Rubella and Congenital Rubella Syndrome
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Direct answer
Rubella virus, a togavirus, causes a mild rash illness in children and adults, but infection in early pregnancy can produce congenital rubella syndrome, whose classic triad is cataract, congenital heart disease (usually patent ductus arteriosus) and sensorineural deafness. The test most often asked is rubella-specific IgM in a newborn's own blood, because maternal IgM does not cross the placenta and so proves intrauterine infection. Prevention rests on measles-rubella vaccination of girls and women before pregnancy.
What you must remember
- Classification: Rubella virus is the sole member of genus Rubivirus in family Togaviridae, an enveloped positive-sense single-stranded RNA virus.
- Transmission: Spread by respiratory droplets; infants with congenital rubella shed virus in nasopharyngeal secretions and urine for months, sometimes up to a year.
- Trimester rule: Risk and number of defects are highest when infection occurs in the first trimester and fall sharply as pregnancy advances.
- Late defects: Sensorineural deafness and some eye involvement can follow infection later in pregnancy, so the classic triad is not the whole story.
- Blueberry muffin rash: Purpuric skin lesions from extramedullary haematopoiesis and thrombocytopenia are a recognised neonatal sign of congenital rubella.
- Diagnosis: Rubella-specific IgM in infant serum establishes congenital infection, whereas IgG in a newborn may simply be transplacental maternal antibody.
- Vaccine: Live attenuated measles-rubella vaccine is contraindicated in pregnancy, and women are advised to avoid conception for about a month after vaccination.
A pathway from maternal rash to confirmed congenital case
A pregnant woman with a rash and lymphadenopathy, possibly exposed to a case, needs a sequence of tests. The order matters because maternal IgG shows immunity, not the current infection.
- Confirm the maternal infection: Test rubella IgM and IgG on paired sera; a rising IgG or low-avidity IgG points to recent infection.
- Establish immunity status: Check IgG at the first antenatal visit, so that any non-immune woman can be vaccinated after delivery.
- Test the newborn: Send infant serum for rubella IgM, since a positive result confirms intrauterine infection.
- Look for the virus: RT-PCR or culture from throat swab, urine or cerebrospinal fluid supports the diagnosis in an infant with suspected congenital infection.
- Screen the organs: Examine the eyes for cataract and pigmentary retinopathy, test hearing, and check the heart with echocardiography for patent ductus arteriosus.
- Protect and report: Infants with congenital rubella shed virus, so use contact precautions and notify surveillance.
Where the exam tries to catch you
Rubella questions usually turn on timing, antibody type and vaccine rules.
- Trap: Treating maternal IgM as proof of fetal infection. Correct: IgM does not cross the placenta, so a positive IgM in the newborn's own serum is the proof.
- Trap: Assuming deafness and cataract are the only defects. Correct: Pigmentary retinopathy, microcephaly, the blueberry muffin rash and hepatosplenomegaly also belong to congenital rubella syndrome.
- Trap: Vaccinating a pregnant woman who is non-immune. Correct: The vaccine is live and contraindicated in pregnancy, so vaccination is given after delivery instead.
Frequently asked questions
Which defects form the classic triad of congenital rubella syndrome?
Cataract, congenital heart disease (most often patent ductus arteriosus) and sensorineural deafness form the classic triad of congenital rubella syndrome. Pigmentary retinopathy, microcephaly and the blueberry muffin rash are other recognised features of the same syndrome.
Which trimester carries the highest risk of defects in congenital rubella?
The first trimester carries the highest risk of defects in congenital rubella, and the risk falls sharply as pregnancy advances. Late maternal infection can still cause deafness and other problems, so exposure at any gestation needs evaluation.
Why is rubella IgM in a newborn more informative than IgG?
Rubella-specific IgM in a newborn's serum is not transferred across the placenta, so its presence shows the infant mounted an immune response to infection in utero. IgG in a newborn may simply be maternal antibody that crossed the placenta.
How should a non-immune woman be protected against rubella?
A non-immune woman should receive the measles-rubella vaccine after delivery, not during pregnancy, because the vaccine is live. She should avoid conception for about a month after vaccination.
Which test establishes congenital rubella in a baby?
Rubella-specific IgM detected in infant serum establishes congenital rubella syndrome, supported by RT-PCR or virus isolation from throat swab or urine. The infant should also be screened for hearing, eye and heart abnormalities.