Measles, rubella and congenital rubella — branching viva

Opening question

Examiner: Take me through this child. What is the most likely diagnosis, and what is your frame for managing it? [5]

Candidate: The most likely diagnosis is measles. The combination of fever, the 3 C's — cough, coryza, conjunctivitis — Koplik spots on the buccal mucosa, and a cephalocaudal maculopapular rash in an unvaccinated child with an exposure history is classic. My frame is two-layered: support this child now, and run the public-health response to stop transmission to other susceptible contacts — particularly pregnant women, infants and immunocompromised people. I would isolate immediately, confirm with IgM and PCR, give vitamin A, provide supportive care, and notify public health on suspicion. [5] [1]

Examiner: Why isolate before you have confirmation? [1]

Candidate: Because measles is one of the most contagious infections in medicine — up to twelve to eighteen secondary cases per case in a susceptible population — and the child is most infectious in the prodrome, before the rash appears. Waiting for laboratory confirmation before isolating allows the exact transmission we are trying to prevent, so we isolate on suspicion with airborne precautions and notify simultaneously. [1] [5]

Branch 1 — pathophysiology

Examiner: Explain the pathophysiology. Why is measles so contagious, and what is immune amnesia? [3]

Candidate: Measles is inhaled in airborne droplets and infects immune and epithelial cells via its receptors — CD150, also called SLAM, on immune cells, and nectin-4 on epithelial cells. So the virus enters through the very cells meant to defend the airway and spreads systemically through lymphoid tissue, which explains its extraordinary transmissibility. The distinctive feature is immune amnesia: measles infects and depletes memory B and T lymphocytes, erasing the child's pre-existing immunological memory. Population analysis of pre-vaccination data showed that measles drove a two-to-three-year rise in overall childhood infectious-disease mortality — not because measles persisted, but because the child had been immunologically reset. [3]

Examiner: And the rash? [5]

Candidate: The rash reflects the cell-mediated immune response to virus in dermal endothelium and epithelium; it appears as the viraemia peaks and spreads cephalocaudally. The same immune-mediated injury is why measles can be more severe in the immunocompromised, who may not mount the typical rash but develop giant-cell pneumonia. [5]

Branch 2 — diagnosis

Examiner: How will you confirm measles? [1]

Candidate: Measles-specific IgM on a single serum taken at first contact is positive in most cases from the first day of the rash. I would send that plus RT-PCR and genotyping on a throat or nasopharyngeal swab and urine, ideally within the first days of the rash — PCR is particularly useful early, when IgM may still be negative, and genotyping links the case to an outbreak. For a late or atypical presentation, paired sera showing a rise in IgG can confirm the diagnosis. [1]

Examiner: What supportive tests would you use? [5]

Candidate: A full blood count often shows a lymphopenia consistent with the viral, immunosuppressive nature of the illness, and a chest X-ray is indicated when pneumonia is suspected — as in this child with basal crackles. I would screen for complications: pneumonia, croup with stridor, dehydration, otitis media, corneal ulceration and any change in conscious state suggesting encephalitis. [5]

Branch 3 — treatment and the vitamin A question

Examiner: What treatment will you give, and why vitamin A? [4]

Candidate: I will give vitamin A in two age-appropriate doses 24 hours apart. The Hussey and Klein randomised trial in children with severe measles showed it reduced mortality and complications, and the Cochrane review confirmed a reduction in measles-related morbidity and mortality. It is cheap, safe and proven, so every child with measles should receive it — especially the young, the malnourished and the vitamin A deficient, in whom it also prevents the keratoconjunctivitis and blindness that complicate measles. [4]

Examiner: What else is in the management bundle? [5]

Candidate: Supportive care — fluids, antipyretics and nutrition, because measles suppresses appetite and worsens with dehydration; eye care to prevent corneal ulceration; and antibiotics for secondary bacterial pneumonia or otitis media when present. Most measles deaths are from pneumonia and its consequences, so I watch the respiratory status closely and escalate to intensive care if the work of breathing rises. [5] [4]

Branch 4 — public-health layer

Examiner: Walk me through the infection-control and public-health response. [1]

Candidate: I maintain airborne precautions and exclude the child from school or childcare until four days after the rash onset. I notify public health on suspicion, which triggers contact tracing. I identify all susceptible contacts — those with fewer than two documented MMR doses — and flag the high-risk contacts: pregnant non-immune women, infants and immunocompromised people. I document the onset and rash dates and the infectious period, from the prodrome to four days after the rash, and the school, childcare and healthcare exposures. [1] [8]

Examiner: Who gets post-exposure prophylaxis, and what? [8]

Candidate: Susceptible contacts should receive MMR within 72 hours of exposure, which can prevent or attenuate disease. High-risk contacts who cannot receive vaccine — pregnant non-immune women, immunocompromised children and young infants — should receive human normal immunoglobulin under protocol. The aim is always to protect the most vulnerable contact, and the same outbreak makes the case for catch-up vaccination to lift coverage above the herd-immunity threshold. [8]

Branch 5 — rubella, CRS and prevention

Examiner: How does rubella fit into this picture, and what is the CRS risk? [6]

Candidate: Rubella is a trivial illness in a child — mild fever, postauricular lymphadenopathy and a fine pink rash — but it is a devastating teratogen before 20 weeks gestation. When a woman catches rubella in early pregnancy, viraemia seeds the placenta and the virus reaches the fetus, where it slows cell division and induces apoptosis during organogenesis. That produces congenital rubella syndrome — the triad of sensorineural deafness, cataracts and PDA, with neurodevelopmental deficit. The CRS risk is governed almost entirely by gestational age: about 90 per cent at 0 to 11 weeks, around 50 per cent at 12 to 17 weeks, and rare after 18 to 24 weeks. [6]

Examiner: How do we prevent CRS? [8]

Candidate: CRS is entirely preventable through vaccination. We screen every pregnancy for rubella IgG at booking, vaccinate non-immune women postpartum — MMR is live and contraindicated in pregnancy — and sustain two-dose MMR coverage above 95 per cent, the herd-immunity threshold, so that rubella does not circulate and women reach pregnancy immune. The MMR schedule is dose one at twelve months and dose two at four to six years. The same high coverage that prevents measles outbreaks prevents CRS, because the unborn child is protected by the mother's immunity and by the community's coverage. [8] [6]

Wrap

Examiner: Summarise the measles, rubella and CRS stance in one sentence. [1]

Candidate: Recognise measles on the prodrome — the 3 C's and Koplik spots — and isolate, confirm with IgM and PCR, give vitamin A, support and notify; trace contacts and offer post-exposure prophylaxis within 72 hours; confirm rubella in pregnancy serologically and counsel by gestational CRS risk; and prevent all three with the two-dose MMR schedule, antenatal rubella screening, postpartum catch-up and two-dose coverage above 95 per cent — because measles kills children, rubella wrecks the unborn, and the vaccine prevents both. [1] [6]