Etiological analysis of discarded measles in the context of a measles outbreak among a highly immunized population.

BACKGROUND
Measles can lead to serious complications and remains an important cause of morbidity and mortality worldwide. We aimed to assess the etiological diagnosis of discarded measles cases in the context of an outbreak among a highly immunized population.


METHODS
We conducted a retrospective observational study of discarded measles cases from an outbreak that occurred from October 2006 to July 2007 in Catalonia. A confirmed case was defined as having a positive measles serum IgM result and/or a positive result by RT-PCR in urine and/or nasopharyngeal swab; or an epidemiological link to a confirmed case. Serum specimens were tested by a commercially available indirect-format and by an in-house capture-format measles IgM enzyme immunoassays.


RESULTS
Testing of 89 samples discarded for measles determined the etiologies for 10 (11.2%), including 1 rubella, 3 human herpes virus 6, and 6 measles infections. Of 381 confirmed cases in the outbreak, 10% had received at least one dose of the measles-mumps-rubella vaccine versus 54% of the discarded for measles (OR=0.09: 95%CI 0.06, 0.14; p<0.001).


CONCLUSIONS
Highly sensitive surveillance systems are critical to identifying cases, responding to outbreaks and verifying progress towards measles elimination. Molecular tools for measles detection and differential diagnosis, and collection of appropriate specimens for molecular and serologic testing are essential to correctly diagnose suspected measles infection.