Source tracking of an anthrax outbreak in northeastern China using complete genome analysis and MLVA genotyping

Anthrax is caused by Bacillus anthracis, an etiological agent behind zoonotic diseases worldwide. B. anthracis is also one of the most dangerous bioterrorism agents. An anthrax outbreak took place in Liaoning Province in northeastern China in August 2012. It resulted in seven human infections and dozens of dead cows. One B. anthracis strain, named Han, was isolated from a dead cow. This strain showed minor pathogenicity in mice and was suspected to be derived from the locally administered vaccine strain, Vac. In order to determine if the Han isolate was derived from the vaccine strain Vac and to track the source of the anthrax outbreak and, so, exclude the possibility of terrorism attack, a complete genome sequencing of these two B. anthracis strains was conducted. With the genome sequencing data, canonical single nucleotide polymorphism (SNP) analysis and whole-genome SNP screening were performed. The results indicate that the Han strain was markedly different from the Vac strain. Further analysis by multiple-locus variable-number tandem repeat analysis (MLVA) showed that Han clustered with previously reported Chinese strains. The result of MLVA15 confirmed that the Han strain is a naturally occurring isolate instead of an engineered agent deliberately distributed by terrorists or other parties. In conclusion, our method used in this study not only facilitates epidemiological studies but also made it easier to distinguish naturally occurring outbreaks from intentionally released pathogens.