MUMPS IN CANADA: THE ALBERTA EXPERIENCE 2007 TO NOW

Background: Canada has infrequently experienced limited mumps outbreaks in a vaccinated population over the past three years, most recently in Nova Scotia. In Alberta only sporadic cases have been detected during this time. However, in the summer of 2007 it became apparent that there were an increasing number of mumps cases occurring in educational establishments, despite high levels of immunization.

Objective: The Provincial Laboratory implemented a mumps nucleic amplification test (NAT) at the outset of the outbreak, to complement traditional serological testing for mumps IgM & IgG antibody. The application of the NAT and serology assays provided a more complete picture of the sensitivity and specificity of the individual assays and their individual pitfalls if used separately.

Results: The mumps outbreaks appear to be confined to just two Health Regions of Alberta at the present time. The beginnings of the outbreak can be traced to the first week of September 2007 in one of the Health Regions. The age cohort most commonly affected is the 16 to 25yr age group, and were most likely to have been a student at an educational establishments located in either of the affected Health Regions. Where vaccination data was available, usually only one dose of MMR was received; although there were individuals with a documented vaccination history of two doses who were also confirmed as mumps cases by the mumps NAT.

At least 50% of cases positive by the he mumps NAT test were negative for mumps IgM antibody at the onset of parotitis, and only 13.5% were both mumps IgM and NAT positive. Approximately 5% of cases were identified solely through IgG or both IgM and IgG seroconversions on paired sera; based upon preliminary data but this figure could be higher once a more extensive database search is completed.

Genotyping on a significant proportion of these cases show that the outbreak genotype circulating is genotype G cluster 1, which is identical to that from Nova Scotia.

Conclusions: The data show the positive impact that nucleic acid assays can have on identifying probable cases of mumps seen in the acute phase of illness that would be missed by serological investigations. However both serology and NAT can identify cases missed by one or the other assays, especially as the duration of mumps virus shedding may be very short in individuals previously vaccinated.