The Flavivirus genus includes a large number of medically relevant pathogens that cycle between humans and arthropods. This host alternation imposes a selective pressure on the viral population. Here, we found that dengue virus, the most important viral human pathogen transmitted by insects, evolved a mechanism to differentially regulate the production of viral non-coding RNAs in mosquitos and humans, with a significant impact on viral fitness in each host. Flavivirus infections accumulate non-coding RNAs derived from the viral 3’UTRs (known as sfRNAs), relevant in viral pathogenesis and immune evasion. We found that dengue virus host adaptation leads to the accumulation of different species of sfRNAs in vertebrate and invertebrate cells. This process does not depend on differences in the host machinery; but it was found to be dependent on the selection of specific mutations in the viral 3’UTR. Dissecting the viral population and studying phenotypes of cloned variants, the molecular determinants for the switch in the sfRNA pattern during host change were mapped to a single RNA structure. Point mutations selected in mosquito cells were sufficient to change the pattern of sfRNAs, induce higher type I interferon responses and reduce viral fitness in human cells, explaining the rapid clearance of certain viral variants after host change. In addition, using epidemic and pre-epidemic Zika viruses, similar patterns of sfRNAs were observed in mosquito and human infected cells, but they were different from those observed during dengue virus infections, indicating that distinct selective pressures act on the 3’UTR of these closely related viruses. In summary, we present a novel mechanism by which dengue virus evolved an RNA structure that is under strong selective pressure in the two hosts, as regulator of non-coding RNA accumulation and viral fitness. This work provides new ideas about the impact of host adaptation on the variability and evolution of flavivirus 3’UTRs with possible implications in virulence and viral transmission.

West Nile virus (WNV) was isolated from the brains of 3 horses that died from encephalitis in February 2006. The horses were from different farms in central Argentina and had not traveled outside the country. This is the first isolation of WNV in South America.

Twenty-six years after it was last detected, Saint Louis encephalitis virus (SLEV) genotype III reemerged in 2005 in Córdoba, Argentina, where it caused an outbreak. Two genotype III SLEV strains were isolated from Culex quinquefasciatus. A 71.43% prevalence for neutralizing antibodies was found in domestic fowl in the homestead of a patient with encephalitis.

The Pantanal is a tropical, seasonal wetland of approximately 140,000 km 2 that is fed by tributaries of the upper Paraguay River in the centre of South America, which covers mainly Brazilian but also Paraguayan and Bolivian territories and is classified as one of the largest freshwater wetland ecosystems in the world (Alho 2005). The Brazilian Pantanal, which represents 85% of the total Pantanal area, is located within the states of Mato Grosso do Sul and Mato Grosso in Central-West Brazil, which are recognised as South and North Pantanal, respectively (Alho et al. 1988). This region is ecologically classified into sub-regions according to vegetation, flooding and physiography (Silva & Abdon 1988). The Nhecolândia sub-region that is situated in South Pantanal is one of the largest, comprising approximately one fifth of the total area, and is characterised by hundreds of shallow lakes that display varying degrees of salinity and coalescence with the system during floods (Adámoli 1982).Recent studies have demonstrated that some crocodilian species may be infected by WNV (Steinman et al. 2003, Jacobson et al. 2005 and that Alligator mississippiensis may also have a viremic load that affords the infection of blood-feeding Culicidae vectors, which suggests that in areas with high population densities of these alligators, juvenile individuals could play an important role in WNV transmission (Klenk et al. 2004). Taking these data into account, the unsuccessful efforts to detect WNV circulation in the avian hosts in Brazil and the widespread, high prevalence of the Crocodilia species, Caiman crocodilus yacare, in the Brazilian Pantanal (Campos et al. 2005), our strategy was to investigate the WNV circulation in the potential vectors, dead-end hosts and natural secondary amplifying hosts through the collection and testing of mosquitoes and serum samples of horses and wild caimans from the Nhecolândia sub-region. No morbidity was observed during the sampling period and only apparently healthy horses and caimans were sampled in the present study. MATERiALS AND METHoDSStudy area -In February 2009, mosquitoes and blood samples from horses and wild caimans were collected in six different cattle ranches of the Nhecolândia sub-region (18º20' 19º40'S and 57º14' 55º00'W) during the rainy season. Considering the free circulation of mosquitoes, horses and wild caimans in large ranches, sample collections for the study were undertaken in a 700-square-kilometre (70,000 ha) area (Figure). The collections for this study were authorised by the Brazilian Institute of Environment and Natural Resources (license IBAMA 18363-1/2009).Samples collections -Adult mosquitoes were captured at sites that were randomly selected using CDC automatic light traps and manual aspirators while landing to blood-feed on horses and research team members as routinely reported. Living mosquito specimens were transported to a field laboratory where they were immobilised by chilling and the species were identified through direct observation of the morphological ...

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