Erythema infectiosum (Fifth disease)

Overview

•Caused by Parvovirus B19 - single-strand DNA virus transmitted via respiratory secretions, blood products, and vertically (placenta)

•Most common in children aged 3-15 years; outbreaks in schools in late winter/spring

•Incubation period 4-20 days

Presentation

•Phase 1 - Prodrome (days 1-7): low-grade fever, malaise, mild coryzal symptoms - child is viraemic and most infectious

•Phase 2 - Slapped cheek rash (~day 10-17): bright bilateral facial erythema with circumoral pallor - viraemia resolved, child is NO LONGER infectious

•Phase 3 - Lacy body rash: symmetrical reticular (net-like) rash on trunk and limbs, spares palms and soles; waxes and wanes with heat, exercise, sunlight over weeks

•Adults: rash less prominent; dominant feature is symmetrical small-joint arthropathy (hands, wrists, knees) - can mimic early rheumatoid arthritis, may persist months

🎯

The child is most infectious BEFORE the rash appears (during viraemia). Once the slapped cheek rash appears, the child can return to school - no exclusion needed.

Investigations

•Immunocompetent child with typical presentation: clinical diagnosis - no investigations required

•High-risk (haemolytic anaemia): FBC + reticulocyte count - reticulocytopenia confirms red cell production arrest

•Pregnancy / uncertainty: Parvovirus B19 IgM and IgG serology - IgM = recent infection; IgG alone = past infection and immunity

•Gold standard (immunocompromised / aplastic crisis): Parvovirus B19 PCR - serology may be falsely negative as patients cannot mount antibody response

Management

•Healthy children: supportive - paracetamol or ibuprofen for fever; reassurance; no school exclusion once rash appears

•Aplastic crisis (haematological patients): hospital admission; red cell transfusion if symptomatic severe anaemia; self-limiting over 7-10 days

•Pregnancy (confirmed/suspected infection): refer to fetal medicine; serial middle cerebral artery Doppler ultrasound to detect fetal anaemia; intrauterine blood transfusion for severe hydrops fetalis

•Immunocompromised (chronic infection): intravenous immunoglobulin (IVIG) - passive antibody to clear virus; may need repeat courses

•Pregnant women exposed (e.g. teacher during school outbreak): check serology promptly - if IgG positive, lifelong immunity, no action; if IgG negative, monitor and refer for fetal surveillance if seroconversion occurs

⚠️

Risk of fetal loss is highest in the first 20 weeks of gestation. No antiviral treatment exists for Parvovirus B19.

Complications

•Transient aplastic crisis - haemolytic anaemia patients (sickle cell, hereditary spherocytosis, thalassaemia, G6PD deficiency); precipitous Hb drop due to 7-10 day arrest of erythropoiesis; these patients are highly infectious - isolate from at-risk contacts

•Hydrops fetalis - fetal anaemia → high-output cardiac failure → generalised fetal oedema → fetal death; risk highest before 20 weeks

•Chronic pure red cell aplasia - immunocompromised patients unable to produce neutralising antibodies; persistent erythropoiesis suppression → chronic anaemia

•Arthropathy - symmetrical small-joint arthritis, predominantly adult women; can persist months to years

•Myocarditis - rare, particularly in adults