Spatial and phylogenetic patterns reveal hidden infection sources of Bacillus anthracis in a Vietnamese anthrax outbreak

Keywords: Anthrax, Vietnam, Phylogenetics, medical geography, One Health
Abstract Type: Paper Abstract

Abstract

Bacillus anthracis, the bacterial cause of anthrax, is a zoonosis affecting herbivores (livestock and wildlife) often spilling over into humans. Anthrax has been nationally reportable in Vietnam since 2015 and most cases occur in northern Vietnam. In April 2022, an outbreak was reported in Son La province following the butchering of a water buffalo, Bubalus bubalis. A total of 137 humans from three villages were likely exposed to contaminated meat. Epidemiological investigations suggested a single animal was involved in all exposures. Five isolates were recovered from clinical cases along with one from the buffalo hide, another from associated maggots, and an 8th from soil at the carcass site. Whole-genome sequencing of these isolates was performed and phylogenetic analysis placed the strains globally, regionally, and locally. All strains from the outbreak belong to the canSNP lineage A.Br.001/002, not previously identified in Vietnam. Though, this lineage has been identified in nearby China, Thailand, India, Indonesia, and Northern Australia. At the local level, a 25 loci multi-locus variable number tandem repeat analysis (MLVA-25) was used to investigate the relationship between human cases, the soil, and buffalo samples. Locally, four MLVA-25 genotypes were identified from the 8 isolates. This level of genetic diversity is unusual for the limited geography and timing of cases and has not been reported in past literature. The coupled spatial and phylogenetic data suggest this outbreak originated from multiple sources, likely undetected animal cases. Though further analysis into mutability of the MLVA-25 loci in vivo is needed to confirm this.