Dorte Bek Folkvardsen
International Reference Laboratory of Mycobacteriology /
Afdeling for Tuberkulose og Mykobakterier.
Denmark is a low-incidence tuberculosis (TB) country, but in the past decades, it has experienced an ongoing outbreak, documented with the genotyping methods RFLP and MIRU-VNTR. The outbreak has spread throughout the Danish Kingdom, including to the high-incidence country Greenland. In this study, the outbreak has been investigated by applying whole genome sequencing (WGS) to all strains in the period 1992-2014 typed to be Cluster 2 or 1112-15 (C2). In total, 952 strains from 892 patients were included. The aim was to decipher the outbreak based on WGS and investigate the new insight gained from applying this method to a previously characterized outbreak.
The outbreak was comprised of two phylogenetic clades, a major and a minor, with a most recent common ancestor dating back to 1959. The mutation rate of 0.24 SNPs/genome/year was consistent with previous findings. Based on the mutation rate, the clades could be divided into three epidemic groups and further into several subgroups. Thus, the outbreak previously known to consist of one clonal strain was divided into several subgroups in accordance with geographic areas. The earlier proposed algorithm to confirm recent transmission cannot be applied to an outbreak of size and setting like C2.
We show that the success of the strain is not due resistance or due to demographic factors only; a list of SNPs found in genes associated with virulence was identified. This list can be used as a tool for investigation of other successful outbreaks. A number of recurrent cases infected with the genotype causing this outbreak was observed and investigated. The previous suggested algorithm for defining a recurrent case as a relapse or a reinfection could not be used. We show that dealing with a big outbreak in a setting of limited size, it is important to know the development of the outbreak. Being infected with different clades of the outbreak, suggests reinfection. Relapse, can only be confirmed, if no other patients are found with identical clones.
All these findings, suggest implications for TB control in Denmark and it is plausible that C2 is more virulent than the average Mtb strain, as it has caused outbreaks wherever it has been introduced throughout the Danish Kingdom, a kingdom consisting of both low- and high incidence settings.