Oropouche virus (OROV) is an emerging arbovirus of growing public health concern, with increasing incidence and geographic spread. Since its discovery in 1955, OROV has caused multiple outbreaks in South and Central America, with a new introduction in Cuba since May 2024. Recent travel-related cases in Europe and the Americas underscore its potential for global dissemination. Assessing vector competence outside endemic regions is critical in the context of global travel and climate change. We evaluated the vector competence of three mosquito species commonly found in Europe— Culex (Cx.) pipiens, Aedes (Ae.) albopictus, and Anopheles (An.) atroparvus —using two OROV strains: the prototype TRVL9760 (1955, Trinidad and Tobago) and a recent isolate OROV-IRCCS-SCDC_1/2024 (2024, imported from Cuba to Italy). Mosquitoes were orally infected and examined at 7- and 14-days post-infection. We assessed infection (body), dissemination (peripheral tissues), and transmission potential (saliva) by measuring infectious virus particles using the gold standard focus-forming assays. Our findings show that Cx. pipiens and An. atroparvus were not susceptible to infection or did not allow transmission with OROV. In Ae. albopictus , low infection rates were observed: 6.7% of mosquitoes showed infection at day 7 with the prototype strain, and 3.1% at day 14 with OROV-IRCCS-SCDC_1/2024. All infected mosquitoes showed viral dissemination, but none had infectious virus in their saliva, indicating low risk for transmission. These results confirm limited vector competence of European mosquito species for OROV and emphasize the importance of continued entomological surveillance to inform future risk assessments.

Background: Mosquito saliva plays a key role in arbovirus transmission and pathogenesis. It was shown that saliva contains several molecules that are essential for blood feeding. Recently, bacteria were also reported to be present in the saliva of Aedes albopictus and Anopheles mosquitoes. Nevertheless, information on the bacterial communities in Aedes and Culex saliva is still scarce. Methods: This study isolated and identified culturable fungal and bacterial colonies from saliva harvested from Aedes aegypti (laboratory strain) and Culex pipiens (field-collected) mosquitoes. 16S metagenomic sequencing was performed to identify bacterial communities in saliva and mosquito organs. Furthermore, it was assessed how these microbial communities were affected upon blood feeding and upon oral treatment with antibiotics and an antifungal drug. Results: The fungal species Penicillium crustosum was identified in mosquito saliva. Culturable bacteria detected in mosquito saliva included Serratia marcescens , Serratia nematodiphila , Enterobacter spp., and Klebsiella spp., which were previously identified as mosquito or insect endosymbionts in the midgut or other organs. Analysis with 16S metagenomics showed that bacterial communities in saliva were more diverse than those in the midgut. Blood feeding did not affect the fungal or bacterial load in mosquito saliva. Oral treatment of adult mosquitoes with antibiotics or an antifungal drug resulted in a significant reduction of bacteria or fungi present in the mosquito saliva. Notably, co-incubation of the mosquito-borne Semliki Forest virus with saliva from antibiotic- or antifungal-treated mosquitoes triggered a decrease in viral infection in human skin fibroblasts compared with nontreated saliva. Conclusions: These findings indicate that bacteria and fungi can be present in mosquito saliva and provide a foundation for further exploration of the impact of salivary fungi and bacteria on both vector competence and arbovirus infection in the mammalian host. Graphical

Background: Alphaviruses, including chikungunya virus (CHIKV), pose a significant global health threat, yet specific antiviral therapies remain unavailable. Methods: We evaluated combinations of three oral directly acting antiviral drugs (sofosbuvir (SOF), molnupiravir (MPV), and favipiravir (FAV)), which are approved for other indications, against CHIKV, Semliki Forest virus (SFV), Sindbis virus (SINV), and Venezuelan Equine Encephalitis virus (VEEV) in vitro and in vivo . We assessed antiviral efficacy in human skin fibroblasts and liver cells, as well as in a mouse model of CHIKV-induced arthritis. Findings: In human skin fibroblasts, synergistic antiviral effects were observed for combinations of MPV + SOF and FAV + SOF against CHIKV, and for FAV + SOF against SFV. In human liver cells, FAV + MPV conferred additive to synergistic activity against VEEV and SINV, while SOF synergized with FAV against SINV. In mice, MPV improved CHIKV-induced foot swelling and reduced systemic infectious virus titres. Combination treatment with MPV and SOF significantly reduced swelling and infectious virus titres compared to monotherapies of each drug. Sequencing of CHIKV RNA from joint tissue revealed that MPV caused dose- dependent increases in mutations in the CHIKV genome. Upon combination therapy of MPV with SOF, the number of mutations was significantly lower compared to monotherapy with several higher doses of MPV. Interpretation: Combining these approved oral nucleoside analogues confers potent suppression of multiple alphaviruses in vitro and in vivo with enhanced control of viral genetic evolution in face of antiviral pressure. These drug combinations may ultimately lead to the development of potent combinations of pan-family alphavirus inhibitors. Funding: This work was supported by a PhD fellowship granted to S.V. by the Research Foundation – Flanders (FWO) (11D5923N). L.D.C. was also supported by Research Foundation – Flanders (FWO) PhD fellowship (11L1325N). Dr. Polyak and Schiffer are partially supported by R01AI121129. Research in Context: Evidence before this study: Alphaviruses such as chikungunya virus (CHIKV), Sindbis virus (SINV), and Venezuelan Equine Encephalitis virus (VEEV) are a major threat for global health. Alphaviruses are responsible for debilitating diseases with major public health implications, yet no antiviral drugs are currently approved for treating these virus infections. Existing treatment options are limited to supportive care and are unlikely to provide protection against future outbreaks of other alphaviruses. Previous studies have shown that oral approved nucleoside analogues such as favipiravir (FAV), molnupiravir (MPV), and sofosbuvir (SOF) have antiviral activity against certain RNA viruses, including alphaviruses. However, systematic in vivo evaluations of these drugs and testing of drug combinations in both in vitro and in vivo settings are limited. Added value of this study: This study provides a comprehensive evaluation of combinations of FAV, MPV and SOF against multiple alphaviruses in two human cell lines and a CHIKV mouse model. We demonstrate that certain combinations of these drugs confer synergistic antiviral activity, effectively suppressing CHIKV, SFV, SINV, and VEEV replication in vitro . Moreover, in vivo , we show for the first time that MPV treatment results in reduced CHIKV-induced foot swelling and systemic virus replication. Combining MPV with SOF enhances antiviral activity in mice as compared to monotherapy. By sequencing the viral genome, we show that MPV increases the number of mutations in a dose-dependent manner. Combination therapy of MPV and SOF reduces the number of mutations compared to higher doses of MPV. These findings highlight the potential of nucleoside analogue combinations as a promising therapeutic strategy against alphavirus infections. Implications of all the available evidence: The results of this study suggest that combination therapy with approved nucleoside analogues could provide an effective treatment strategy for alphavirus infections. The observed increased efficacy of drug combinations supports the potential for dose optimization to enhance efficacy while reducing toxicity and development of resistance. Future research should focus on clinical evaluation of these drug combinations, pharmacokinetic studies, and further exploration of their impact on viral evolution. Given the expanding geographical distribution of alphaviruses and the lack of available treatments, these findings provide a foundation for developing pan-alphavirus antiviral therapies that could improve patient outcomes and global outbreak preparedness.

Background/Objectives: Mosquito-borne viruses cause substantial global morbidity and mortality. Mosquito saliva, secreted during biting, facilitates blood feeding and can enhance virus infection. Here, we evaluated whether combining virus-specific antigens with mosquito salivary protein-derived peptides improves protection in Zika virus (ZIKV) and chikungunya virus (CHIKV) mouse models. Methods: Peptides from Aedes and Culex salivary proteins were selected based on conservation, predicted T-cell reactivity, and cellular and humoral immunogenicity in mice. Mice were immunised subcutaneously with a prime and boost two weeks apart using adjuvanted salivary peptides (SAL), commercial ZIKV- or CHIKV-like particles (VLPs), or salivary peptides combined with the relevant VLPs. Vaccine doses contained 0.5 µg VLPs and/or 10 nmol of each salivary peptide, formulated with Montanide ISA-51; when combined, VLPs and SAL were administered on opposite flanks. Two weeks after boosting, mice were challenged with ZIKV or CHIKV plus Aedes aegypti saliva. Challenge teams were blinded to group allocation until data analyses were complete. Results: SAL enhanced the protective effect of VLP vaccination compared with VLPs alone. In the CHIKV model, CHIKV VLP + SAL prevented joint swelling and reduced infectious virus in the inoculated ankle and viral RNA in the contralateral ankle. In the ZIKV model, ZIKV VLP + SAL reduced viral RNA in skin and brain tissue and improved survival. Conclusions: Mosquito salivary peptide antigens improved the protective efficacy of VLP vaccines against ZIKV and CHIKV in mouse models. Salivary antigens may provide a complementary vaccine component for multiple arboviruses transmitted by Aedes and Culex mosquitoes.

Oropouche virus (OROV) is an emerging arbovirus of growing public health concern, with increasing incidence and geographic spread. Since its discovery in 1955, OROV has caused multiple outbreaks in South and Central America, with a new introduction in Cuba since May 2024. Recent travel-related cases in Europe and the Americas underscore its potential for global dissemination. Assessing vector competence outside endemic regions is critical in the context of global travel and climate change. We evaluated the vector competence of three mosquito species commonly found in Europe— Culex (Cx.) pipiens, Aedes (Ae.) albopictus, and Anopheles (An.) atroparvus —using two OROV strains: the prototype TRVL9760 (1955, Trinidad and Tobago) and a recent isolate OROV-IRCCS-SCDC_1/2024 (2024, imported from Cuba to Italy). Mosquitoes were orally infected and examined at 7- and 14-days post-infection. We assessed infection (body), dissemination (peripheral tissues), and transmission potential (saliva) by measuring infectious virus particles using the gold standard focus-forming assays. Our findings show that Cx. pipiens and An. atroparvus were not susceptible to infection or did not allow transmission with OROV. In Ae. albopictus , low infection rates were observed: 6.7% of mosquitoes showed infection at day 7 with the prototype strain, and 3.1% at day 14 with OROV-IRCCS-SCDC_1/2024. All infected mosquitoes showed viral dissemination, but none had infectious virus in their saliva, indicating low risk for transmission. These results confirm limited vector competence of European mosquito species for OROV and emphasize the importance of continued entomological surveillance to inform future risk assessments.