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?
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.”
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.
The disease took its name from the West Nile district of Uganda, which reported the first cases of West Nile Virus in 1937.
Until the early 1990s, outbreaks of West Nile virus remained consigned to Africa and parts of Asia and Europe — mostly in the Mediterranean basin.
Then, the virus started spreading to Eastern Europe, and finally, in 1999, the first officially recognized cases of West Nile virus appeared in New York City. However, some researchers suggest that the virus may have reached North America a year earlier, though it remained undetected until 1999.
Many scientists have since argued that climate change is the most significant factor contributing to the spread of the West Nile virus to the U.S. and around the world.
At present, the U.S. Environmental Protection Agency list the West Nile virus as an indicator of climate change.
In 2015, Prof. Shlomit Paz — a climatologist and climate change specialist and head of the Department of Geography and Environmental Studies at the University of Haifa in Israel — conducted a review of the existing literature linking climate change to changes in spread patterns of the West Nile virus.
Prof. Paz writes in her review, “Multiple factors impact the complex epidemiology of [West Nile virus] besides its transmission and distribution.” She also notes that weather conditions are some of the top determinants in this equation.
The climatologist also labels temperature, level of precipitation and humidity, as well as wind, as some of the key elements affecting the spread of West Nile virus.
Higher temperatures, she explains, can foster the population growth of disease-carrying mosquitoes, as well as “decrease the interval between blood meals, [and] shorten the [virus’s] incubation time.”
With precipitation and humidity, the situation is a lot more complicated, Prof. Paz explains. Traditionally, higher numbers of mosquitoes have links to an increase in rainy weather. However, according to some studies, lack of rain may do even more to accelerate virus spread.
Some species of Culex mosquitoes spread out more widely when precipitation is scarce as their regular feeding and breeding grounds — wetlands — become less amenable.
Moreover, Prof. Paz writes, “Drought leads to close contact between avian hosts and mosquitoes around remaining water sources and therefore accelerates the epizootic [virus spread among animals] cycling and amplification of [West Nile virus] within these populations.”
Observations about the links between changes in the spread of the West Nile virus and events related to global heating are also consistent with reports from U.S. regions.
“Generally,” Prof. Paz points out, “during the past 50 years, the average temperature across the [U.S.] has risen, while precipitation has increased by an average of about 5%.”
“Some extreme weather events, such as heat- and cold-waves, intense precipitation events, and regional droughts, have become more frequent and intense,” she also adds.
Since it first entered the U.S., the West Nile virus has continued to spread, affecting states from coast to coast. In a relatively short time, writes Prof. Paz, the virus “became endemic [regularly recurring] across most temperate regions of North America.”
According to research published last year in PLOS Pathogens, estimates suggest that “the [West Nile virus ‘wave’ moved from the East Coast to the West Coast at an average dispersal velocity of approximately 1,000 [kilometers per year] during the first few years,” between 1999–2003.
The investigators note that the virus spread the fastest and most widely between 2001–2002. They argue that this is likely because, since it first reached the U.S., West Nile virus strains had enough time to diversify by “jumping” between different hosts. They write that:
“The rapid geographical expansion of [West Nile virus] between 2001–2002 is consistent with a large increase in virus genetic diversity (i.e., a ‘polytomy,’ as a result of many new transmission chains being introduced) and a significant jump in human cases (66 human cases in 2001 to 4,156 in 2002).”
Data from the CDC indicate that in 2019, 958 people across the U.S. had contracted West Nile virus. The virus affected the brain and central nervous system in 626 of these people.
In that year, Maricopa County, AZ, reported the highest number of cases — 155 in total. The highest concentrations of cases occurred in Western regions of the States in California, Oregon, New Mexico, Colorado, and Arizona.
CDC data indicate that the number and density of West Nile virus occurrences have ebbed and flowed across the U.S. However, from 2004 until 2019, the virus has been a constant in the Southwest, and particularly in California.
Over the past few years, temperatures in the Southwest have been consistently high, on average, compared with other U.S. regions, and the levels of precipitation have remained low, leading to a dryer climate.
In a paper published in 2016 in WIREs Climate Change, researchers from the University of Alaska Fairbanks and the University of California in Davis argue that the spread of West Nile virus-carrying vectors in the U.S. is likely explained by temperature extremes and, in some areas, the increased frequency of droughts.
“Above-normal temperatures have been among the most consistent factors associated with [West Nile virus] outbreaks,” the authors write. “This has been found in both the Americas and Europe, for both of the main [West Nile virus strains].”
This paper also suggests that the relationship between West Nile virus and precipitation and humidity is complex, with outbreaks [having] followed both unusually dry and unusually wet conditions.”
“Particularly in urban areas, extended periods of drought can result in increased mosquito abundance that may enhance transmission.”
– Kara Hoover, Ph.D. and Christopher Barker, Ph.D.
Although researchers have already pinpointed many of the climatic factors that influence the spread of the West Nile virus, they generally agree that it is currently difficult to estimate how this infectious disease will affect the U.S. in the years to come.
That is because many data are still lacking, and there are too many variables to consider. However, research published last year in the CDC’s journal, Emerging Infectious Diseases, has concluded that the West Nile virus will most likely continue to be a top health risk for most of the U.S. population.
“We estimate that [approximately] 7 million (95% [confidence interval]) persons in the continental [U.S.] were infected with [West Nile virus] during 1999–2016, more than double the 2010 estimate of 3 million infections,” they write.
“As the cumulative incidence continues to climb, our findings provide additional support for the economic benefit of insecticide and vaccine interventions, especially in the Midwest, Southwest, and West of the [U.S.]; nearly 98% of the U.S, population remains vulnerable to [West Nile virus] infection,” its authors warn.
Given this situation, there is a need for better monitoring of public health events and better education regarding infectious diseases, such as the West Nile virus.
This is what Prof. Paz told MNT when we reached out to her for comment:
“Reinforcing West Nile virus control efforts and population health resilience under the current and future impacts of climate change should include monitoring and surveillance on a regular basis; risk assessment processes, which are adapted to different climatic and environmental conditions, as well as for populations in different socioeconomic levels; data and information sharing and collaboration between regions and countries; health system preparedness for possible outbreaks; and education that raises the public’s awareness, particularly of mosquito bite prevention.”
At the level of public educational campaigns, Prof. Paz said that these “should aim to involve the public in combating and preventing vector-borne diseases through means of identifying, reporting, and managing breeding sites, and information about individual protection in daily routines and in case of an outbreak.”
At an individual level, people could “significantly reduce [their] exposure risk by taking simple measures, such as eliminating small breeding sites and using mosquito traps and mosquito nets,” she told us.
Yet these interventions are akin to symptomatic treatments — they aim to keep the effect under control, but what about the cause?
“It’s hard to separate these problems [the wider spread of infectious diseases] from the broader health issues that climate change causes, and a good example of that would be nutrition,” Dr. Bernstein commented for MNT.
“One of the clearest problems for health [that is] related to climate change is that climate change is making it too hot for plants to be grown where they usually grow, causing more heavy downpours and more droughts,” he noted. He also suggested that this could eventually affect individuals’ access to food, which, in turn, could cause malnutrition.
“People who are undernourished are much more vulnerable to infections,” he emphasized. When climate change affects one factor that is critical for the well-being of our planet’s inhabitants, that can cause a snowball effect and impact all aspects of life.
“Climate change is affecting the full breadth of what it means to be leading a healthy life,” Dr. Bernstein emphasized.
The Harvard University expert also stressed that an important first step in mitigating the effects of climate change on the spread of infectious diseases — and on people’s health, in general — is to reduce pollution.
“Something like 80% of global carbon pollution is because of our reliance on fossil fuels,” he told MNT. “So if we’re going to prevent more carbon pollution, we’d better focus our attention on fossil fuels.”
“The good news with that is that many of the things we need to do to reduce our reliance on fossil fuels also will bring better health right now in the communities in which they are taken, and particularly benefit people whose health may be most vulnerable already.”
– Dr. Aaron Bernstein
Changes to ecological cycles are bound to affect insect populations and other vectors. In turn, this is likely to change how diseases spread. But humans can control their impact on the environments that they inhabit, and that is, perhaps, the most crucial step in ensuring that we do not accelerate the arrival of another future health crisis.
