Toscana virus - an emerging Mediterranean arbovirus transmitted by sand flies

Abstract

Toscana virus (TOSV) is an emerging arthropod-borne virus (arbovirus) of medical importance that is increasing its range across much of the Mediterranean Basin, Europe and the Middle East. Transmitted by Phlebotomus spp. sand flies, it is the most clinically relevant sand fly-borne phlebovirus. Initially isolated in the Tuscany region of Central Italy, it has now been detected in multiple countries that surround this geographical area. Infection of the vertebrate host can cause fever and neurological disease, following the dissemination of the virus to the brain. The prevalence is high in some regions, with a notable percentage of individuals showing seroconversion. TOSV can be a leading cause of acute meningitis and encephalitis (AME) during the summer months. In this comprehensive review, we will focus on several key topics. We discuss how TOSV has spread to establish outbreaks of infection in both humans and animals around the Mediterranean and the wider region. Clinical aspects of TOSV infection in humans are described, along with the best standards in diagnosis. Finally, we focus our discussion on the role of the sand fly vector, describing their biology, vector competency, implications for putative vertebrate reservoirs, the effect of the climate emergency on sand fly distribution and the putative role that sand fly-derived salivary factors may have on modulating host susceptibility to TOSV infection.

Keywords: arbovirus; diagnostics; epidemiology; pathogenesis; vector-borne disease.

Fig. 1.. Genome organisation (a) and replication cycle (b) of TOSV. (a) TOSV contains a tripartite single-stranded RNA genome. L segment (negative-sense) encodes the viral component of the RNA-dependent RNA polymerase (RdRp). M segment (negative-sense) encodes Gn/Gc and a non-structural protein (NSm). S segment encodes (ambisense) the nucleocapsid (N) protein and a non-structural protein (NSs). (b) (1) Phleboviruses, including TOSV, use virus-encoded glycoproteins, Gn and Gc, to bind cell surface molecules DC-SIGN, L-SIGN and heparan sulphate (glycosaminoglycan). This is similar to the related phenuiviruses that also target DC-SIGN. Receptor use is likely cell specific, e.g., with DC-SIGN mediated entry occurring in dendritic cells (DCs). (2) Virus enters the cell via receptor-mediated endocytosis. (3) Once internalized, the viral particles move through early and late endosomes. (4) In late endosomes, acidification induces the membrane fusion activity of the Gc protein with the endosomal membrane. This fusion triggers the encapsidation of the viral genome, and virus RdRps are released into the cytoplasm, where primary transcription and replication occur. (5) N interactions with RdRp allow access to the ribonucleoprotein (RNP), which serves as a template for the transcription of new mRNA. Following the translation of the viral mRNAs and genome replication (6–7), viral Gn/Gc are cleaved by host cell proteases in the endoplasmic reticulum, allowing Gn–Gc glycoprotein heterodimers to reach the Golgi apparatus, (8) where viral assembly occurs. (9) Newly formed virions decorated with Gn and Gc in the Golgi bud via vesicles to the plasma membrane and (10) are released from the host cell by exocytosis. Replication within arthropod cells, although likely similar, is not well studied and requires definition. The figures were created on BioRender.com.

Fig. 2.. Phylogenetic tree of arboviruses of medical and veterinary importance. Maximum likelihood tree of the RdRp aligned using MAFFT and reconstructed using IQTREE2. Tips are annotated with the virus name, the species name in brackets and the arthropod vector. Values at the nodes represent the bootstrap support, only values above 70 are shown and 100 bootstrap support is represented with a *. The tree is mid-point rooted, the branch lengths are drawn to scale and the scale bar represents the number of amino acid changes per site.