Pathologic and genomic characterization of an outbreak of anthrax-like disease caused by <i>Bacillus tropicus</i> (formerly atypical <i>Bacillus cereus</i> ) in red kangaroos ( <i>Macropus rufus</i> )

Atypical <i>Bacillus cereus</i> strains (currently classified as <i>Bacillus tropicus</i>) capable of causing anthrax-like disease in humans and animals have emerged in the last 2 decades. These emerging strains are characterized by the acquisition of virulence plasmids pBCXO1 and pBC210, which are homologous to the pXO1 and pXO2 virulence plasmids of <i>Bacillus anthracis</i>, the agent of anthrax. The aim of this study was to describe the gross, histologic, microbiologic, and molecular features of an outbreak of anthrax-like septicemia caused by <i>B. tropicus</i> in red kangaroos (<i>Macropus rufus</i>). Three red kangaroos from a wild animal preserve were found dead with no premonitory clinical signs. No changes in husbandry were reported prior to the outbreak. The peracute disease process was characterized by severe splenomegaly, associated with fibrinonecrotizing splenitis in all affected animals, in addition to segmental suppurative enteritis in 2 kangaroos and cutaneous excoriations, with underlying necrotizing cellulitis and lymphadenitis, in 1 kangaroo. Numerous intralesional, gram-positive and capsulated bacilli were identified as the formerly known <i>B. cereus</i> group via bacteriologic culture. Whole-genome sequencing from one of the bacterial isolates (designated 11844) revealed numerous anthrax-like virulence factors, including the pBCXO1 and pBC210 virulence plasmids. This isolate also had a close phylogenetic relationship with other <i>B. tropicus</i> strains carrying these virulence plasmids, including <i>B. tropicus</i> (formerly <i>B. cereus)</i> G9241. This is the first report of <i>B. tropicus</i> leading to anthrax-like disease in kangaroos. This disease form carries significant public health risks due to potential zoonotic transmission.