Sick, hot world: Climate change favors disease vectors, threatening to unleash more pandemics

Global heating has so profoundly altered our planet that some experts argue it's no longer about a changing climate and instead about a changed climate. In other words, the hotter, more chaotic world predicted by climate scientists is part of our present, not just our future. And those changes extend beyond rising sea levels and heat waves to how diseases spread and impact society.

For example, climate change is a breeding ground for intensified cholera outbreaks — one of only many diseases that could become full-fledged pandemics as humans continue to overheat the planet by burning fossil fuels.

Cholera patients experience many symptoms, ranging from unpleasant to deadly. Once a mosquito bite transmits a dangerous Vibrio cholerae strain like O1 and O139 to a human, that person will experience painful leg cramps, insatiable thirst and constant nausea punctuated by vomiting. An infected individual usually feels restless and spends a lot of time defecating watery diarrhea. Even healthy adults will die within a few hours if they do not get the correct treatment, with hundreds of thousands suffering that fate every year.

This is in spite of the fact that, as the Harvard Global Health Institute's faculty director Dr. Louise Ivers said, the disease is "completely preventable and also treatable."

Cholera is a particularly horrible consequence of this trend, generally afflicting those least responsible for global warming, given that Western nations disproportionately release the most carbon emissions.

"Typically the people who are affected by cholera are impoverished, distant from medical care, underserved communities — those without access to clean water and sanitation are the most vulnerable," Ivers said. "Those with food insecurity also have a disproportionate risk of death."

Given that cholera disproportionately impacts marginalized communities, it is perhaps unsurprising that Ivers (who works directly to address the Haitian outbreaks) described it as an "underreported pandemic." It has been circulating since the 1960s, and in Haiti alone it took 10,000 lives while impacting almost 1 million others from 2010 to 2018. The outbreak returned a few years later. Global heating is only making things worse.

"Climate change is important for cholera in that extreme weather events can cause displacement of people and also disruption of their safe water supplies through flooding, putting pressure on water sources making people vulnerable to pathogens in the water, effectively this is increasing the dispersal of pathogens," Ivers said. "We see routinely that cholera peaks in Haiti during seasons when rainfall is highest. We see that some Vibrios are very susceptible to environmental temperatures and the impact of that directly on Vibrio cholerae is being studied."

Ivers added, "As a doctor, my team and I have cared for thousands of patients with cholera over the years. It is a dramatic and painful death – and totally unnecessary."

Other pathogens are also likely to benefit from global heating. Dr. Ben Beard, deputy director of the Division of Vector-Borne Diseases at the Centers for Disease Control and Prevention (CDC), said that climate change will create a number of conditions conducive to dangerous pathogens: Longer and warmer summers, shorter and milder winters and increasingly frequent/unusually severe extreme weather events (such as heat waves, storms and droughts).

All of these climate alterations can cultivate pathogens like viruses, bacteria, fungi and parasites by helping them spread and multiply more quickly, widening their geographical distribution and influencing behaviors such as when they feed and their preferred choice of host.

Beard noted that the geographic distribution for mosquito and tick vectors is already expanding, including blacklegged ticks (Ixodes scapularis) responsible for Lyme disase or yellow fever mosquitos (Aedes aegypti) that don't just carry their namesake illness but also dengue, Zika and Chikungunya.

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"As environmental conditions change, it is likely that certain diseases will appear in areas where they previously had not occurred," Beard said. "Likewise, we might expect that some diseases may become less common in places where they had been of great importance."

In the case of tick-borne diseases, for example, Beard noted that the geographic ranges have already expanded in recent years for ticks that spread Lyme, anaplasmosis, ehrlichiosis and spotted fever rickettsiosis.

"While the exact reasons for the geographic spread of ticks and the diseases they carry are unclear, a number of factors may contribute," Beard said, such as how "the spread of Lyme disease over the past several decades has been linked to changes in land use patterns, including reforestation in the northeastern United States."

For example, suburban developments that attempt to shove civilization into recently wild areas put humans in close contact with tick hosts like mice and chipmunks. It is hardly unusual in the modern era for human activity to inadvertently cause pathogen-carrying animals to more closely interact with our own species. Climate change is just one more example of that happening.

Ivers referred Salon to a pair of papers from the journal Nature, both of which illuminate the growing threat of a climate change-induced pandemic. In a 2022 paper titled "Climate change increases cross-species viral transmission risk," they projected how 3,139 mammal species will shift their geographical ranges by 2070 due to climate change and human use of wild land, with their study including a number of possible outcomes. The authors anticipate that species will repopulate at higher elevations while interacting with each other in new ways, as well as entering so-called "biodiversity hotspots" with lots of various organisms. Inevitably this will bring them into highly populated areas, particularly in Asia and Africa, "causing the cross-species transmission of their associated viruses an estimated 4,000 times."

The other 2022 paper by Nature, titled "Over half of known human pathogenic diseases can be aggravated by climate change," involved scientists investigating empirical examples of how each known human pathogenic disease responds to ten different types of climatic hazards sensitive to greenhouse gas emissions.

"We found that 58% (that is, 218 out of 375) of infectious diseases confronted by humanity worldwide have been at some point aggravated by climatic hazards," the authors wrote. "16% were at times diminished."

If there is any good news, it is that the problem of addressing pandemics caused by climate change can be solved. It will simply require the same kind of concerted, science-informed human activity that got our species into this mess in the first place.

"Addressing the continuing and increasing threat of diseases spread by mosquitoes, ticks and other vectors requires a multi-faceted approach at federal, territorial, state and local levels," Beard said. "To prepare for, prevent and respond to this growing threat, human and ecological surveillance and research need to be expanded; state, local and federal capacity enhanced; safe and effective prevention tools validated for use; and public and health care provider awareness increased."

Beard added, "Everyone can play a role in helping prevent themselves and their loved ones from vector-borne diseases by preventing mosquito bites by wearing EPA-registered insect repellents and taking other prevention steps."

In addition to treating the vectors of these diseases directly, Ivers also urged people to tackle two of the roots of the problem — humanity's overuse of fossil fuels and the consequent climate change and systemic social inequalities.

"Our generation has an important role to play in mitigating climate change — and that includes the healthcare industry which is responsible for 8.5% of the US’ greenhouse gas emissions — and adapting," Ivers said. "We should be building the health systems that we need to respond not just to pandemics but also to the most basic health needs of the global population. In some ways the call to action around climate change is an opportunity to truly transform health systems to meet the needs of people now and in the future."