Parvovirus B19 and Viral Exanthems
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Direct answer
Parvovirus B19 is a single-stranded DNA virus that causes erythema infectiosum, the fifth disease of childhood, and it is uniquely toxic to erythroid progenitor cells. In haemolytic anaemias it triggers a transient aplastic crisis, and infection in early pregnancy can lead to fetal anaemia and hydrops. Diagnosis in immunocompetent patients rests on IgM serology, while PCR is used when immunity is compromised.
What you must remember
- Receptor: the P antigen, or globoside, on erythroid precursors explains why the virus targets red cell production, and it is also found on fetal myocardial cells.
- Erythema infectiosum: a slapped-cheek facial erythema followed by a lacy, reticular rash on the trunk and limbs in children, which can recur with heat, sunlight or exercise.
- Adult arthropathy: symmetric small-joint arthralgia or arthritis, most common in adult women, usually settling without erosions.
- Aplastic crisis: a sudden fall in haemoglobin and reticulocytes in haemolytic anaemias such as sickle cell disease and hereditary spherocytosis, because the red cell lifespan is shortened.
- Chronic infection: persistent anaemia or pure red cell aplasia in immunodeficiency, treated with intravenous immunoglobulin.
- Hydrops fetalis: fetal anaemia leads to high-output heart failure and generalised oedema, with risk greatest when maternal infection occurs in the first half of pregnancy.
- Diagnosis: IgM anti-B19 for acute infection in immunocompetent people, and PCR for immunocompromised patients or fetal samples.
- Self-limiting course: erythema infectiosum needs no antiviral therapy in immunocompetent children.
The classic exanthems in order
Parvovirus B19 is one of the historical childhood exanthems, each numbered in the order it was described. The table places it among its siblings so the rash pattern can be recognised quickly.
| Classical name | Causative agent | Rash and key clue |
|---|---|---|
| First disease: measles | Measles virus | Koplik spots, cough, coryza, conjunctivitis, then a blotchy rash |
| Second disease: scarlet fever | Group A streptococci | Sandpaper rash, strawberry tongue, circumoral pallor |
| Third disease: rubella | Rubella virus | Fine pink maculopapular rash with postauricular nodes |
| Fifth disease: erythema infectiosum | Parvovirus B19 | Slapped-cheek erythema, then a lacy reticular rash |
| Sixth disease: roseola infantum | Human herpesvirus 6 | High fever for several days, then a rash as the fever settles |
The clue is the rash pattern and its timing, not the number. A slapped-cheek face points to parvovirus, while a rash that appears only after the fever ends points to roseola. Measles and scarlet fever are separated by their prodrome and throat findings, and rubella is often mild enough to be missed in children.
Where the exam tries to catch you
Parvovirus questions usually test the haematological complications and the reassurance trap in pregnancy.
- Trap: restricting aplastic crisis to sickle cell disease Correct: any haemolytic anaemia with a high red cell turnover, including hereditary spherocytosis, is at risk.
- Trap: treating erythema infectiosum with antivirals Correct: the disease is self-limiting in immunocompetent children, and management is supportive.
- Trap: missing parvovirus in an adult with symmetric arthritis Correct: it is a recognised cause of symmetric small-joint arthritis in women, and serology can confirm it.
- Trap: reassuring a pregnant woman after one normal scan Correct: exposed pregnancies need serial fetal Doppler assessment, since anaemia and hydrops can develop after an initially normal ultrasound.
Frequently asked questions
Why does parvovirus B19 cause aplastic crisis in haemolytic anaemia?
Parvovirus B19 destroys erythroid progenitor cells, so marrow output stops for a few days. In haemolytic anaemia the red cell lifespan is already short, so the sudden halt in production causes a sharp fall in haemoglobin.
Which test confirms acute parvovirus B19 infection in an immunocompetent patient?
IgM antibody to parvovirus B19 confirms acute infection in an immunocompetent patient, and it remains detectable for some months. PCR is reserved for immunocompromised patients and for fetal samples in suspected hydrops.
How does parvovirus B19 cause hydrops fetalis?
Parvovirus B19 infects fetal erythroid progenitor cells, causing severe fetal anaemia that leads to high-output heart failure and generalised oedema. Serial Doppler monitoring and intrauterine transfusion are used in severe cases.
Which viral exanthem produces a rash after the fever has settled?
Roseola infantum, caused by human herpesvirus 6, produces several days of high fever in young infants and a rash that appears once the fever subsides. The rash is typically pink and maculopapular and does not need treatment.
How does rubella rash differ from the rash of erythema infectiosum?
Erythema infectiosum causes a slapped-cheek facial erythema followed by a lacy reticular rash on the trunk and limbs. Rubella produces a fine pink maculopapular rash that starts on the face and is accompanied by postauricular lymphadenopathy.