Pathogenesis of Hazara orthonairovirus infection in type I interferon receptor-deficient mice and resolution of disease following 4'-fluorouridine therapy.

The Crimean-Congo hemorrhagic fever orthonairovirus (CCHFV) is a widespread tick-borne agent that causes severe disease in humans. Its expanding geographic range poses a significant public health threat. Although animal models for CCHFV infection have been developed, the requirement for maximum biosafety level 4 facilities limits the development of countermeasures against the virus. Hazara virus (HAZV) is a closely related orthonairovirus that can be handled in biosafety level 2 containment and has been proposed as a prototype virus for the Orthonairovirus genus to facilitate early-stage countermeasure development against the highly pathogenic CCHFV. Here, we present a detailed characterization of HAZV infection in mice deficient in type I interferon signaling (Ifnar-/-), providing insights into the natural history of orthonairovirus disease and highlighting similarities and differences between the HAZV and previously described CCHFV mouse infection models. We then utilized the HAZV mouse infection model to test the efficacy of a promising broad-spectrum antiviral, 4'-fluorouridine (4'-FlU). Our findings demonstrate that delayed intervention with orally administered 4'-FlU can rescue clinically ill mice following challenge with a lethal dose of HAZV, supporting further investigations of the compound's efficacy in CCHF disease models. IMPORTANCE: The Crimean-Congo hemorrhagic fever orthonairovirus poses a significant public health threat, underscored by the expansion of Hyalomma genus tick vectors and the lack of clinically proven therapeutic options. The related Hazara orthonairovirus (HAZV), which has not been reported to cause human disease, has been proposed as a prototype virus for the Nairoviridae family. Here, we characterize in detail the mouse model of lethal HAZV disease to gain further insight into nairovirus pathogenesis and use the model for the preclinical development of a promising broad-spectrum antiviral drug candidate, 4'-fluorouridine (4'-FlU). Our findings highlight the value of HAZV as a surrogate for proof-of-concept studies supporting early-stage antiviral drug studies and the therapeutic potential of 4'-FlU for the treatment of often-fatal Crimean-Congo hemorrhagic fever.