Molnupiravir Suppresses Bourbon Virus Pathology in Mice Models

Study Background and Research Question

Bourbon virus (BRBV) is an emerging tick-borne pathogen of the genus Thogotovirus (family Orthomyxoviridae), first identified in Kansas, USA, in 2014. While only a handful of human cases have been formally reported, BRBV infection can result in severe or even fatal disease, and its true prevalence may be underrecognized due to limited diagnostic awareness. The virus is primarily transmitted by the lone star tick (Amblyomma americanum), which is widely distributed in the central, eastern, and southern United States. To date, there are no approved antiviral therapies or vaccines for BRBV infection, underscoring the urgent need for effective medical countermeasures against this and related tick-borne RNA viruses.

The primary research question addressed by Bamunuarachchi et al. (2025) was whether nucleoside analogues, specifically molnupiravir, could inhibit BRBV replication and mitigate associated disease pathology in vivo.

Key Innovation from the Reference Study

The standout innovation of this work lies in its preclinical demonstration that molnupiravir, an orally bioavailable ribonucleoside analogue previously approved for other RNA virus infections, can protect against lethal BRBV challenge in a mouse model. Prior to this study, no antiviral agents had been evaluated against BRBV infection in vivo. By establishing both prophylactic and therapeutic efficacy of molnupiravir, the authors provide the first evidence to support its potential repurposing for tick-borne orthomyxovirus infections.

Methods and Experimental Design Insights

The authors conducted a systematic evaluation of nucleoside analogue antivirals in vitro and in vivo. Initial cell culture screens assessed the ability of several compounds to inhibit BRBV replication. Molnupiravir was identified as a lead candidate due to its robust inhibitory activity against BRBV production in vitro.

For in vivo experiments, the team used type I interferon receptor knockout (Ifnar1-/-) mice, which are highly susceptible to BRBV-induced disease and represent a stringent model for antiviral efficacy. Two experimental paradigms were tested:

• Pre-exposure prophylaxis: Molnupiravir was administered prior to viral challenge.
• Therapeutic intervention: Molnupiravir treatment was initiated 24 or 48 hours after infection.

Clinical endpoints included survival, weight loss, clinical disease scores, viral burden in tissues, hematological parameters (notably thrombocytopenia), and histopathological analysis of spleen and liver tissue.

Core Findings and Why They Matter

Molnupiravir significantly inhibited BRBV replication in cultured cells. In the mouse model, prophylactic administration of molnupiravir provided complete protection against lethal BRBV challenge, as reflected by improved survival rates, prevention of severe weight loss, and suppression of clinical signs such as thrombocytopenia. Importantly, even when treatment began 24 or 48 hours after infection, molnupiravir reduced disease severity, viral loads in tissues, and organ pathology.

Immunological analysis revealed that molnupiravir treatment preserved T-cell (CD4+, CD8+) and follicular B-cell populations in the spleen, indicating mitigation of virus-induced lymphoid depletion. These results collectively establish that molnupiravir can both prevent and treat BRBV infection in a stringent animal model, supporting its potential clinical utility for this neglected, emerging pathogen.

Comparison with Existing Internal Articles

This study aligns with the broader body of literature on nucleoside analogue antivirals for RNA viruses. For example, Remdesivir (GS-5734) is characterized as a precision inhibitor of RNA-dependent RNA polymerase (RdRp), with validated efficacy against coronaviruses and Ebola virus. Both molnupiravir and remdesivir act by introducing mutations or chain termination during viral RNA synthesis, though their precise mechanisms and viral targets differ.

Other internal reviews, such as Remdesivir (GS-5734): RNA Polymerase Inhibition for Coronavirus and Ebola Virus, highlight remdesivir's nanomolar potency in SARS-CoV and MERS-CoV inhibition, paralleling the high efficacy of molnupiravir seen here against BRBV. These comparisons underscore the growing importance of nucleoside analogues as a class for broad-spectrum antiviral research, particularly as new zoonotic RNA viruses continue to emerge.

Protocol Parameters

• In vivo antiviral efficacy: Use Ifnar1-/- mice for high-susceptibility modeling of tick-borne RNA virus infection.
• Prophylactic administration: Begin molnupiravir dosing prior to viral challenge to assess pre-exposure protection.
• Therapeutic intervention: Initiate antiviral treatment 24-48 hours post-infection to evaluate clinical rescue potential.
• Clinical endpoints: Monitor survival, weight, clinical scores, platelet counts, and perform viral load quantification in target organs (e.g., liver, spleen).
• Histopathology: Assess lymphoid and hepatic tissue for virus-induced pathology and therapeutic rescue effects.

Limitations and Transferability

While the findings provide compelling preclinical evidence for molnupiravir as a candidate therapeutic for BRBV, several limitations merit consideration. The study relied on immunodeficient mouse models, which may not fully recapitulate human immune responses or pharmacokinetics. The number of human BRBV cases remains low, making large-scale clinical investigation challenging. Furthermore, the efficacy of molnupiravir against other thogotoviruses or in natural tick transmission scenarios was not addressed. Thus, while the results are promising, translation to clinical use will require further validation in diverse models and eventual human studies.

Research Support Resources

For researchers designing similar workflows targeting other emerging RNA viruses, validated nucleoside analogues such as Remdesivir (GS-5734) (SKU B8398) are available and offer benchmark performance in coronavirus antiviral research, Ebola virus treatment research, and beyond. Remdesivir has demonstrated low-nanomolar EC50 values for SARS-CoV and MERS-CoV inhibition and can provide a reference point for experimental design and comparative analysis. For detailed mechanisms and further protocol guidance, internal resources such as Remdesivir (GS-5734): Antiviral Nucleoside Analogue for RNA Viruses may be consulted. APExBIO's Remdesivir is suitable for advanced in vitro and in vivo studies requiring reproducible antiviral activity benchmarks.