If you live in the US, you might know that this year the country suffered the largest outbreak of West Nile virus (WNV) since 2003. As of December 11th, infections with the virus have been reported in humans, birds and mosquitos in all of the lower 48 states. In humans, there have been a reported 5,387 cases of WNV disease with 2,734 or 51% of these cases classified as West Nile neuroinvasive disease (WNND), a severe manifestation of the infection resulting in encephalitis, chronic mental sequelae and occasionally death, and there have been 243 deaths; 80% of these cases have occurred in 13 states with a third of all cases reported in Texas.
Though it’s unknown why 2012 proved to be a particularly noxious and geographically expansive year for WNV in the States, there were several environmental factors that precipitated the outbreak’s emergence in Texas that may have influenced its severity. A short, mild winter at the tail end of 2011 may have allowed for greater numbers of mosquitos to survive the winter, an occurrence known as “overwintering”.
In the spring and summer that followed, bouts of rain provided standing water for mosquito breeding and egg laying while droughts and higher than average temperatures may have also accelerated the replication of the WNV in birds and increased its transmissibility to mosquitoes, thereby augmenting its spread across the country.
Dallas county in Texas has disproportionately suffered the majority of WNV cases and fatalities - 405 total cases of WNV disease and 18 deaths.
Aside from auspicious environmental conditions for the mosquito population and the virus, the scale and severity of the outbreak can partly be attributed to the county’s inadequate dedication to mosquito surveillance and control, which led to several challenges to WNV disease prevention within the local population.