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Q1: A 5-year-old unvaccinated boy is brought with fever, cough, coryza, conjunctivitis and a rash spreading head-to-toe. Take me through the diagnosis and the pathogen. (2 min)
This is the classic presentation of measles (rubeola) — the prodrome of high fever with the 3 Cs (cough, coryza, conjunctivitis) followed by a cephalocaudal maculopapular rash that becomes confluent. The pathognomonic sign is Koplik spots — small (1-3 mm) bluish-white 'grains of salt' papules on bright-red erythematous buccal mucosa opposite the lower second molars — appearing in the late prodrome, 1-2 days BEFORE the rash.
The pathogen is the measles morbillivirus, an enveloped negative-sense single-stranded RNA virus of the family Paramyxoviridae, genus Morbillivirus. It is one of the most contagious human pathogens — the basic reproduction number (R0) is 12 to 18 — and is transmitted by respiratory droplets and airborne aerosols that remain infectious for up to 2 hours after the patient leaves a room. The incubation period is 10 to 14 days (range 7-21), and the patient is most infectious during the prodrome — from 4 days before to 4 days after the rash appears.
Q2: Walk me through the pathophysiology. Why does the rash appear, and why is an immunocompromised patient sometimes 'rashless' yet severely ill? (3 min)
Measles enters the respiratory tract, binds two receptors — CD150 (SLAM, signalling lymphocyte activation molecule) on activated T and B lymphocytes, monocytes, and dendritic cells, and nectin-4 (PVRL4) on the basolateral surface of respiratory and other epithelial cells. After local replication, the virus spreads to regional lymph nodes, then via a primary viraemia (days 2-3) to the reticuloendothelial system, and via a secondary viraemia (days 5-7) to skin, conjunctivae, respiratory and GI tracts. The pathognomonic Warthin-Finkeldey giant cells (multinucleated fused infected epithelial/lymphoid cells) appear in lymphoid and epithelial tissue.
The rash is NOT direct viral damage — it is a cell-mediated (type IV) immune attack by cytotoxic T lymphocytes on measles-infected capillary endothelial and epithelial cells. This explains two key facts: (1) the rash coincides with defervescence (the immune system is clearing virus), and (2) the immunocompromised patient — lacking intact cell-mediated immunity — may have NO rash yet severe disease (giant-cell pneumonia, measles inclusion-body encephalitis). The same mechanism explains why Koplik spots are the buccal-mucosal analogue: tiny foci of epithelial necrosis with neutrophil infiltrate on an erythematous base.
A pivotal recent discovery is measles-induced immune amnesia (Mina et al., Science 2015 and 2019): measles depletes 11-73 percent of the pre-existing antibody repertoire, erasing immunity to OTHER pathogens and predisposing the host to otitis, pneumonia, and diarrhoea for 2-3 years after the acute illness.
Q3: How would you confirm the diagnosis, and what are your immediate actions? (2 min)
Diagnosis is primarily clinical in an outbreak — typical prodrome with Koplik spots plus a cephalocaudal rash is diagnostic — but laboratory confirmation is mandatory for sporadic and atypical cases. I would send measles-specific IgM serology (positive 1-2 days after the rash, peaks at 1-3 weeks, persists 30-60 days) and RT-PCR on a throat swab or urine (most sensitive in the first 3-4 days of the rash; allows genotyping to identify the outbreak source). The WHO case definition of a suspected case is fever plus maculopapular rash plus at least one of cough, coryza, or conjunctivitis.
My two non-negotiable immediate actions are: (1) isolate with AIRBORNE precautions — single room, ideally negative-pressure, with FFP3/N95 for staff, for at least 4 days after the rash appeared (the ENTIRE illness if immunocompromised) — and (2) notify public health immediately (measles is statutorily notifiable). I would then start supportive care (paracetamol, fluids, oxygen as needed), give vitamin A (200,000 IU orally for a child over 12 months; repeat in 24 h), and treat any complication (pneumonia, otitis, dehydration, keratitis).
Q4: Discuss the complications and the late sequelae. (2 min)
Measles has a high complication burden, especially in the under-5s, adults over 20, the malnourished, the vitamin A deficient, the immunocompromised, and pregnant women. The commonest cause of death is pneumonia — either viral giant-cell (Hecht) pneumonia or secondary bacterial pneumonia (pneumococcus, Hib, Staphylococcus aureus). Other respiratory complications include otitis media (the commonest bacterial complication, especially in children), croup, sinusitis, and reactivation of tuberculosis with transient tuberculin skin-test anergy. Diarrhoea and dehydration worsen malnutrition. Eye complications — keratitis, corneal ulceration, xerophthalmia, and blindness — are devastating in vitamin A deficiency.
Neurological complications are examinable in detail: (1) acute measles encephalitis (~1 in 1000; mortality 15 percent; sequelae in 25-40 percent); (2) post-infectious encephalomyelitis (ADEM), a demyelinating autoimmune process within 2-14 days of the rash; (3) measles inclusion-body encephalitis (MIBE) in the immunocompromised within 1-10 months; and (4) subacute sclerosing panencephalitis (SSPE) — a persistent defective (M-protein-mutant) measles virus infection of neurons appearing 5-15 years after natural measles, with progressive cognitive and behavioural decline, myoclonus, seizures, EEG periodic complexes with burst-suppression, raised measles antibody in CSF and serum, and uniform fatality in 1-3 years. The risk is highest when measles was acquired under 2 years of age.
Other systemic complications include thrombocytopenia, myocarditis, hepatitis, and in pregnancy miscarriage, premature labour, and low birthweight (measles is NOT teratogenic). The immune amnesia (Mina) predisposes to other infections for 2-3 years.
Q5 (examiner's probe): Her mother is 14 weeks pregnant, never vaccinated, never had measles. She has been caring for the child throughout the prodrome. What do you do?
She is susceptible and pregnant, so live MMR is CONTRAINDICATED. I would give human normal immunoglobulin (HNIG) intramuscularly within 6 days of exposure — dose 0.5 mL/kg IM, maximum 15 mL — to provide passive protection and attenuate disease. I would counsel that measles is NOT teratogenic (unlike rubella) but raises the risk of severe maternal measles, miscarriage, and premature labour. I would monitor her closely for fever and prodromal symptoms.
I would also trace and vaccinate the playschool contacts — MMR within 72 hours of exposure for susceptible contacts who can receive a live vaccine. For other contacts in whom live vaccine is contraindicated — immunocompromised, infants under 6 months — HNIG within 6 days. After delivery, the mother should receive two doses of MMR separated by at least 4 weeks, with a 1-month contraception interval before a future pregnancy.
Q6 (final probe): Why does measles keep coming back when we have a highly effective vaccine?
Because the herd-immunity threshold is approximately 92-95 percent two-dose coverage — derived from H = 1 - 1/R0, with R0 of 12-18 giving 91.7 to 94.4 percent. Two-dose MMR effectiveness is about 97 percent, so effectively near-universal two-dose coverage is required. Any drop below that threshold — from missed vaccination, vaccine refusal, or cold-chain failure — predictably triggers outbreaks (UK loss of elimination in 2018, US Disneyland 2014-15 outbreak, recent resurgence in regions with falling coverage, and post-COVID-19 coverage disruptions).
The dominant driver of vaccine refusal is the retracted 1998 Wakefield Lancet paper that falsely linked MMR with autism. The paper was retracted in 2010 and Wakefield's medical licence was struck off. Subsequent large cohort studies — including Hviid et al. 2019 in 657,461 children (Annals of Internal Medicine) — confirm no association between MMR and autism, even in children at high familial risk. Combating this with clear, evidence-based communication; mandatory vaccination in some jurisdictions (Italy, France, California SB277); and catch-up campaigns are central to measles elimination. WHO's 2017 position paper endorses two doses of measles-containing vaccine, the first at 9 months (6 months in outbreaks, then repeated) and the second at 15-18 months.
Q7 (examiner's probe): A child who had one MMR dose at 9 months develops a mild fever and sparse rash 10 days after exposure to measles. IgM is negative. What is your approach?
This is likely modified measles. It occurs in people with partial immunity — here, one MMR dose may provide incomplete protection. The prodrome is attenuated, Koplik spots may be few or absent, and the rash is sparse and non-confluent. Despite the mild presentation, the child is still infectious and must be isolated with airborne precautions.
Because IgM may be delayed or absent in modified measles, the preferred confirmatory test is RT-PCR on a throat swab or urine. I would still give vitamin A to all children with measles (two doses 24 hours apart), and I would trace and offer post-exposure prophylaxis to contacts within the 72-hour (MMR) or 6-day (HNIG) windows. This case illustrates why modified measles is a clinical trap: it can be mistaken for a non-specific viral illness, yet it can seed outbreaks.
The child should still receive the routine second MMR dose at 15 to 18 months as part of the primary schedule.
References5ShowHide
- [1]Rota PA, Moss WJ, Takeda M, et al. Measles. Nature Reviews Disease Primers, 2016.PMID 27411684
- [2]Moss WJ, Griffin DE. Measles. Lancet, 2012.PMID 21855993
- [6]Hussey GD, Klein M. A randomized, controlled trial of vitamin A in children with severe measles. N Engl J Med, 1990.PMID 2194128
- [7]World Health Organization. Measles vaccines: WHO position paper - Recommendations. Vaccine, 2019.PMID 28760612
- [10]Hviid A, Hansen JV, Frisch M, Melbye M. Measles, Mumps, Rubella Vaccination and Autism: A Nationwide Cohort Study. Annals of Internal Medicine, 2019.PMID 30831578