Fatal Case of Diphtheria and Risk for Reemergence, Singapore

Abstract

We report a fatal autochthonous diphtheria case in a migrant worker in Singapore. This case highlights the risk for individual cases in undervaccinated subpopulations, despite high vaccination coverage in the general population. Prompt implementation of public health measures and maintaining immunization coverage are critical to prevent reemergence of diphtheria.

Keywords: Corynebacterium diphtheriae; Singapore; bacteria; contact tracing; diphtheria; public health; reemergence; toxigenic; transients and migrants; vaccines.

Figure 1

Phylogenetic analysis of Corynebacterium diphtheriae isolate from a 23-year-old man who died from diphtheria (OTH-17-20; bold) and 9 other isolates collected from hospitals in Singapore during 2013–2017. The tree was constructed by using 7 concatenated housekeeping gene sequences corresponding to the C. diphtheriae multilocus sequence typing scheme (https://pubmlst.org/cdiphtheriae/). Sequences were extracted from whole-genome sequences of each isolate. Concatenated sequences were aligned by using ClustalW (http://www.clustal.org/). Phylogeny was inferred by using the maximum-likelihood method, neighbor-joining algorithm based on the Jukes-Cantor model, and MEGA7 software (10). There were 2,544 positions in the final dataset. Numbers next to branches show bootstrap values calculated by using 1,000 reiterations. Scale bar indicates nucleotide substitutions per site. ST, sequence type.

Figure 2

Core-genome single-nucleotide polymorphism (SNP) phylogeny of Corynebacterium diphtheriae isolate from a 23-year-old man who died from diphtheria (OTH-17-20; bold) and 4 publicly available ST50 genomes (TH1141, GenBank accession no. GCA_001723455.1; CMCTH1443, accession no. GCA_001981275.1; CMCTH1112, accession no. GCA_001981275.1; and CDCE8392, accession no. GCA_000255215.1) of isolates collected from hospitals in Singapore during 2013–2017. Phylogeny was deduced by alignment with Parsnp (http://harvest.readthedocs.io/en/latest/content/parsnp.html), a rapid core-genome multialignment tool for genome assemblies. Extracted SNPs were concatenated, and phylogeny was inferred as described in Figure 1. There were 26,785 positions in the final dataset. Numbers next to branches show bootstrap values calculated by using 1,000 reiterations. Scale bar indicates nucleotide substitutions per site. ST, sequence type.