Predicting West Nile Virus Outbreaks

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Geographic Scale:
  • National
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What's the likelihood of a West Nile virus outbreak where you live? Since the 1930s, West Nile virus (WNV) has been the cause of human asymptomatic infection and fevers in Africa, west Asia, and the Middle East (Center for Integration of Natural Disaster Information 2001). In 1999, outbreaks of WNV encephalitis (inflammation of the brain) and other febrile illnesses were first reported in the Western Hemisphere in the New York City metropolitan area. Between 1999 and 2001, 83 cases of West Nile illness in humans were reported, and nine people died. In 2001, the first U.S. cases of WNV encephalitis outside the New York City metro area occurred (Center for Integration of Natural Disaster Information 2001). How does this disease propagate? Are there any triggers to this propagation? WNV is transmitted to humans through mosquito bites. Mosquitoes become infected by feeding on infected birds that have high levels of WNV in their blood. Infected mosquitoes can then transmit WNV when they feed on humans or other mammals. WNV is not transmitted from person to person, and there is no evidence that a person can get infected by handling live or dead infected birds (Center for Integration of Natural Disaster Information 2001). Humans get WNV through bites from infected mosquitoes, so areas that have high mosquito breeding activity are more likely areas for contracting WNV. One of the factors linked to areas of high mosquito breeding and subsequent incidents of WNV is areas that exhibit a mild winter followed by a summer having a higher-than-normal number of days with a maximum temperature above 25 degrees Celsius (°C) (El Adlouni et al. 2007). A 5°C increase in mean maximum weekly temperature is also associated with a statistically significant 32–50 percent higher incidence of reported WNV infection (Soverow et al. 2009). The WNV outbreak indicator created in this lab is based on these two research findings. By using monthly climate data for the contiguous 48 states in the United States, you can calculate county minimum and maximum temperature averages. Then, aggregating and averaging short-term monthly temperature data for any given month and comparing this to the climatic averages, you can determine counties that have mild winters, significantly warmer-than-average summer months, or a combination of these indicators of WNV.

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