Evidence for local and international spread of Mycobacterium avium subspecies paratuberculosis through whole genome sequencing of isolates from the island of Ireland

We describe application of whole genome sequencing (WGS) to a collection of 197 Mycobacterium avium subsp paratuberculosis (MAP) isolates gathered from 122 cattle herds across 27 counties of the island of Ireland. We compare WGS to MAP diversity quantified using mycobacterial interspersed random unit - variable number tandem repeats (MIRU-VNTR). While MIRU-VNTR showed only two major types, WGS could split the 197 isolates into eight major groups. We also found six isolates corresponding to INMV 13, a novel MIRU-VNTR type for Ireland. Evidence for dispersal of MAP across Ireland via cattle movement could be discerned from the data, with mixed infections present in several herds. Furthermore, comparisons of MAP WGS data from Ireland to data from Great Britain and continental Europe revealed many instances of close genetic similarity and hence evidence for international transmission of infection. BEAST MASCOT structured coalescent analyses, with relaxed and strict molecular clocks, estimated the substitution rate to be 0.10-0.13 SNPs/site/year and disclosed greater transitions per lineage per year from Europe to Ireland, indicating transmission into Ireland. Our work therefore reveals new insight into the seeding of MAP infection across Ireland, highlighting how WGS can inform policy formulation to ultimately control MAP transmission at local, national and international scales.

Automated summary

This study applied whole genome sequencing (WGS) to analyze 197 Mycobacterium avium subsp paratuberculosis (MAP) isolates from cattle herds in Ireland. The results showed that WGS provided more detailed grouping compared to mycobacterial interspersed random unit - variable number tandem repeats (MIRU-VNTR). The study also found evidence for the transmission of MAP across Ireland via cattle movement and international transmission of infection. The findings contribute new insights into the spread of MAP infection and highlight the potential of WGS in informing policy formulation to control transmission at various scales.