Epidemiology of WNV

With West Nile Virus originating in African tropics and sub-tropics, it was first documented in the United States in New York City in 1999. Since then, it has seen an explosion, expansion, and then a relative normalization. Initially this spread away from New York in a  general spread out fashion among the south, east, and west. Then there was a massive convergence of incidence in the Midwest to the East face of the Rocky Mountains. This eventually normalized again with a more even spread nationwide, with just a slight higher incidence in the west.

West Nile Virus does not seem to affect any age range more prevalently than others. There is an even age distribution with the median age being 49. There is a higher risk of neuroinvasive WNV and death associated with an increase in age. The median age range for all other neuroinvasive diseases is 69 years of age. This is a sharp increase from the 49 years of age seen with WNV.

The transmission is seen as generally seasonal for the more temperate areas of the United States. here, the majority of cases are seen in late spring, and summer. If we shift our view to further south to more humid and tropical climates (more conducive to mosquito populations) we see the season as a much wider swath. There are cases reported as late in the year as December, and as early as April the next year.

 

 

Fig 1. 2014 West Nile Virus Epidemiology Map

As for mortality, there is approximately a 10% fatality rate among properly reported and diagnosed WNV cases. This is seen as higher in the elder population, as is common with most neuroinvasive diseases. It is rarely reported as fatal in children, but there have been a handful of sporadic cases.

Today, we are still currently seeing the bulk of the cases in the west, such as California, the southern states, and northern Midwest states such as North and South Dakota. Other than these areas there is a lesser, but still even spread. The only areas that aren't showing any WNV at all, or are only showing non-human WNV are in the upper extremes of the Northeast. Here in Maine and surrounding states we see a relative disease free region. This is undoubtedly due to the inability for the main vector (mosquitos) to survive in such a year round temperate climate.

Search Terms: West Nile Virus on Centers for Disease Control and Prevention

References:

West Nile Virus. (2015, January 30). Retrieved April 6, 2015, from http://www.cdc.gov/westnile/

West Nile Virus. (2011, July 1). Retrieved April 6, 2015, from http://who.int/mediacentre/factsheets/fs354/en/