Phylogeography of West Nile Virus: from the Cradle of Evolution in Africa to Eurasia, Australia, and the Americas

Abstract

West Nile virus (WNV) is the most widely distributed of the encephalitic flaviviruses and is a major cause of encephalitis, with isolates obtained from all continents, apart from Antarctica. Subsequent to its divergence from the other members of the Japanese encephalitis virus complex, presumably in Africa, WNV has diverged into individual lineages that mostly correspond with geographic distribution. Here we elucidate the phylogeography and evolutionary history of isolates from lineage 1 of WNV. Interestingly, there are many examples of the same amino acid having evolved independently on multiple occasions. In Africa, WNV exists in an endemic cycle, whereas it is epidemic in Europe, being reintroduced regularly from Africa either directly (in western Europe) or via the Middle East (in eastern Europe). Significantly, introduction into other geographic areas has occurred on one occasion only in each region, leading to subsequent establishment and expansion of the virus in these areas. Only one endemic genotype each is present in India and Australia, suggesting that WNV was successfully introduced into these locations once only. Each introduction occurred many centuries ago, probably due to trade and exploration during the 19th century. Likewise, in the Americas, WNV was successfully introduced in 1999 and subsequently became endemic across most temperate regions of North America (NA). In contrast to previous suggestions, an isolate from the epidemic in Israel in 1998 was not the direct progenitor of the NA epidemic; rather, both epidemics originated from the same (unknown) location.

Climate change impacts on West Nile virus transmission in a global context

“…Serologically, it is a member of the Japanese encephalitis serocomplex. The viruses can be designated into at least five phylogenetic lineages but only lineages 1 and 2 have been associated with significant disease outbreaks in humans [12,13]. The enzootic cycle is driven by continuous virus transmission to susceptible bird species through adult mosquito blood-meal feeding, which results in virus amplification.…”

“…Since then, new viral strains, probably of African origin, have increased human disease incidence in parts of Russia and southern and eastern Europe, with large outbreaks of increased clinical severity occurring in Romania, Russia, Israel and Greece [12,26].…”

“…Currently, WNV has an extensive distribution throughout Africa, the Middle East, southern and eastern Europe, western Asia and Australia, which derives from its ability to infect numerous mosquito and bird species. Today, as WNV is the most widely distributed of the encephalitic flaviviruses, it is a vector-borne pathogen of global importance [12].…”

“…Later, the virus moved across the continent, reaching Canada and Central America by 2002 and was isolated in California in July 2003 [80]. In fact, WNV became endemic across most temperate regions of North America [12]. The effects of weather fluctuations on WNV transmission in the USA and Canada have been analysed by several researchers who showed that increased temperatures influence North American WNV distribution and play an important role in the maintenance and amplification of human infection [43,81].…”

IS-98-ST1 West Nile virus derived from an infectious cDNA clone retains neuroinvasiveness and neurovirulence properties of the original virus

“…Phylogenetic analysis has permitted identification of lineages 1 (divided into clades 1a to 1b) to 5 [12,13]. Lineage 1 WNV strains have been reported in many regions, including Africa, Europe, the Middle East, Russia, India, and North America since 1999 [3].…”