Mumps is a contagious infectious disease that was common in Canadian school-age children before the measles, mumps, and rubella (MMR) vaccine was introduced starting in 1983. Two MMR doses have been recommended for BC children since 1996. Mumps causes parotitis, but symptoms can also include fever, headache, and myalgia. At the BCCDC Public Health Laboratory, mumps is diagnosed by detecting the mumps virus RNA in buccal swabs or urine using PCR or by serology. Mumps-RNA-positive samples are genotyped to track transmission clusters.
Despite vaccination programs, outbreaks of mumps continue to occur and are becoming more frequent. In the US there were 5748 mumps cases last year, which was the highest number reported in the last decade, and as of March 2017 a total of 1965 cases have been reported this year. In 2016 and 2017 there have been reported mumps outbreaks in Ontario, Manitoba, Saskatchewan, Alberta, and BC. Outbreaks likely reflect secondary vaccine failure from waning vaccine immunity and/or a mismatch between the vaccine strain and circulating outbreak strains.
In BC outbreaks have occurred in two populations: an unimmunized faith-based community in 2008, and in young adults likely to have received only one MMR dose. From 2008 to 2015, the number of BC mumps cases ranged from 8 to 132 per year. In spring 2016, a cluster of mumps cases was identified in Fraser Health and spread to two other regions. From 1 April to 31 October, 139 confirmed mumps cases were identified with a median age of 27 years; only 23 (17%) cases had two documented doses of MMR vaccine, and 14 (10%) were born prior to 1970, an age group for which mumps immunity due to prior infection is generally assumed. The outbreak strain was identified as genotype G, related to the strain that is endemic in North America. From January to March 2017, 39 confirmed mumps cases have been reported in a genotypically distinct new cluster.
Interpretation of diagnostic tests can be challenging because results are affected by the time of specimen collection relative to symptom onset and vaccination history. In unvaccinated individuals, detection of IgM antibodies becomes reliable only 3 to 4 days after symptom onset. For vaccinated individuals, IgM may be absent or short-lived, and detection of IgG, traditionally associated with immunity, does not imply protection. Compared with serology, mumps-virus-specific RT-PCR is more sensitive and specific. While mumps virus RNA can be detected in buccal swabs up to 9 days, it is recommended that specimens be collected within 5 days of symptom onset when viral load is highest. Simultaneous collection of a urine sample improves the detection yield, as mumps RNA is typically shed in urine for about 14 days after the onset of symptoms.
For suspected acute mumps cases, health care providers should collect buccal swabs after expressed parotid massage and urine, especially for individuals who have received prior MMR vaccination where serological testing is less reliable. Oral and urine specimens are also less invasive for the patient, and allow for viral genotyping, which can aid in epidemiological investigations. Serology can be useful if more than 5 days have passed for optimal buccal swab collection.
Clinicians should always interpret negative test results in the context of sample collection time and vaccination history; a negative result cannot rule out mumps infection. Furthermore, clinicians should consider alternative infectious etiologies when mumps tests are negative (Epstein-Barr virus, parainfluenza viruses, adenovirus, and enterovirus, as well as influenza).
—Agatha Jassem, PhD, (D)ABMM, FCCM
—Alexandra Nunn, MPH
—Monika Naus, MD, MHSc, FRCPC, FACPM
—Mel Krajden, MD, FRCPC
This article is the opinion of the BC Centre for Disease Control and has not been peer reviewed by the BCMJ Editorial Board.
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