Animal modelling with the Francisella tularensis subspecies holarctica strain OR96-0246

Tularemia is a zoonotic disease caused by Francisella tularensis. Most human cases are caused by F. tularensis ssp. tularensis (type A) or F. tularensis ssp. holarctica (type B), with the former considered more virulent. For this reason, type A isolates are often the benchmark for the testing of new vaccines or antibiotics. However, both subspecies cause considerable disease and can differ in their responsiveness to medical countermeasures. Accordingly, there is a need to identify and characterize representative type B isolates that are available to qualified research institutions to ensure the development of future vaccines or antibiotics is efficacious against both subspecies. The type B isolate OR96-0246 was identified as a strain that can address this need and was subsequently characterized. For in vitro characterization, the OR96-0246 strain was examined for growth in media and for its ability to form biofilm. As the LPS is an essential virulence factor, the O-antigen was characterized through western analysis. For future medical countermeasure testing for biodefence concerns, pneumonic challenges with animal modelling would be required. Therefore, using the OR96-0246 strain, we implemented animal models that encompassed BALB/c mice, Fischer 344 rats and cynomolgus macaques. Mice were challenged via intranasal instillation with varying doses of OR96-0246, and the LD₅₀ was determined to be 1 c.f.u. We progressed to Fischer 344 rats, which are a better-suited rodent model to gauge vaccine efficacy. When challenging the rats by whole body aerosolization with various doses of OR96-0246, the LD₅₀ was determined to be 138 c.f.u. Finally, a staircase challenge design was applied to three cynomolgus macaques, each receiving a different aerosolized dose of OR96-0246 to determine an estimated LD₅₀ for non-human primates (NHPs). Two out of the three NHPs succumbed to the challenge. The animal that received the lowest dose (2.1×10⁴ c.f.u.) survived but did demonstrate clinical signs of infection. Samples from the challenged rats and NHPs were collected for histopathology characterization. Generally, the pathological changes observed in both models were similar, consisting primarily of multifocal bronchopneumonia in the lung and necrotic lesions in the spleen. This animal model development with type B strains of F. tularensis will be essential to properly evaluate new antimicrobials and vaccines to protect against tularemia.