Common viral exanthems — formative SAQs

SAQ 1 (10 marks)

A 26-year-old woman who is 22 weeks pregnant presents to her general practitioner. Two days ago her 6-year-old son was diagnosed with erythema infectiosum (fifth disease) after developing slapped cheeks and a lacelike rash. She has no history of parvovirus B19 infection and is unsure whether she has had it before. She is currently well with no rash or fever. ^[1]

Question: Outline the assessment and management of this pregnant woman, including the risk to the fetus, the investigations, the surveillance pathway and the role of treatment. (10 marks) ^[3]

Model answer

Risk to the fetus (2 marks). Parvovirus B19 crosses the placenta and is tropic for fetal erythroid precursors via the P blood group antigen. In a non-immune pregnant woman who acquires primary infection, the fetus is at risk of severe anaemia, high-output cardiac failure and hydrops fetalis, and of non-hydropic late intrauterine fetal death. The risk is concentrated in the second trimester because fetal erythropoiesis is massive and the red-cell lifespan is short. Critically, these are risks of anaemia and hydrops, not of a congenital malformation syndrome — so termination is not indicated, and a hydropic fetus can be rescued by intrauterine transfusion if recognised in time. ^{[3] [1]}

Investigations (3 marks). The first step is to determine the mother's immune status, because an immune woman needs no further action. Send parvovirus B19 IgG and IgM. IgG positive with IgM negative means past infection and immunity — reassure her, the fetus is protected. IgG negative means she is susceptible and at risk of primary infection, and a rising IgM or seroconversion over the next two to four weeks confirms infection. PCR has a role where the picture is atypical or the woman is immunocompromised and may not seroconvert. The single most important point is that she should not be reassured without serology. ^{[1] [3]}

Surveillance pathway (3 marks). If she is non-immune and seroconverts, refer her to fetal medicine for serial fetal ultrasound for ten to twelve weeks after the infection. Ultrasound looks for the earliest signs of hydrops — ascites, pleural or pericardial effusions, skin oedema, polyhydramnios, and raised middle cerebral artery peak systolic velocity as a marker of fetal anaemia. The surveillance window reflects the time over which fetal anaemia can develop after maternal infection. Liaise early with obstetrics and fetal medicine so that a plan is in place should hydrops emerge. ^[3]

Treatment (2 marks). If ultrasound surveillance detects hydrops, the intervention is intrauterine transfusion to correct the fetal anaemia, which can rescue a fetus that would otherwise die. There is no vaccine and no role for antiviral prophylaxis or immunoglobulin in the immunocompetent pregnant woman. Counselling is central: explain the risk honestly, the surveillance plan, and the fact that timely transfusion gives an excellent outcome — the aim is to detect and treat, not to abort. ^{[3] [1]}