Overview
The laboratory confirmation of suspected cases of measles and rubella becomes more challenging in settings where significant progress has been made toward elimination (near-elimination) or where the absence of endemic circulation of these diseases has been successfully demonstrated and verified (post-elimination). Because the strategy for achievement and maintenance of elimination requires a case-based surveillance programme, greater demands are placed on the laboratory to confirm suspected cases of measles or rubella or to discard a suspected case [1,2]. When a suspected case is detected, rapid laboratory confirmation and implementation of control measures are critical elements for minimizing additional exposures and onward transmission of virus. However, where there is a low prevalence (or absence) of measles and rubella, there is a greater likelihood that time and resources directed toward investigations of suspected cases may include a higher proportion of IgM positive cases that are not true cases.
As discussed in Section 8.1.1, the reliability of an IgM positive EIA result to accurately identify a true case of measles or rubella is significantly diminished due to the low prevalence of disease in elimination settings. However, the investigation for a clinically suspicious case should not be delayed pending the completion of additional testing or collection of additional specimens that may be planned to resolve lingering questions about the case.
This chapter provides guidelines for additional testing strategies including specialized testing that may provide helpful for the final classification of suspected cases of measles and rubella in an elimination setting. In most circumstances, routine testing for IgM will provide sufficient laboratory evidence to confirm a suspected case, as most cases will have an epidemiologic association with another confirmed case or other known risk factor for exposure to virus.
The epidemiologic context is extremely important in deciding which cases warrant further testing. To resolve the true disease status for a challenging case, it is essential that all information regarding the suspected case are shared between health care providers, epidemiologists, and laboratory staff. Countries in elimination settings should consider simultaneous testing for prevalent diseases that can mimic measles or rubella infections, particularly where diseases such as dengue and chikungunya are endemic. Network laboratories are encouraged to consult with the regional laboratory coordinator (RLC) to discuss the best approach when confronted with a difficult case, particularly those without a known source of infection.