Laboratory testing and phylogenetic analysis during a mumps outbreak in Ontario, Canada

Abstract

Background: In September 2009, a mumps outbreak originated in New York and spread to Northeastern USA and Canada. This study compares the performance of different diagnostic testing methods used in Ontario and describes molecular characteristics of the outbreak strain.

Methods: Between September 2009 and February 2010, specimens from suspect cases were submitted to Public Health Ontario Laboratory for mumps serology, culture and/or real-time reverse-transcriptase PCR (rRT-PCR) testing. rRT-PCR-positive specimens underwent genotyping at Canada's National Microbiology Laboratory. Whole genome sequencing was performed on four outbreak and three sporadic viral culture isolates.

Results: Six hundred ninety-eight patients had IgM serology testing, of which 255 (37%) had culture and rRT-PCR. Among those, 35/698 (5%) were IgM positive, 39/255 (15%) culture positive and 47/255 (18%) rRT-PCR-positive. Buccal swabs had the highest rRT-PCR positivity (21%). The outbreak isolates were identical to that in the New York outbreak occurring at the same time. Nucleotide and amino acid identity with the Jeryl Lynn vaccine strain ranged from 85.0-94.5% and 82.4-99.4%, depending on the gene and coding sequences. Homology of the HN protein, the main immunogenic mumps virus protein, was found to be 94.5 and 95.3%, when compared to Jeryl Lynn vaccine major and minor components, respectively.

Conclusions: Despite higher sensitivity than serology, rRT-PCR testing is underutilized. Further work is needed to better understand the suboptimal match of the HN gene between the outbreak strain and the Jeryl Lynn vaccine strain.

Keywords: Diagnosis; Mumps; Outbreak; PCR; Phylogeny; Serology; Viral culture.

Fig. 1

Whole genome sequencing (WGS) phylogenetic tree. The WGS phylogenetic tree was constructed using the nearly complete genomes (15,264 nucleotides) of four genotype G mumps outbreak isolates from Ontario (GenBank accession numbers: KY680537.1, KY680538.1, KY680539.1 and KY680540.1), three sporadic Ontario isolates (GenBank accession numbers: KY006856.1, KY006857.1 and KY006858.1) and representatives of other genotypes available in NCBI’s GenBank sequence database. The phylogenetic tree containing 37 strains was reconstructed using the Neighbor-Joining method and Maximum Composite Likelihood was used to compute the evolutionary distances. The statistical significance of the phylogenies constructed was estimated by bootstrap analysis of 1000 pseudo-replicate data sets (bootstrap values of > 70% are given at each node). The horizontal length of the bar denotes the nucleotide changes per site (MEGA version 6.06 software package). Outbreak and non-outbreaks isolates identified at Public Health Ontario Laboratory in 2010 are labelled with red and blue circles respectively

Fig. 2

Small hydrophobic (SH) gene sequencing phylogenetic tree. The SH phylogenetic tree was constructed based on the 316 nucleotides of the entire SH gene of four genotype G mumps outbreak isolates from Ontario (GenBank accession numbers: KY680537.1, KY680538.1, KY680539.1 and KY680540.1), three sporadic Ontario isolates (GenBank accession numbers: KY006856.1, KY006857.1 and KY006858.1) and representatives of other genotypes available in NCBI’s GenBank sequence database. The phylogenetic tree containing 37 strains was reconstructed using the Neighbor-Joining method and Maximum Composite Likelihood was used to compute the evolutionary distances. The statistical significance of the phylogenies constructed was estimated by bootstrap analysis of 1000 pseudo-replicate data sets (bootstrap values of > 70% are given at each node). The horizontal length of the bar denotes the nucleotide changes per site (MEGA version 6.06 software package). Outbreak and non-outbreaks isolates identified at Public Health Ontario Laboratory in 2010 are labelled with red and blue circles respectively