Ecology of West Nile virus in North America

Abstract

The introduction, dispersal and establishment of West Nile virus in North America were reviewed, focusing on factors that may have enhanced receptivity and enabled the invasion process. The overwintering persistence of this tropical virus within temperate latitudes was unexpected, but was key in the transition from invasion to endemic establishment. The cascade of temporal events allowing sporadic amplification to outbreak levels was discussed within a future perspective.

Figure 1

Global distribution of West Nile virus, 2006 [Figure courtesy of the US Centers for Disease Control and Prevention.].

Figure 2

Sequence of maps showing the rapid expansion in the distribution of West Nile virus activity and human cases in the eastern United States from 1999 through 2002. Circles roughly circumscribe epicenters. [http://www.cdc.gov/ncidod/dvbid/westnile/USGS_frame.html].

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 4

Seasonal activity of West Nile virus in (A) Coachella Valley and (B) Los Angeles, Southern California summarized from 2003–2011. Graphs plot the total number of mosquito pools tested and positive for WNV per month. Mosquitoes were collected host-seeking at CO₂ traps or attempting oviposition at gravid female traps and therefore were reproductively active. Red arrows show months when there were no WNV positive pools.

Figure 5

Annual number of human cases and mosquito infection rate per 1000 tested from 2003–2012 for the (A) Greater Los Angeles County Vector Control District [GRLA]; (B) Kern Mosquito and Vector Control District [KERN] and (C) Sacramento-Yolo Mosquito and Vector Control District [SAYO]. Mosquito infection rate was calculated by the maximum likelihood estimate. (confirm the quality of the figure)

Figure 6

Number of human cases of WNV reported by the Canadian government per year, 2003–2008, and the mean monthly temperature in Regina Saskatchewan in °C for the 50 year average [1950–2000], 2003 and 2007. Horizontal line in bottom panel shows the 14.3 °C threshold for WNV replication.