1.7 Infection, immune response and laboratory diagnosis of rubella and CRS

After the rubella virus infects the nasopharynx, it multiplies in the lining of the respiratory tract and in local lymph nodes before passing into the bloodstream. Viraemia begins 5–7 days after infection, spreading throughout the rest of the body, including the skin. As with measles, the rash is immunologically mediated and coincides with the development of rubella-specific antibodies. Virus can be isolated from the nasopharynx from up to 1 week before, and for up to 2 weeks after the onset of rash.

Humoral and cell-mediated immunity develop following a rubella infection. Rubella-specific IgG and IgM antibodies are observed about 14–18 days after rubella infection, at about the time when the rash appears. Refer to Figure 1.6, Immune response in postnatal rubella infection.

Rubella IgM antibodies wane quickly and are usually undetectable after 2 months, whereas rubella IgG antibodies persist. A rubella-specific, cell-mediated lymphocyte response begins 1 week after the humoral response and persists for a lifetime. Although natural rubella infection generally confers lifelong immunity, rare cases of serologically confirmed reinfections after earlier infection (or immunization) have been reported. There have also been cases of CRS following reinfection in pregnant women with natural or vaccine-induced immunity, but this is extremely rare. Although maternal rubella antibodies provide protection against rubella for the first months of life, vaccination of infants prior to the waning of maternal antibody can result in primary vaccination failure or an attenuated immune response.

Laboratory confirmation of a primary infection of rubella is generally performed by detection of rubella-specific IgM by EIA. Although IgM is sometimes detectable at rash onset, by 5 days after rash >90% of cases are IgM positive [22]. However, false-positive rubella IgM results are known to occur, particularly among persons infected with parvovirus B19. Blood collected from pregnant women for antibody screening purposes may be inappropriately tested for rubella IgM. Positive results obtained under these circumstances will require additional testing. An approved or certified laboratory will make use of the most appropriate method(s) to confirm suspected cases of rubella according to the details of the suspected case including the epidemiologic setting and vaccine history. Refer to Chapter 4. Antibody detection methods for laboratory confirmation of measles, rubella, and CRS for additional information on serologic testing.

The detection of rubella-specific RNA by RT-PCR can be used to complement serologic testing to confirm suspected cases. The use of RT-PCR to confirm rubella infection is described in chapter 6. Although virus isolation confirms rubella infection, propagation of rubella virus in cell culture is a labour-intensive and time-consuming method. Moreover, rubella virus does not generally produce cytopathic effect (CPE) in cell culture and must be confirmed by virus detection methods such as immunohistochemistry, immunofluorescence or RT-PCR. However, the ability to produce virus isolates is important for providing ample sources of RNA for the molecular characterization of viruses and for the collection of novel strains for virus banks (Chapter 7. Molecular epidemiology of measles and rubella).

If rubella infection occurs in a pregnant woman after 20 weeks of pregnancy, the infant may be infected with rubella and yet not develop the signs and symptoms of CRS. An infant born without clinical signs of CRS but with laboratory confirmation of rubella must be monitored since virus can be shed for up to 1 year. A laboratory confirmed CRS case is defined as a clinically confirmed CRS case that has a positive blood test for rubella-specific IgM. Although IgM antibodies may persist for up to 1 year, about 50% of CRS cases are IgM negative at 6 months of age. Because negative IgM results may be obtained from CRS cases tested shortly after birth, the IgM assays should be repeated at 1 month of age.

For infants over 6 months of age, a negative IgM should not be relied upon to rule out CRS. In such cases, serial IgG testing should be included. A sustained level of IgG antibody over several months confirms CRS. The recommendations for sample collection for laboratory confirmation of suspected CRS cases is provided in chapter 3, section 3.6. Laboratory confirmation may also be performed through detection of rubella virus RNA from a suitable specimen (i. e., throat swab, urine). Specific chapters in this manual address the collection of samples, antibody testing and RT-PCR for confirmation of CRS cases.

The publication, WHO-recommended standards for surveillance of selected vaccine-preventable diseases [26], includes guidance and definitions for suspected, clinically confirmed, and laboratory confirmed CRS cases. Regional surveillance field guides are also available which include specific activities and technical information for the investigation, laboratory testing, and classification of suspected cases of CRS and CRI.