Arthropod-borne viruses (arboviruses) have a long history of emerging to infect humans, but during recent decades, they have been spreading more widely and affecting larger populations. This is due to several factors, including increased air travel and uncontrolled mosquito vector populations. Emergence can involve simple spillover from enzootic (wildlife) cycles, as in the case of West Nile virus accompanying geographic expansion into the Americas; secondary amplification in domesticated animals, as seen with Japanese encephalitis, Venezuelan equine encephalitis, and Rift Valley fever viruses; and urbanization, in which humans become the amplification hosts and peridomestic mosquitoes, mainly Aedes aegypti, mediate human-to-human transmission. Dengue, yellow fever, chikungunya, and Zika viruses have undergone such urban emergence. We focus mainly on the latter two, which are recent arrivals in the Western Hemisphere. We also discuss a few other viruses with the potential to emerge through all of these mechanisms.

In 2016, the order Mononegavirales was emended through the addition of two new families (Mymonaviridae and Sunviridae), the elevation of the paramyxoviral subfamily Pneumovirinae to family status (Pneumoviridae), the addition of five free-floating genera (Anphevirus, Arlivirus, Chengtivirus, Crustavirus, and Wastrivirus), and several other changes at the genus and species levels. This article presents the updated taxonomy of the order Mononegavirales as now accepted by the International Committee on Taxonomy of Viruses (ICTV).

Sylvatic dengue virus (DENV) strains are ecologically and evolutionarily distinct, ancestral DENV lineages that circulate in a forest cycle between non-human primates and arboreal canopy-dwelling mosquitoes of the genus Aedes in Southeast Asia and West Africa. Each of the four serotypes of human DENV represents an independent cross-species transmission of sylvatic DENV into humans. As demonstrated experimentally, there is a low to non-existent adaptive barrier for the emergence of sylvatic DENV in humans, indicating that re-emergence of sylvatic DENV in the human transmission cycle is a clear and present danger. Evolutionarily recent epidemiological events such as the slave trade, the development of global travel networks, uncontrolled urbanization and human population increases, and the colonization of most tropical cities by the highly efficient mosquito vector Aedes aegypti, have contributed to the global emergence of DENV. Sylvatic DENV comes into regular contact with humans in Southeast Asia and West Africa and has the potential to cause individual cases of dengue haemorrhagic fever or transient spillover in urban settings. If additional DENV serotypes that have not yet emerged into human transmission cycles exist in as-yet-undetected sylvatic cycles, there could be major implications for the long-term control of dengue using the vaccines that are currently under development. The absence of efficient peridomestic vectors and the occurrence of widespread herd immunity may be important factors in limiting the spread of sylvatic-DENV transmission in urban settings. There are currently no serological diagnostic tools available to distinguish between infections with sylvatic and human DENV, further limiting an accurate assessment of human exposure to sylvatic DENV.

Arthropod-borne viruses (arboviruses), especially those transmitted by mosquitoes, are a significant cause of morbidity and mortality in humans and animals worldwide. Recent discoveries indicate that mosquitoes are naturally infected with a wide range of other viruses, many within taxa occupied by arboviruses that are considered insect-specific. Over the past ten years there has been a dramatic increase in the literature describing novel insect-specific virus detection in mosquitoes, which has provided new insights about viral diversity and evolution, including that of arboviruses. It has also raised questions about what effects the mosquito virome has on arbovirus transmission. Additionally, the discovery of these new viruses has generated interest in their potential use as biological control agents as well as novel vaccine platforms. The arbovirus community will benefit from the growing database of knowledge concerning these newly described viral endosymbionts, as their impacts will likely be far reaching.

Dengue viruses (DENV) are by far the most important arboviral pathogens in the tropics around the world, putting at risk of infection nearly a third of the global human population. DENV are members of the genus Flavivirus in the Family Flaviviridae and comprise four antigenically distinct serotypes (DENV-1-4). Although they share almost identical epidemiological features, they are genetically distinct. Phylogenetic analyses have revealed valuable insights into the origins, epidemiology and the forces that shape DENV evolution in nature. In this review, we examine the current status of DENV evolution, including but not limited to rates of evolution, selection pressures, population sizes and evolutionary constraints, and we discuss how these factors influence transmission, pathogenesis and emergence.