Nav: Home

Drought identified as key to severity of West Nile virus epidemics

February 07, 2017

A study led by UC Santa Cruz researchers has found that drought dramatically increases the severity of West Nile virus epidemics in the United States, although populations affected by large outbreaks acquire immunity that limits the size of subsequent epidemics.

The study, published February 8 in Proceedings of the Royal Society B, involved researchers from UC Santa Cruz, Stanford University, and the New York State Department of Health. They analyzed 15 years of data on human West Nile virus infections from across the United States and found that epidemics were much larger in drought years and in regions that had not suffered large epidemics in the past.

"We found that drought was the dominant weather variable correlated with the size of West Nile virus epidemics," said first author Sara Paull, who led the study as a post-doctoral researcher at UC Santa Cruz and is now at the National Center for Atmospheric Research.

West Nile virus was introduced into North America in 1999 and has caused yearly epidemics each summer since. The intensity of these epidemics, however, has varied enormously. In some years, there were only a few hundred severe human cases nationally, whereas in each of three years (2002, 2003, and 2012), approximately 3,000 people suffered brain-damaging meningitis or encephalitis, and almost 300 died. The variation at the state level has been even higher, with yearly case numbers varying 50-fold from year to year, on average. The causes of this enormous variation were unknown and had led scientists at the Centers for Disease Control and Prevention to suggest that predicting the size of future epidemics was difficult or impossible.

In the new study, Paull and Marm Kilpatrick, an associate professor of ecology and evolutionary biology at UC Santa Cruz, analyzed patterns in the number of severe West Nile virus infections each year in each state and nationally. They examined a number of weather variables, including summer temperature, precipitation, winter severity, and drought. They also tested a long-standing hypothesis that the disease shows a wave-like pattern in causing large outbreaks in the first year and few cases subsequently due to a build-up of immunity in bird populations, which are the main hosts for the virus.

"We found strong evidence that in some regions the spread of West Nile virus was indeed wave-like, with large outbreaks followed by fewer cases," Paull said. "However, our analyses indicated that human immunity--not just bird immunity--played a large part in the decrease in human cases by reducing the number of people susceptible to the disease."

Kilpatrick said the links with drought were unexpected. In collaboration with Dr. Laura Kramer from the New York State Department of Health, his lab had developed a very careful method of mapping the influence of temperature on the biology of both the virus and the three different mosquitoes that are most important in transmitting the virus.

"We thought epidemics would coincide with the most ideal temperatures for transmission," Kilpatrick said. "Instead, we found that the severity of drought was far more important nationally, and drought appeared to be a key driver in the majority of individual states as well."

It's not yet clear how drought increases transmission of the virus, he said. Data from Colorado indicate that drought increases the fraction mosquitoes infected with West Nile virus, but not the abundance of mosquitoes. Drought might affect transmission between mosquitoes and birds by stressing birds or changing where they congregate.

With the help of climatologists Dan Horton and Noah Diffenbaugh at Stanford University, Paull used the links between drought, immunity, and West Nile virus to project the impacts of climate change on future epidemics. Over the next three decades, drought is projected to increase in many regions across the United States due to increased temperatures, despite increases in precipitation in some of the same areas.

Model projections indicated that increased drought could double the size of future West Nile virus epidemics, but that outbreaks would be limited to regions that have yet to sustain large numbers of cases. These findings provide a tool to help guide public health efforts to regions most likely to experience future epidemics.
-end-
Additional coauthors of the paper include Moetasim Ashfaq and Deeksha Rastogi from Oak Ridge National Laboratories. This work was supported by the National Institutes of Health and the National Science Foundation.

University of California - Santa Cruz

Related Climate Change Articles:

Mapping the path of climate change
Predicting a major transition, such as climate change, is extremely difficult, but the probabilistic framework developed by the authors is the first step in identifying the path between a shift in two environmental states.
Small change for climate change: Time to increase research funding to save the world
A new study shows that there is a huge disproportion in the level of funding for social science research into the greatest challenge in combating global warming -- how to get individuals and societies to overcome ingrained human habits to make the changes necessary to mitigate climate change.
Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.
Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.
A CERN for climate change
In a Perspective article appearing in this week's Proceedings of the National Academy of Sciences, Tim Palmer (Oxford University), and Bjorn Stevens (Max Planck Society), critically reflect on the present state of Earth system modelling.
Fairy-wrens change breeding habits to cope with climate change
Warmer temperatures linked to climate change are having a big impact on the breeding habits of one of Australia's most recognisable bird species, according to researchers at The Australian National University (ANU).
Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.
Older forests resist change -- climate change, that is
Older forests in eastern North America are less vulnerable to climate change than younger forests, particularly for carbon storage, timber production, and biodiversity, new research finds.
Could climate change cause infertility?
A number of plant and animal species could find it increasingly difficult to reproduce if climate change worsens and global temperatures become more extreme -- a stark warning highlighted by new scientific research.
Predicting climate change
Thomas Crowther, ETH Zurich identifies long-disappeared forests available for restoration across the world.
More Climate Change News and Climate Change Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Climate Mindset
In the past few months, human beings have come together to fight a global threat. This hour, TED speakers explore how our response can be the catalyst to fight another global crisis: climate change. Guests include political strategist Tom Rivett-Carnac, diplomat Christiana Figueres, climate justice activist Xiye Bastida, and writer, illustrator, and artist Oliver Jeffers.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Speedy Beet
There are few musical moments more well-worn than the first four notes of Beethoven's Fifth Symphony. But in this short, we find out that Beethoven might have made a last-ditch effort to keep his music from ever feeling familiar, to keep pushing his listeners to a kind of psychological limit. Big thanks to our Brooklyn Philharmonic musicians: Deborah Buck and Suzy Perelman on violin, Arash Amini on cello, and Ah Ling Neu on viola. And check out The First Four Notes, Matthew Guerrieri's book on Beethoven's Fifth. Support Radiolab today at Radiolab.org/donate.