherocytosis, are at risk of aplastic crisis with parvovirus B19. The virus suppresses red cell production for a short period, causing severe anaemia. Transfusion may be needed during the crisis.
What is the risk of parvovirus B19 infection in pregnancy?
Parvovirus B19 infection in pregnancy can cause fetal anaemia, hydrops fetalis and, rarely, fetal loss. A pregnant woman exposed to the infection needs serology. Fetal ultrasound monitoring is used if the infection is confirmed.
Can adults develop arthropathy from parvovirus B19 infection?
Adults with parvovirus B19 infection, particularly women, can develop a symmetrical arthropathy affecting the small joints of the hands. It is usually self-limiting. Arthropathy is a common adult presentation, unlike the mild rash seen in children.