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Spatial and phylogenetic patterns reveal hidden infection sources of Bacillus anthracis in an anthrax outbreak in Son La province, Vietnam

A map of Son La province in Vietnam is shown along with district and commune boundaries. Communes are shaded purple to indicate the level of SBS cumulative incidence of human anthrax. The darkest purple shows an incidence rate of over 80 per ten thousand people. The map is annotated in three places by a line pointing to a region on the map. The top shows a buffalo with text saying one animal case in 2005. The middle shows a buffalo with text saying the confirmed animal case in 2022 outbreak in Nong Lay Commune. The third shows a buffalo and cattle with text saying 5 animal cases in 2005.
Fig. 3. Cumulative human anthrax incidence in Son La province from 2003 to 2022 adapted from Luong et al. (In Review). Communes with no human anthrax cases are shown in white. Communes with animal cases of anthrax are indicated with year, number of animals, and whether buffalo and/or domestic cattle were involved. Within the province (black outline) there are administrative districts (thin black outline) which are further sub-divided into administrative communes (gray outline). The approximate locations of the 2022 villages are illustrated as gray dots (dithered to protect the communities).

METRAILER, JIRANONTASOK, LUONG, HAMERLINCK, NORRIS, BLACKBURN – Spatial and phylogenetic patterns reveal hidden infection sources of Bacillus anthracis in an anthrax outbreak in Son La province, Vietnam

Morgan C. Metrailer, Thi Thu Ha Hoang, Treenate Jiranantasak, Tan Luong, Luong Minh Hoa, Do Bich Ngoc, Quang Thai Pham, Van Khang Pham, Tran Thi Mai Hung, Vu Thi Lan Huong, Thanh Long Pham, Jos ́e Miguel Ponciano, Gabriela Hamerlinck, Duc Anh Dang, Michael H. Norris, Jason K. Blackburn

Article first published online: 7 September 2023

DOI: https://doi.org/10.1016/j.meegid.2023.105496

ABSTRACT: Bacillus anthracis, the bacterial cause of anthrax, is a zoonosis affecting livestock and wildlife often spilling over into humans. In Vietnam, anthrax has been nationally reportable since 2015 with cases occurring annually, mostly in the northern provinces. 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 from the animal. Early epidemiological investigations suggested a single animal was involved in all exposures. Five B. anthracis isolates were recovered from human clinical cases along with one from the buffalo hide, another from associated maggots, and one from soil at the carcass site. The isolates were whole genome sequenced, allowing global, regional, and local molecular epidemiological analyses of the outbreak strains. All recovered B. anthracis belong to the A.Br.001/002 lineage based on canonical single nucleotide polymorphism analysis (canSNP). Although not previously identified in Vietnam, this lineage has been identified in the nearby countries of China, India, Indonesia, Thailand, as well as Australia. A twenty-five marker multi-locus variable number tandem repeat analysis (MLVA-25) was used to investigate the relationship between human, soil, and buffalo strains. Locally, four MLVA-25 genotypes were identified from the eight isolates. This level of genetic diversity is unusual for the limited geography and timing of cases and differs from past literature using MLVA-25. The coupled spatial and phylogenetic data suggest this outbreak originated from multiple, likely undetected, animal sources. These findings were further supported by local news reports that identified at least two additional buffalo deaths beyond the initial animal sampled in response to the human cases. Future outbreak response should include intensive surveillance for additional animal cases and additional molecular epidemiological traceback to identify pathogen sources.

Read the full publication in Infection, Genetics and Evolution.