Emerging arboviruses: Why today?

Abstract

The recent global (re)emergence of arthropod-borne viruses (arboviruses), such as chikungunya and Zika virus, was widely reported in the media as though it was a new phenomenon. This is not the case. Arboviruses and other human microbial pathogens have been (re)emerging for centuries. The major difference today is that arbovirus emergence and dispersion are more rapid and geographically extensive, largely due to intensive growth of global transportation systems, arthropod adaptation to increasing urbanisation, our failure to contain mosquito population density increases and land perturbation. Here we select examples of (re)emerging pathogenic arboviruses and explain the reasons for their emergence and different patterns of dispersal, focusing particularly on the mosquito vectors which are important determinants of arbovirus emergence. We also attempt to identify arboviruses likely to (re)emerge in the future.

Keywords: Anthropology,; Arthropods,; Dispersal,; Emerging arboviruses,; Evolution,; Global distribution; Mosquitoes,.

Fig. 1

Phylogenetic analysis of representative arthropod-borne (and no-known vector) flaviviruses based on the entire open-reading frames. An amino acid alignment was constructed using Mafft v.7.266. Following alignment and model-testing, a maximum likelihood tree was computed using PhyML v.3.1 employing the GTR + gamma model of nucleotide substitution with 1000 bootstraps and using the Subtree Pruning and Regrafting branch-swapping algorithm. All bootstraps exceeded 80%. The tree was edited and visualised with FigTree v.1.4.2 (http://tree.bio.ed.ac.uk/software/figtree). The tree was mid-point rooted for visual purposes only. New World and Old World refers to the most likely geographic origin of the viruses. “ISFV-like” refers to insect-specific flaviviruses which show close relationships with conventional arboviruses. The primary arthropod (or no-known vector) with which the viruses are associated is shown for each group of viruses identified by the vertical lines on the right hand side of the Figure.

Fig. 2

Phylogenetic tree for Asian, Oceanic and American ZIKV isolates. The accompanying Table indicates the estimated time in years from the present (22/11/2016) to the most recent common ancestor (MRCA) for each of the nodes identified as A to J. The base of the tree is represented by the Malaysian isolate which is the first recognised descendant of the African lineages (not shown in this tree). To reconstruct the temporal evolution of the Asian lineage of ZIKV, 165 unique complete or near complete open reading frame genomes were retrieved on 27/03/2017 from NCBI GenBank (www.ncbi.nlm.nih.gov/genbank) and aligned using Mafft v.7.266 keeping the reading frame intact. Evolutionary rates and time to most recent common ancestor were estimated using BEAST 1.8.3, employing the GTR nucleotide substitution model with gamma distribution, a strict molecular clock with a CTMC prior, and a Bayesian skyline coalescent tree prior with a piecewise-constant demographic model. The dataset was run twice for 100 million generations each, sampling every 10,000 generations to ensure sufficient mixing of chains (ESS > 1000). After burn-in for each run, a consensus tree was produced using LogCombiner and TreeAnnotator (BEAST package). The consensus tree was then viewed and annotated in FigTree v.1.4.2 (http://tree.bio.ed.ac.uk/software/figtree/). All computations were performed at the CIPRES web portal (www.phylo.org).