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

Archival and Newly Isolated Historical Bacillus anthracis Strains Populate the Deeper Phylogeny of the A.Br.075(Sterne) Clade

The anthrax pathogen Bacillus anthracis can remain dormant as spores in soil for many years. This applies to both natural foci and to sites of anthropogenic activity such as tanneries, abattoirs, or wool factories. The A.Br.075 (A-branch) clade (also known as A.Br.Sterne) is prominent not only because it comprises several outbreak strains but even more so because spore preparations of its namesake, the Sterne strain, are counted among the most utilized anthrax animal vaccines. In this study, we genome-sequenced and analyzed 56 additional B. anthracis isolates of the A.Br.075 clade. Four of these we recently retrieved from soil samples taken from a decades-long abandoned tannery. The other 52 strains originated from our archival collection from the 20th century. Notably, the extended phylogeny of the A.Br.075 clade indicated that many of the newly added chromosomes represent basal members, some of which are among the most basal strains from this lineage. Twelve new strains populate a very deep-branching lineage we have named A.Br.Ortho-Sterne (also known as A.Br.076). A further 11 isolates amend the clade named A.Br.Para-Sterne (A.Br.078). Finally, some of the terminal clusters of the clade named A.Br.Eu-Sterne appear to be replete with (near) identical isolates, possibly a result of widespread use of the Sterne vaccine and of its re-isolation from vaccination-related animal anthrax outbreaks. From the accrued new phylogenetic information, we designed and tested a variety of new SNP-PCR assays for rapid and facile genotyping of unassigned B. anthracis genomes. Lastly, the successful isolation of live B. anthracis from a long-abandoned tannery reemphasizes the need for continued risk awareness of such sites.

Toxin and capsule production by Bacillus cereus biovar anthracis influence pathogenicity in macrophages and animal models

Bacillus cereus biovar anthracis (Bcbva) causes anthrax-like disease in animals, particularly in the non-human primates and great apes of West and Central Africa. Genomic analyses revealed Bcbva as a member of the B. cereus species that carries two plasmids, pBCXO1 and pBCXO2, which have high sequence homology to the B. anthracis toxin and polyglutamate capsule encoding plasmids pXO1 and pXO2, respectively. To date, only a few studies have investigated the effect of variations in Bcbva sporulation, toxin, and capsule synthesis on animal and macrophage pathogenicity compared to B. anthracis, therefore more research is needed to gain a better understanding of the pathogenesis of this emerging infection. Here, we report that Bcbva can multiply and vegetatively survive on nutrient-rich media for a minimum of six days while generating spores. Sporulation of Bcbva occurred faster and more extensively than B. anthracis Ames. Bcbva tended to secrete less protective antigen (PA) than B. anthracis Ames when cultured in growth medium. We found Bcbva produced a substantially higher amount of attached poly-ƴ-D-glutamic acid (PDGA) capsule than B. anthracis Ames when grown in medium supplemented with human serum and CO2. In a phagocytosis assay, Bcbva spores showed reduced internalization by mouse macrophages compared to B. anthracis Ames. Our research demonstrated that Bcbva is more virulent than B. anthracis Ames using two in vivo models, Galleria mellonella larvae and guinea pigs. Following that, the efficacy of the veterinary vaccine Sterne strain 34F2 against anthrax-like disease was assessed in guinea pigs. Sterne vaccinated guinea pigs had significantly increased anti-PA titers compared to the unvaccinated control group. Toxin neutralizing antibody titers in vaccinated guinea pigs correlated with anti-PA titers. This indicates the Sterne vaccine provides adequate protection against Bcbva infection in laboratory animals.

Spatial clusters of human and livestock anthrax define high-risk areas requiring intervention in Lao Cai Province, Vietnam 1991-2022

Anthrax, a widespread zoonosis in low and middle-income countries with low disease awareness and insufficient livestock vaccination coverage, has been known in Lao Cai Province in northern Vietnam for years before its apparent absence in 2009, which requires investigation as this infection is frequently reported from neighbouring provinces and countries. We aimed to describe the seasonal patterns of anthrax (1991-2008), compare livestock anthrax vaccine coverage to disease occurrence (1991- 2022), and delineate the high-risk areas to inform local disease surveillance in the province. We illustrated the seasonal pattern of anthrax and provided a comparison between livestock vaccine coverage and disease occurrence by purely spatial SaTScan (Poisson model, 25% population at risk) to detect spatial clusters of human and livestock anthrax using population derived from zonal statistics routines. The number of cases, crude cumulative incidence, and spatial clusters of human and livestock anthrax were mapped in QGIS. Results indicate peak anthrax incidence from May to October. Buffalo, domestic cattle, and horses accounted for 75% of total animal cases. Horse anthrax was more common in Lao Cai than in its neighbours and often occurred in years with human mortality. Vaccination covered less than 30% of the livestock population. We found an apparent pattern where anthrax was controlled from 1998-2003 with higher vaccine coverage (>20%) and identified spatial clusters of human and livestock anthrax in Muong Khuong, Bao Thang, and Bac Ha districts of Lao Cai. The local public health and veterinary agencies are recommended to revisit the high-risk areas and communicate with neighbouring provinces for a regional approach to anthrax surveillance and control.