In recent decades, increases in temperature and tropical rainfall have facilitated the spread of mosquito species into temperate zones. Mosquitoes are vectors for many viruses, including West Nile virus (WNV) and dengue virus (DENV), and pose a serious threat to public health. This review covers most of the current knowledge on the mosquito species associated with the transmission of WNV and DENV and their geographical distribution and discusses the main vertebrate hosts involved in the cycles of WNV or DENV. It also describes virological and pathogenic aspects of WNV or DENV infection, including emerging concepts linking WNV and DENV to the reproductive system. Furthermore, it provides an epidemiological analysis of the human cases of WNV and DENV reported in Europe, from 1 January 2018 to 31 December 2023, with a particular focus on Italy. The first autochthonous cases of DENV infection, with the most likely vector being Aedes albopictus, have been observed in several European countries in recent years, with a high incidence in Italy in 2023. The lack of treatments and effective vaccines is a serious challenge. Currently, the primary strategy to prevent the spread of WNV and DENV infections in humans remains to limit the spread of mosquitoes.

The SARS-CoV-2 protease (3CLpro) is one of the key targets for the development of efficacious drugs for COVID-19 treatment due to its essential role in the life cycle of the virus and exhibits high conservation among coronaviruses. Recent studies have shown that flavonoids, which are small natural molecules, have antiviral activity against coronaviruses (CoVs), including SARS-CoV-2. In this study, we identified the docking sites and binding affinity of several natural compounds, similar to flavonoids, and investigated their inhibitory activity towards 3CLpro enzymatic activity. The selected compounds were then tested in vitro for their cytotoxicity, for antiviral activity against SARS-CoV-2, and the replication of other coronaviruses in different cell lines. Our results showed that Baicalein (100 μg/mL) exerted strong 3CLpro activity inhibition (>90%), whereas Hispidulin and Morin displayed partial inhibition. Moreover, Baicalein, up to 25 μg/mL, hindered >50% of SARS-CoV-2 replication in Vero E6 cultures. Lastly, Baicalein displayed antiviral activity against alphacoronavirus (Feline-CoV) and betacoronavirus (Bovine-CoV and HCoV-OC43) in the cell lines. Our study confirmed the antiviral activity of Baicalein against SARS-CoV-2 and demonstrated clear evidence of its pan-coronaviral activity.

Contradictory results have been reported regarding interferon (IFN) lambda (λ1–3) and IFN gamma (γ) production in COVID-19 patients. To gain insight into the roles played by these IFNs in SARS-CoV-2 infection, IFNλ1–3 and IFNγ mRNA expression was evaluated in peripheral blood mononuclear cells (PBMCs) (n = 32) and in cells of paired bronchoalveolar lavages (BALs) (n = 12). Lower IFNλ1–3 values (p < 0.001 for IFNλ1 and 3 and p = 0.013 for IFNλ2) in the PBMCs of severely ill patients were found compared to healthy donors (n = 15). Reduced levels of IFNγ were also detected in patients’ PBMCs (p < 0.01) and BALs (p = 0.041) compared to healthy donors. The presence of secondary bacterial infections was associated with decreased IFNλ amounts in PBMCs (p = 0.001, p = 0.015 and p = 0.003, respectively) but increased concentrations of IFNλ3 (p = 0.022) in BALs. Patients with alterations in C-reactive protein, lactate dehydrogenase and D-dimer levels had decreased IFNλ1 and 3 (p = 0.003 and p < 0.001) and increased IFNγ (p = 0.08) in PBMCs. Analyzing Toll-like receptors (TLRs) involved in IFN production, we found that TLR3 was highly expressed (p = 0.033) in patients with bacterial superinfections, while TLR7 and 8 (p = 0.029 and p = 0.049) were reduced in BALs of deceased patients. Overall, severe COVID-19 might be characterized by dysregulation in IFNγ, IFNλ and TLR3, 7 and 8 production.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is recognized by pattern recognition receptors (PRRs), which play a vital role in triggering innate immune responses such as the production of type I and III interferons (IFNs). While modest PRR activation helps to defend against SARS-CoV-2, excessive or sustained activation can cause harmful inflammation and contribute to severe Coronavirus Disease 2019 (COVID-19). Altered expression of Toll-like receptors (TLRs), which are among the most important members of the PRR family members, particularly TLRs 2, 3, 4, 7, 8 and 9, has been strongly linked to COVID-19 severity. Furthermore, retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), collectively known as RLRs (RIG-I-like receptors), act as sensors that detect SARS-CoV-2 RNA. The expression of these receptors, as well as that of different DNA sensors, varies in patients infected with SARS-CoV-2. Changes in PRR expression, particularly that of TLRs, cyclic GMP-AMP synthase (cGAS), and the stimulator of interferon genes (STING), have also been shown to play a role in the development and persistence of long COVID (LC). However, SARS-CoV-2 has evolved strategies to evade PRR recognition and subsequent signaling pathway activation, contributing to the IFN response dysregulation observed in SARS-CoV-2-infected patients. Nevertheless, PRR agonists and antagonists remain promising therapeutic targets for SARS-CoV-2 infection. This review aims to describe the PRRs involved in recognizing SARS-CoV-2, explore their expression during SARS-CoV-2 infection, and examine their role in determining the severity of both COVID-19 and long-term manifestations of the disease. It also describes the strategies developed by SARS-CoV-2 to evade PRR recognition and activation. Moreover, given the considerable interest in modulating PRR activity as a novel immunotherapy approach, this review will provide a description of PRR agonists and antagonists that have been investigated as antiviral strategies against SARS-CoV-2. This review aims to explore the complex interplay between PRRs and SARS-CoV-2 in depth, considering its implications for prognostic biomarkers, targeted therapeutic strategies and the mechanistic understanding of long LC. Additionally, it outlines future perspectives that could help to address knowledge gaps in PRR-mediated responses during SARS-CoV-2 infection.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is recognized by pattern recognition receptors (PRRs), which play a vital role in triggering innate immune responses such as the production of type I and III interferons (IFNs). While modest PRR activation helps to defend against SARS-CoV-2, excessive activation can cause harmful inflammation and contribute to severe Coronavirus Disease 2019 (COVID-19). Altered expression of Toll-like receptors (TLRs), which are among the most important PRR family members, particularly TLRs 2, 3, 4, 7, 8 and 9, has been strongly linked to disease severity. Furthermore, retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) — collectively known as RLRs (RIG-I-like receptors) — act as sensors that detect SARS-CoV-2 RNA. The expression of these receptors, as well as that of different DNA sensors, varies in patients infected with SARS-CoV-2. Changes in PRR expression, particularly that of TLRs, cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING), have also been shown to play a role in the development and persistence of long COVID (LC). However, SARS-CoV-2 has evolved strategies to evade PRR recognition and subsequent signaling pathway activation, contributing to the IFN response dysregulation observed in SARS-CoV-2-infected patients. Nevertheless, PRR agonists and antagonists remain promising therapeutic targets for SARS-CoV-2 infection. This review aims to describe the PRRs involved in recognizing SARS-CoV-2, explore their expression during SARS-CoV-2 infection and examine their role in determining the severity of both acute and long-term manifestations of the disease. It also describes the strategies developed by SARS-CoV-2 to evade PRR recognition and activation. Finally, given the considerable interest in modulating PRR activity as a novel immunotherapy approach, this review will describe PRR agonists and antagonists that have been investigated as antiviral strategies against SARS-CoV-2.