West Nile virus in the US: A case study on climate change and health
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Scientists around the world agree that we are currently facing a climate crisis in which global heating is threatening wildlife and primary resources. This crisis also affects humans by impacting the spread of infectious diseases. In this feature, we look at one such prominent example: the spread of the West Nile virus.
Because of the new coronavirus pandemic, people worldwide are becoming aware of how powerful viruses can be.
Scientists are trying to understand how SARS-CoV-2, the new coronavirus, behaves, and why. This involves looking into all the factors that might influence its spread, including climate change.
Some researchers have hypothesized that the virus spreads at different rates depending on humidity levels. In contrast, others have argued that temperature and other climate factors probably do not influence its epidemiology (pattern of spread).
These questions tie into the larger issue of how and why climate change might influence the spread of viruses. One mediating factor that illustrates how climate change can speed up the spread of viral and other infectious diseases is the mosquito.
According to the Centers for Disease Control and Prevention (CDC), the mosquito is responsible for spreading more diseases among humans than any other animal.
In light of World Mosquito Day (August 20) — which commemorates the discovery that mosquitoes carry and transmit malaria to humans — Medical News Today delves into a case study that caused concern in the United States before the new coronavirus became an issue: the mosquito-borne West Nile virus.
So what is the link between climate, mosquitoes, and viral spread? And why is the West Nile virus such an interesting case study?
An impending global health crisis?
Climate change, triggered by the negative impact of human action, has become a crisis that could adversely affect all aspects of life on earth.
Globally, local climates have become unbalanced over the past century. The National Aeronautics and Space Administration (NASA) indicate that the Earth’s average surface temperature has increased by approximately 1.62oF (0.9oC) since the end of the 19th century.
They add that this increase in temperature has not taken place little by little, spread over an entire century. In fact, it has happened quite abruptly, mostly over the past 35 years.
These changes have demonstrably affected many aspects of natural life. News outlets frequently provide updates on how global heating is melting glaciers and ice sheets — some of which are thousands of years old — at an alarming rate.
But such changes are not self-contained. Melting glaciers have contributed to the steady rise of the global sea level over the past quarter of a century. This impacts marine ecosystems and endangers the life of people living in coastal communities because of a higher risk of catastrophic floods.
According to the official 2019 report from The Lancet Countdown, the climate crisis may soon become synonymous with a health crisis. The researchers in charge of putting together the report warn that “a child born today” will face the reality of “climate change impacting human health from infancy and adolescence to adulthood and old age.”
Increasingly frequent extremes of heat and cold do and will continue to affect vulnerable populations — particularly young children and adults aged 65 and over.
According to the report, “Over 220 million additional exposures to heatwaves (with each exposure defined as one person aged 65 years or older exposed to one heatwave) occurred in 2018” alone compared to such occurrences in 1986–2005. This number of exposures, the researchers emphasize, is “higher than ever previously tracked.”
Climate change and disease spread
Changing trends in global climates have also affected patterns of infectious disease transmission. How? Viruses cause many infectious diseases, and insects, such as mosquitoes, often carry these viruses. As mosquitoes migrate within one territory or between territories, they can transmit the pathogens they carry to human populations.
But changes in weather — increased rainfall, extreme weather events, such as flooding, and more violent heatwaves have impacted patterns of insect activity. These changes have also created environments that better suit the transmission of viruses.
According to The Lancet Countdown report, worldwide, “suitability for disease transmission has increased for dengue, malaria, Vibrio cholerae [the cholera bacterium],” and these are just a few examples.
Dr. Aaron Bernstein — the interim director of the Center for Climate, Health, and the Global Environment at Harvard University in Cambridge, MA — told MNT that “understanding how climate change matters to vector-borne diseases is an important part of figuring out what climate change means for health.”
He explained that often it can be difficult to pinpoint exactly how climate change affects public health, given that — in the current context of globalization and constant human migration — people move around all the time. This movement increases the risk of disease spread, as we have also recently seen with the new coronavirus.
Still, he noted, “there is plenty of evidence that insect-transmitted diseases that affect animals [that] haven’t changed their global travel plans and don’t […] participate in trade [have spread more widely].”
“[Some examples are] diseases, such as West Nile, Zika, [and] malaria. We should have every reason to expect that climate is going to make places [that used to be] less suitable to these diseases more suitable [to them] and vice versa.”
– Dr. Aaron Bernstein
Indeed, over the past 20 years, some virus-carrying mosquitoes have seemingly altered their migration patterns, emerging in continents that they had never previously reached. Perhaps the most telling example is the case study of Culex mosquitoes, which carry the West Nile virus.
But why and how have these disease-carrying insects migrated between continents, and what does this mean for the U.S.?
The West Nile virus is a flavivirus belonging to the same family as the Zika, dengue, and yellow fever viruses. Culex mosquitoes contract this virus when they ingest the blood of infected birds and can transmit it to humans by biting them. Mosquitoes can also pass the virus back to bird hosts.
When humans contract the virus, they do not typically experience any symptoms. However, in some individuals, the West Nile virus can become life threatening, leading to encephalitis or meningitis. Severe inflammation of the brain or other elements of the central nervous system characterizes both of these conditions.
The particular danger of an outbreak of West Nile virus is that there are currently no specific treatments. This means that, rather than treating the disease, doctors have to focus on managing the symptoms.
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