Climate Change and West Nile Virus

Climate Change

West Nile Virus is transmitted by mosquitoes and has been increasingly mentioned in news headlines and discussed around dining tables lately. The threat of West Nile Virus is making the bloodsucking pest ever more annoying and worrisome. Some people have expressed the fear that climate change will produce an environment in which these summer parasites are going to thrive, causing more West Nile Virus cases and other mosquito-borne illnesses in the future. But is this true?

Mosquitoes have changed very little in the last 46 million years, as revealed by fossils in Eocene deposits from northwestern Montana last October. Previously, the consensus was that 10 million years is the longest time one species of insects could maintain major features without changing. The extreme similarities between these prehistoric mosquitoes to the modern ones amazed scientists.

This discovery did not make the Jurassic Park more plausible, as scientists have not discovered any fossil of mosquito with blood meal remain from the time of dinosaurs, and DNA cannot survive this long. However, knowing the mosquito today has such formidable “age”  helps people to face the limited successes from the battle against them. There are over 3500 species of them, suitable for almost every climate and ecosystem available.

The warming climate may lead to a harder battle against mosquitoes, as they are assumed to breed more and have a longer season to spread West Nile Virus and other diseases. This assumption is not true in southern U.S., said Cory Morin and Andrew Comrie in University of Arizona. In a paper published last September in Proceedings of the National Academy of Sciences, using a climate-driven model to predict the population of one species of  mosquito that transmits West Nile virus, they concluded that in the next 40 years, many locations across southern U.S. will have a reduced mosquito population.

The reason for the shrinking number of mosquitoes in southern parts is that hotter and drier conditions resulted from climate change are hostile to larvae. Larvae develops faster in warmer temperature, but there is a threshold of temperature above which mortality increases.

The temperature and precipitation of various regions are affected by the climate change differently. Southern U.S. is not impacted equally. For example, in southwestern U.S., mosquito season would start later due to the hotter and drier summer conditions, but would last into fall potentially because of the precipitation in the fall. In south-central U.S., the amount of rain would reduce for both summer and early fall, resulting in fewer mosquito days.

Northern U.S. would most likely face a longer mosquito season, as the general assumption from climate change would hold true. The higher temperature projected for spring and fall will not significantly reduce the precipitation in these regions, so enough rain will still maintain summer breeding grounds for mosquitoes. Residents and authorities in these regions are likely to face a harder battle to control the pests and illnesses.

Although the model in this paper did not take birds which are West Nile virus hosts into the consideration for disease-carrying mosquito population, the paper highlighted the vast regional differences in the mosquito population and its season length in the future of climate change. While northern U.S. can be spared from the extreme temperatures, which southern parts are projected to experience from a changing climate, the potentially longer mosquito season and larger population, thanks to climate change, need to be taken seriously. The pesticide spraying, ineffective in killing mosquitoes while causing harm to human and ecological system, needs to be replaced by a new set of mosquito control methods which are already showing results in many cities.

Opinion by Tina Zhang


Smithsonian Science
Science Daily

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