4.1. Setting the Stage
The colonization of NA by Europeans greatly changed the landscape and markedly increased the size of the human population. Intensive agriculture and the need for construction supplies fragmented the deciduous forests. Points of trade by sailing ships produced cities along the eastern seaboard and then along major waterways. With a large number of humans and animals concentrated into permanent urban settings, waste disposal became a major problem and produced refuse dumps and highly eutrophic municipal water systems for waste and storm run-off. With urbanization came a reduction in avian species diversity [
59,
60], but an expansion of the number and range of commensal species such as American Robins, House Finches and American Crows that were able to exploit peridomestic habitats. In addition, House Sparrows, European Starlings and Rock Doves were intentionally released into Eastern NA and rapidly exploited the expanding urban environment throughout the continent. This reduction in urban avian diversity [
61] left a guild of commensal species, many of which were competent hosts for WNV [
62]. The
Culex vectors of WNV seemed to be opportunistic feeders able to exploit whatever avian or mammalian hosts were abundant in the environment [
63,
64,
65,
66,
67]. Simplification of avian diversity therefore focused vector blood meal acquisition on a few competent species, facilitating infection and transmission and increasing the efficiency of viral amplification [
68,
69].
The need to carry drinking water for long ocean voyages allowed the unintentional transport and introduction of several mosquito species, including members of the
Culex pipiens complex that included the Northern and Southern House Mosquitoes, aptly named for their close association with humans. This complex apparently arose within the Ethiopian region [
70], but now is distributed circumglobally [
71], being able to survive cold northern winters as well as exploit warm southern latitudes, with hybrids found at intervening latitudes [
72,
73,
74]. A third member of the complex,
Cx. pipiens form molestus seems to have evolved from above ground
Cx. pipiens populations [
75] to exploit underground collections of water in temperate [
36], but not tropical latitudes, where these underground habitats are exploited by
Cx. quinquefasciatus. Females in this complex typically blood feed on birds, but southern and admixed populations also feed on humans and dogs [
64,
67]. Other rural
Culex such as
Culex nigripalpus in the southeast and
Culex tarsalis in the west have exploited irrigated agricultural landscapes, managed wetlands and some urban habitats. Both species feed on both avian and mammalian hosts, but shift to more frequent mammal feeding during late summer, thereby functioning effectively as both enzootic and bridge vectors [
67,
76,
77].
This mixture of urban birds and peridomestic mosquitoes living in close proximity to humans created situations conducive for the transmission of arboviruses, especially SLEV. Although this virus is apparently endemic to the New World, it was not discovered until 1933, when there was a large outbreak of human disease associated with lower socio-economic housing, poor waste water management, large
Cx. pipiens populations, and exceptionally hot and dry weather in St Louis Missouri [
78], conditions now associated with WNV outbreaks. Subsequently, SLEV was found throughout NA, where it caused extensive epidemics of human neurological disease, especially in the Ohio River Valley during the 1970s [
44]. Improved intervention through organized mosquito control and urban waste water management seems to have eliminated large epidemics, leaving most human and avian populations without acquired flavivirus immunity.
4.2. The Invasion
Prior to its discovery in NA, WNV had been a virus on the move, with small outbreaks recorded in the Mediterranean region and epidemics of neuroinvasive disease documented in Romania [
79] and Russia [
9,
80,
81,
82]. In 1998 there was an outbreak of WNV in Israel, and this virus strain was similar genetically to that introduced into NYC [
40]. There is frequent air travel between NYC and Israel, and it was most likely that the virus was introduced by this frequent and repeated route of travel.
Similar to the SLEV outbreak in 1933, multiple factors in NYC during 1999 set the stage for the successful invasion and outbreak of WNV. The decrease in endemic arbovirus activity in prior years resulted in the closing of arbovirus surveillance and most mosquito control programs in the NYC area, except for a small program on Long Island retained to control pestiferous salt marsh mosquitoes. The summer and especially July of 1999 was the hottest in NYC recorded history and was associated with below average rainfall. These weather conditions were conducive for the production of large numbers of Cx. pipiens complex mosquitoes from storm water systems partially dammed with debris and enriched with leaves. Warm weather typically speeds larval mosquito development, shortens population generation times and thereby accelerates growth of mosquito populations. In addition, the commensal avian population and most of the human population had no immunity against flaviviruses.
During the summer of 1999, large numbers of American Crows were observed dead and dying in and around NYC [
83], and exotic birds from collections at the Bronx zoo were dying [
20]. A virus isolated from a deceased Chilean flamingo grouped with lineage 1 of WNV [
40]. Interestingly, the NY99 virus strain, as well as multiple isolates from outbreaks in Europe, carried the T249P mutation in the NS3 region of the viral genome that was associated experimentally with elevated viremias and 100% mortality in American Crows [
57], but not necessarily other corvids. Concurrently, a small cluster of neuroinvasive disease cases was recognized and diagnosed serologically, initially as SLEV, and then as WNV. Although the actual mechanism or date of WNV introduction probably will never be known, it seems likely that the virus was introduced by air traffic from Israel. Humans and equines (that also travel frequently by air) are considered to be ‘dead end’ hosts for the virus; however, some
Culex can infrequently become infected after feeding on fairly low doses of WNV [
84]. Alternatively, mosquitoes often are inadvertently transported on aircraft that are not routinely and thoroughly dis-insected. There also is a lucrative trade in smuggled pets, so multiple routes of introduction may have been potentially possible.
Studies done during the summer of 1999 rapidly incriminated the
Cx. pipiens complex as the likely urban vector [
85] and House Sparrows as an important maintenance host [
86]; highly infectious American Crows [
62] likely were important in virus amplification [
87]. Seemingly, the combination of previously introduced urban mosquitoes and birds exploiting periurban habitats in combination with extraordinarily hot weather facilitated the introduction and establishment of WNV.
Despite a large scale adulticiding response, WNV managed to overwinter successfully and then spread slightly in the NE USA during the following summer (
Figure 2). During late summer/fall of 2000, the virus seemed to have been carried by southbound migrant birds along the Atlantic flyway, by-passing the midAtlantic states, and becoming established in the Southeast, especially Georgia and Florida, where it amplified during 2001, again in association with hot, dry conditions. However, the peak of the epidemic occurred following years after the virus invaded the west, with epicenters in Chicago and New Orleans in 2002, Colorado in 2003 and Los Angeles in 2004, where
Cx. pipiens, Cx. quinquefasciatus, Cx. tarsalis/Cx. pipiens, and
Cx. quinquefasciatus, respectively, were the likely vectors. A hallmark of all these urban epidemics were the huge numbers of American Crows and other bird species dying from infection [
22,
35] as well as large numbers of horse cases with neuroinvasive disease with high case fatality (
http://www.cdc.gov/ncidod/dvbid/westnile/index.htm). Interestingly, like humans, most equine infections seemed to remain subclinical, resulting in high levels of acquired immunity [
88]. The equine epidemic was rapidly halted subsequent to 2003 by widespread natural and/or intentional vaccination.
During 2002, WNV acquired another mutation, E159A in the envelope region of the genome, that rapidly replaced the invading NY99 genotype. This strain, known in the literature as WN02, appeared to enhance
Culex transmission by allowing the virus to invade the salivary glands sooner after infection than the NY99 strain, especially under warm temperatures [
89,
90]. Therefore, the virus that invaded the western USA contained both the NS3 mutation causing high viremias and mortality in American Crows and the E mutation that may have enhanced
Culex transmission, as well as other genetic differences whose functions were not well understood [
91].
Climate variation has been an important factor historically driving SLEV and now WNV transmission to outbreak levels. Typically elevated transmission has been associated with hot, dry weather events [
92]. In urban landscapes with a large percentage of impervious groundcover, high rainfall volumes result in rapid run-off that typically ‘flushes-out’ urban waste water systems [
93]. Conversely, drought conditions stimulate residence and park landscape irrigation that creates a low volume ‘curb drizzle’, daily refreshing underground systems and catch basins without flushing-out developing larval mosquito populations. In addition, drought conditions may force avian populations into suburban areas where water is more freely available, thereby bringing competent hosts into contact with competent urban vectors. Drought conditions typically are associated with elevated temperatures (
http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html), and these conditions are further exacerbated by urban heat island formation [
94]. Because mosquitoes are poikilotherms, their body temperature approximates ambient conditions, although
Culex may behaviorally adjust their temperature by seeking daytime refugia and altering evening activity rhythms [
95,
96]. In general, warm temperatures increase the rate of larval development and reduce generation time [
97], thereby rapidly increasing mosquito population size, and reducing the duration of the gonotrophic [
98] and the extrinsic incubation [
99] periods. Therefore, during warm temperature anomalies, there frequently are more female mosquitoes, taking more frequent blood meals, thereby increasing host-vector contact and the probability of infection, and infected females are able to transmit virus earlier in adult reproductive life [
100] than during cooler seasons. The dramatic shortening of the extrinsic incubation period by warming temperature also compensates for the concurrent decrease in adult survival with warming temperature [
101]. The impact of warm temperature has been most noticeable in the US prairie states where the incidence of human infection markedly increases during warm weather anomalies, such as experienced in 2012 (
Figure 3).
Figure 3.
West Nile virus activity and climate analomies in the USA during 2012. West Nile cases: (
A) Distribution; (
B) reported case incidence per 100,000; Seasonal climate departures from the 1950–1995 average for the Jan–Nov 2012 period: (
C) temperature in °C, (
D) precipitation in cm. (Please confirm the
Figure 3 and
Figure 4)
Figure 3.
West Nile virus activity and climate analomies in the USA during 2012. West Nile cases: (
A) Distribution; (
B) reported case incidence per 100,000; Seasonal climate departures from the 1950–1995 average for the Jan–Nov 2012 period: (
C) temperature in °C, (
D) precipitation in cm. (Please confirm the
Figure 3 and
Figure 4)
Landscape heterogeneity has had a marked impact on the distribution of vector and avian host populations and therefore WNV transmission. Epidemiologically, the greatest number of cases have been detected in urban areas where the most peoplereside, whereas the risk of infection as expressed by case incidence has been highest in rural areas with low human population density, such as the northcentral prairie states. Frequently, the distribution of urban human cases has been delineated spatially by high mortality rates among periurban or urban corvid populations, especially American Crows [
102]. All corvids produce elevated viremias and frequently succumb from infection [
62,
103], and this provides a virus source for effective vector infection, especially when the birds are ill, less mobile and less defensive. In one case control study, residences with dead corvids reported on their property were 19.8 times more likely to also have infected mosquitoes than residences without dead corvids [
104]. When the number of dead American Crows around a large communal roost in Los Angeles was delimited spatially using SatScan statistics, the
Cx. quinquefasciatus minimum infection rate was 8.0 per 1000 within areas circumscribed by dead corvid clusters as opposed to 2.1 per 1000 outside of these clusters; also, only 41% of the human population resided within clusters of dead corvids, but 75% of the laboratory confirmed human cases were reported from within these clusters (incidence within = 5.9, without = 1.3 per 100,000 population, respectively) [
105]. As pointed out previously, transmission within these urban areas with reduced avian species richness tends to be more efficient than in rural areas with high species richness, because more blood meals are taken from competent hosts. This was shown in recent surveys of
Culex blood meal host diversity [
67], where blood meal host species richness in urban Los Angeles was half that observed in rural wetlands near Sacramento, where almost every other blood meal came from a different host species, including many that were incompetent hosts for WNV.
4.3. Movement
The rapid dispersal of WNV throughout the New World from NYC to Los Angeles and from Saskatoon, Canada, to Buenos Aires, Argentina, was unexpected, demonstrated the inability of public health interventions to contain an invading vector-borne zoonoses [
106], and may have occurred even faster than recorded by surveillance programs, based on an analysis of genetic change in time and space among available isolates [
27]. Long distance movement of WNV initially was attributed to migratory birds [
28,
29,
30], and in support, viremic migrants were collected repeatedly during southbound flights from temperate transmission foci [
55,
58,
107]. In contrast, few infected birds were detected during northbound flights from the tropics [
107,
108], thereby questioning this as a mechanism of rapid east to west movement. In addition, although evidence of WNV presence has been reported repeatedly in the Neotropics and Caribbean [
9], foci of human or equine disease have not been detected, indicating limited amplification to levels allowing tangential transmission, and therefore a limited source of virus to be inserted into northbound migrants. Subsequent modeling studies indicated that rapid east-west dispersal could result from post-nesting movement by resident birds and perhaps host-seeking by mosquitoes [
109]. During 2004, for example, the movement of WNV from the Los Angeles Basin across the Tehachapi Mountains and into the Central Valley of California occurred after the arrival of Neotropical migrants, but concurrent with post-fledging dispersal by resident birds such as House Finches [
110]. Mosquito movement by prevailing storm tracks [
111] also would not seem important for east-west WNV dispersal, because weather fronts in NA typically move from west to east and opposite to the dispersal direction of WNV. An unknown was the possible role of commerce moving infected mosquitoes by ground or air transport. The infection of a Los Angeles International Airport employee with WNV in 2002 before the detection of WNV in California by the surveillance program in 2003 may have indicated the long distance transport by an infectious mosquito.