Abstract
In December 2015, six cases of Shiga toxin (Stx)-producing Escherichia coli (STEC) O157:H7 stx2a/stx2c phage type (PT) 24 were identified by the national gastrointestinal disease surveillance system at Public Health England (PHE). Frozen grated coconut imported from India was implicated as the vehicle of infection. Short and long read sequencing data were interrogated for genomic markers to provide evidence that the outbreak strain was from an imported source. The outbreak strain belonged to a sub-lineage (IIa) rare in domestically acquired infection in the United Kingdom, and indicative of an imported strain. Phylogenetic analysis identified the most closely related isolates to the outbreak strain were from cases reporting recent travel not to India, but to Uganda. Phylo-geographical signals based on travel data may be confounded by the failure of local and/or global monitoring systems to capture the full diversity of strains in a given country. This may be due to low prevalence strains circulating in-country under the surveillance radar, or a recent importation event involving the migration of animals and/or people. Comparison of stx2a-encoding prophage harbored by the outbreak strain with publicly available stx2a-encoding prophage sequences revealed that it was most closely related to stx2a-encoding prophage acquired by STEC O157:H7 that caused the first outbreak of STEC-hemolytic uremic syndrome (HUS) in England in 1982–83. Animal and people migration events may facilitate the transfer of stx2a-encoding prophage from indigenous STEC O157:H7 to recently imported strains, or vice versa. Monitoring the global transmission of STEC O157:H7 and tracking the exchange of stx2a-encoding phage between imported and indigenous strains may provide an early warning of emerging threats to public health.
Highlights
Outbreaks of foodborne, gastrointestinal disease caused by Shiga toxin (Stx)-producing Escherichia coli (STEC) serotype O157:H7 are regarded as a significant threat to public health (Riley et al, 1983; Michino et al, 1999; Cowley et al, 2016; Launders et al, 2016a; Gobin et al, 2018)
Additional isolates sequenced from the archive collection submitted to Gastrointestinal Bacterial Reference Unit (GBRU) between 2006 and 2016, included eight isolates belonging to the same phage type (PT) as the outbreak strain, PT24, all of which reported recent travel to Uganda, and all cases reporting recent travel to the Indian Sub-Continent (ISC; n = 56) (Supplementary Table S1)
The epidemiological analysis provided evidence that the outbreak strains originated from India, while the phylogenetic analysis of the sequencing data indicated the strain was most closely related to isolates from Uganda, and the stx2a-encoding phage was most closely related to stx2a-encoding bacteriophage harbored by the strains of STEC O157:H7 that emerged in the United Kingdom, as the most common cause of STEC-hemolytic uremic syndrome (HUS) in early 1980s
Summary
Gastrointestinal disease caused by Shiga toxin (Stx)-producing Escherichia coli (STEC) serotype O157:H7 are regarded as a significant threat to public health (Riley et al, 1983; Michino et al, 1999; Cowley et al, 2016; Launders et al, 2016a; Gobin et al, 2018). STEC O157:H7 is zoonotic and can be transmitted to humans via direct contact with animals and/or their environment, contaminated food or water, or person-toperson spread (Byrne et al, 2015). There are three main lineages of STEC O157:H7 (I, II, and I/II) and eight sub-lineages (Ia, Ib, Ic, IIa, IIb, IIc, I/IIa, and I/IIb) (Dallman et al, 2015a). The majority of human cases and outbreaks of STEC O157:H7 in the United Kingdom are caused by sub-lineages Ic, I/IIa, and IIc (Dallman et al, 2015a). The sub-lineages that are infrequently isolated from human cases in the United Kingdom are more likely to be associated with returning travelers or outbreaks associated with contaminated food imported to the United Kingdom from elsewhere (Gobin et al, 2018)
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