West Nile Virus in the USA: Risks, Tactics and Research

Six additional mosquitoes testing positive for the West Nile Virus infection in Suffolk County. Their Health Services Commissioner Dr. James L. Tomarken announced this unnerving development on July 24th. That means the 2015 total for the country up to 13 positive samples. The mosquito samples are regularly gathered from well known mosquito breeding sites.


Alarmingly, the west nile virus has already been detected in several areas of New York. Seven state counties and three boroughs in New York City reported the presence of the west nile virus. Fortunately, as of July 23rd, no human infections with the virus have been reported yet the state of New York.


However, Texas already announced that as of July 18th, 221 mosquito pools have tested positive for the west nile virus, including two birds and one horse. and one human incident of west nile fever and two more of west nile neurologic illness.

As of July 18th in Florida seven counties were found to house sentinel chickens carrying west nile. In all, 19 chickens tested positive, but horses and humans have so far been spared.

Recently the Centers for Disease Control and Prevention (CDC) was made aware of 23 patients with illnesses related to the west nile virus. The most vicious form of the west nile virus, known as neuroinvasive disease, has been found in nine patients. Formerly, the CDS thought between 150 and 250 patients have either a clinical west nile virus illness or west nile fever for each case of neuroinvasive disease. If correct, this would put the total number of west nile virus cases in 2015 alone somewhere between 1,350 and 2,250.


Culex pipiens and a closely related Culex species are known to carry the west nile virus. By hard work, scientists are becoming better at predicting which specific conditions are most responsible for increases in the mosquito population and the increase in carried illnesses among such populations. A paper published in 2013 called “Predicting Culex pipens/restuans population dynamics by interval lagged weather data” explained some of the key factors: Culex pipiens carries “West Nile fever, St. Louis encephalitis, Japanese encephallitis, Western equine encephalitis, and Rift Valley fever.” It seems the length of day has a great effect on the mosquito concentration. The maximum population occurs roughly four weeks after the year’s longest day.


Female mosquitoes’ (the only kind that bite) activity is greatly effected by air temperature. It seems like quicker increases in temperature cause them to hasten their search for hosts to suck.

Rain affects mosquito activity in diverse ways, but is basically determined by the frequency and quantity of rain. If it’s too heavy, the baby mosquitoes are washed away from their breeding pools, which can cause activity to decrease momentarily. Light rain, which merely refreshes any standing water (e.g. sewer catch basins), or that fills a volume of stagnant water that will last for weeks, greatly increases the mosquito population.


By combining this data with data from other, less recent studies, we can begin to model the mosquito population using variables set to predict ahead of their own lag time, so that we can have a real time estimation of what’s going on with the mosquito population, and what is yet to come. This will help mosquito control tactics like fogging and larvacide become more effective and precise.


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