Emerging infectious diseases: West Nile Virus

West Nile cases as of 21 August 2012

If you’ve seen the news lately, you’ve probably seen that this summer marks the highest level of West Nile infections yet recorded. According to the Centers for Disease Control, just over 1100 cases have been recorded to date in 2012, and analysis of the surveillance tables indicates all of those cases have been reported since the beginning of June. The map to the right indicates where cases have been reported, with 75% of the cases coming from South Dakota, Texas, Louisiana, Mississippi, and Oklahoma.

West Nile is a mosquito transmitted disease that is also found in a number of animal reservoirs including many bird species. In fact, veterinary cases are an important datum that is monitored by the CDC to follow outbreaks of the disease. The spike in cases this summer is the result of higher levels of mosquito activity during the summer, although it is not clear at this time whether the increased numbers in humans is due to increases in the numbers of infected animals.

West Nile Virus is currently know to be transmitted to humans either by the bite of an infected mosquito, and a small number of cases have been demonstrated to be caused by blood transfusion. Consequently, screening of donor blood in done to prevent that route of infection. Most people who become infected with West Nile Virus (80%) show no symptoms at all, and become immune to further infection. The majority of the remaining people may develop mild disease 3 to 14 days following infection, with symptoms including fever, headache, and body ache. These symptoms generally resolve on their own with no intervention within a few days. Approximately 1 in 150 patients develop serious encephalitis, or infection of the brain, with symptoms ranging from high fever and stupor, to coma and death. Patients with serious disease may remain ill for several weeks, and there is currently no treatment beyond mitigation of the symptoms. Some neurological symptoms may be permanent.

Summer 2012 drought conditions, via NOAA

Some news reports have conjectured on why this disease appears to be significantly more serious this summer than in past summers, and have raised the possibility of anthropomorphic climate change as being responsible, and the very mild winter, early spring, and hot initial summer months may have helped to stimulate mosquito breeding. This may be possible, but mosquitoes also require standing water for their reproductive cycle. The map at the left reports current drought conditions throughout the continental United States, and shows how much of the interior of the country is experiencing “extreme” drought conditions, and have been for several months now, which presumably would not be conducive for mosquito population growth. Interestingly, areas which have reported the highest levels of West Nile are highly correlated with areas of very low rainfall, which would seem to be counter intuitive. Mosquito control would certainly be an appropriate measure of restricting transmission. However if the frequency at which the mosquitoes are infected, due to a higher incidence in animal reservoirs, this could explain the increased transmission to humans even with a presumed decrease in the mosquito population due to low rainfall levels. The CDC collects data on mosquito and animal infection rates, but that data is currently under analysis for trends.


About ycpmicro

My name is David Singleton, and I am an Associate Professor of Microbiology at York College of Pennsylvania. My main course is BIO230, a course taken by allied-health students at YCP. Views on this site are my own.

Posted on August 23, 2012, in Danger danger danger!, Death from the skies and tagged , , . Bookmark the permalink. Comments Off on Emerging infectious diseases: West Nile Virus.

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