Distribution of West Nile Virus, United States

by Sarah King

West Nile virus (WNV) is a relatively new disease in North America, and consequently there is very little information available about how climate change will affect its distribution. In order to gain a better understanding, Ryan J. Harrigan and his colleagues modeled the incidence of the disease under current climate conditions (2003–2011) to predict how it will spread in the future (2013). The models proved to give a significantly accurate prediction for 2012 WNV distributions. They also projected the range of WNV for 2050 and 2080, which showed that predicted warmer temperatures and decreased precipitation would expand the range of WNV beyond its current bounds. The model and its predictive capabilities may help public health and policy officials prepare for and mitigate possible future outbreaks of WNV.

West Nile virus is one of the most prevalent mosquito-borne viruses in the world. It was not introduced to North America until 1999, but since then it has spread across the United States and Canada and has infected thousands of people. Researchers know that climate and environmental conditions influence the distribution of the disease vector, mosquitoes, but it is not clear how climate change will ultimately affect the range of WNV. To gain a better picture of how climate change will affect WNV in the United States, Harrigan et al. combined WNV incidence data of vectors and hosts, identified the primary climate factors that influence WNV, predicted annual WNV hotspots, and predicted how current risks will change with climate change.

First, Harrigan and his team predicted the distribution of WNV from 2003–2011 with available data. These models confirmed that the researchers could accurately project the special distribution of WNV using climate variables. They also used multiple ensemble models that weighted different variables in different orders to successfully predict human cases of infection for 2012 and accurately map high-risk areas in Texas and Southern California in that year. Through their modeling, Harrigan et al. discovered that maximum temperature of the warmest month and seasonal precipitations levels were the strongest predictors for the occurrence of WNV. They found that higher maximum temperatures in the warmest months are directly associated with the presence of WNV while precipitation has an inverse relationship it.

Next, the group used future climate change models to predict the future distribution of WNV. They found that there should be a northward and altitudinal spread of suitable WNV climate. It is believed that there will be less rainfall across the United States in the future, although this data is less certain, and it is convincingly predicted that temperatures will increase in the northern United States and at higher altitudes. Therefore, it makes sense for the model to project a significant spread of WNV due to climate change.

Climate change will facilitate the spread of WNV in several forms. First, the changes in temperature and rainfall will create a greater habitable region for mosquitoes, the virus vector. Harrigan et al. also believe that the climatic changes may affect vector lifecycles. Not only will mosquitoes be present in places where they currently are not, but also their lifecycles may become faster, which would allow their population to grow.

The predicted immergence of WNV in areas that have so far never been exposed is crucial information for public health officials in the United States. Without the proper preparation and preemptive control, populations that are currently not exposed to WNV may undergo devastating epidemics. This study does not include some important variables such as socio-economic conditions, land-use changes, and host diversity, which all must be considered in preventative decision-making. Ultimately, Harrigan and his colleagues have provided one of the first climate change prediction models for the spread of WNV in the United States that can and should be used to help officials and civilians prepare for possible future outbreaks of the disease.

Harrigan, R.J., Thomassen, H.A., Buermann, W., Smith, T.B., 2014. A continental risk assessment of West Nile virus under climate change. Global change biology 20, 2417–2425.

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