We’ve discussed a few basic concepts in epidemiology before, such as R0 and predicting strains. We’ve also discussed climate change on a few occasions, specifically how our civilization has impacts on the planet that in turn have complex, difficult-to-predict impacts on our civilization. So, I thought it could be interesting to talk about both at once, specifically what factors epidemiologists are now paying attention to that could impact disease prevention in the future if the climate continues to shift.
Growing Population
Perhaps the biggest concern impacting future pandemics is the planet’s growing population and population density. The human population is expected to reach 10 billion by 2056 and most of that population will be located in urban areas.
Now, before I go further, I feel I should dispel a few myths about overpopulation. While the Earth’s population is still growing and will continue to grow for some time, the growth rate of humanity is actually slowing down. The growth rate peaked in the mid 1960s at about 2.1% increase in global population each year and has been steadily declining ever since.
The UN predicts that the planet’s human population will never exceed 12 billion, which is theoretically sustainable with the right technological advancement. The phenomenon behind this is called the demographic transition. For most of history, every society on Earth had an extremely high birth rate and an extremely high death rate. Across cultures, roughly a quarter of children died before they turned one and roughly a half died before adulthood. To compensate, the average parent would have between 4.5 and 7 children in their lifetimes. Infant death was so common that in almost every pre-modern culture, it was normal to not name one’s children until a few months or even years after birth. (It’s believed the practice of baptism or christening evolved from this.)
But the industrial revolution brought about advances in medicine and agriculture that radically reduced disease and famine, which radically lowered death rates in industrializing societies. This caused a population explosion as infant mortality dropped while birth rates remained high, with England’s population increasing from about 6 million to 18 million between 1760 and 1870. But as low infant mortality becomes the norm, birth rates decline to a little over 2 children per parent, with increased education, gender equality, and access to contraception also contributing to this drop.
We can now see that in most industrialized nations, birth and death rates are roughly equal again, so the population has plateaued or begun to shrink. This pattern has been observed in every nation with each country falling somewhere on this development curve. The global population is continuing to increase because there are still developing nations going through this process, but their populations are expected to plateau as well in the coming decades.
Now that that’s cleared up, let’s get back to epidemiology. Even though Earth’s population won’t grow to the point of apocalyptic overpopulation, it will still continue to grow over the next few decades. And while diseases associated with poverty (dysentery, malnutrition) are falling worldwide (though still prevalent in some regions), the continuing population growth has the potential to create its own problems. For one, most of this population growth is occurring in urban areas due to economic opportunities. Densely packed areas are more prone to epidemics due to their easier spread. And since this growth can sometimes outstrip the rate at which infrastructure such as sewer systems can be built, issues of sanitation can crop up.
But perhaps the biggest way that our growing population can make epidemics easier is our expansion into formally wild territory. As natural spaces are developed for agriculture and other human needs, which is happening very quickly in developing countries, people are put into closer contact with animals. Since about 75% of new diseases come from animals, this closer contact with animals puts humans in greater contact with these diseases. And there’s one species in particular that contributes a lot of these new viruses; bats.
Bats make up about a quarter of all distinct mammalian species and despite severe population decline due to human activity, they’re still the most populus group of mammals after rodents. Flight gives them the ability to spread disease over wide areas and their close social structure (see image) makes transmission between individuals easy. Most importantly, the ability to fly requires the fastest metabolism of any other mammal. Just like a car’s engine, running a metabolism this much will cause internal wear damage in the form of DNA-damaging free radicals. So bats had to evolve mechanisms to repair and resist this damage, which simultaneously gave them extreme disease resistance. Bats can still be infected with viruses, but they rarely get sick from them. And the viruses have to evolve to be hardier in order to survive bat’s body heat. The list of human viruses that originally came from bats includes ebola, nipah, hendra, several flu strains, and coronaviruses such as SARS, MERS, and of course, Covid-19. And as new residential and especially agricultural areas are built inside bat territory, more opportunities are created for these viruses to jump from animal to human. The film Contagion, perhaps the most realistic depiction of a deadly pandemic by Hollywood, had its virus jump from bats to pigs via pig farms in bat territory before jumping to humans via pork. And we know this transmission method is feasible because this is exactly how the Nipah virus was first transmitted to humans in 1998.
Climate Change
We’ve talked before about how man-made climate change will have detrimental effects on our society in ways that aren’t always immediately intuitive. One effect you might not have thought about is with transmissible diseases. The CDC has already begun to focus on how climate change will have effects on human health. There are several ways this could happen, such as warmer weather making heat stroke more common, to poorer air quality worsening diseases like asthma, malnutrition caused by changes to the food supply, to even mental health issues caused by increased civil conflict. But since we’re focused on epidemiology, which is the study of transmissible diseases, let’s focus on how a warmer climate will make certain diseases more prevalent.
Insect-borne illnesses, such as Dengue fever, West Nile virus, Lyme disease, and malaria, have historically been some of the deadliest. There is an oft-repeated statistic that of the 109 billion humans to have ever lived, half of them died of malaria. And while this statistic is questionable at best, malaria is the deadliest disease in the world today, and would have been even deadlier in the past. The fact that these diseases are most common in the tropics and subtropics is believed to be part of why today’s developed nations tend to be located in the temperate regions. A significant amount of modern epidemiology focuses on the spread of these diseases, both due to their severity and due to the difficulty in containing insect-borne illnesses. And if the range of these illnesses were to expand, these diseases could spread to regions that haven’t seen them before.
Being cold-blooded, insects require warm temperatures to survive, and the pathogens they harbor in their bodies develop quicker in warmer temperatures. As the climate warms, summers in the subtropics and even temperate regions become longer and warmer. Mosquitos require standing water to breed, so increased humidity and precipitation could lead to an increase in mosquito populations. If warming continues, we may begin to see significant outbreaks of insect-borne diseases in higher latitudes, subjecting these regions to stressors they’re not prepared for and further straining resources meant to combat these outbreaks. The 2015-6 outbreak of the Zika virus could be seen as a model of future outbreaks. The summer of 2015 was the warmest on record at the time, leading to an uptick in mosquito populations. Heavy rains in Brazil and Uruguay created standing water, though drought has also been associated with upticks in mosquito-borne disease due to people without access to piped water storing it in basins. These conditions allowed the virus to spread far faster than expected, and a warmer summer in higher latitudes allowed it to spread as far north as New York. This is not to say that climate change caused the Zika outbreak; it should be noted that 2015 was an El Nino year, resulting in naturally warmer temperatures across South America. But as these kinds of super-warm summers become more common, these kinds of outbreaks could become more common as well.
A Side Note…
This is at best tangentially related to the rest of this post, but I feel it is worth addressing. As a general rule, I don’t like debunking conspiracy theories. I believe that doing so treats these “theories” too much like they are legitimate ideas that are worthy of being disputed with logic, instead of malarky that’s only believed because the truth bruises one’s ego. That said, I have heard versions of this theory being discussed by a broad range of people and there is enough reason for rational concern here that I think it’s worth discussing.
Gain-of-function research is when a virus is genetically modified to enhance it in some way, such as increasing its transmissibility, mutation rate, or host range. There are multiple reasons one would do this, such as making a virus mutate faster in order to see what parts of the virus are more prone to mutating or increasing the speed of infection to make it easier to observe the virus as it infects cultured cells. This method of research is controversial even among scientists as there is always some risk that these modified viruses could escape the lab and infect people. So most governments have regulations and restrictions on this research with a lot of debate over what these rules should look like. The United State’s National Institute of Health (NIH) put a moratorium on this GoF research funding from 2014 to 2017 so that the process of reviewing and regulating these experiments could be overhauled. It should be noted that GoF research is a very broad description of multiple types of studies and there is some debate over what should count as GoF research for the purposes of these regulations. The research that was recently revealed to be conducted in Wuhan using NIH funding was with viruses modified to infect rats,* a species they normally couldn’t infect. Should this count as GoF research for the purposes of regulation since it doesn’t make the virus more dangerous to humans? That’s a worthwhile conversation for scientists and regulators to have, but it’s arguably something that reasonable people could disagree on.
EcoHealth Alliance is a non-government organization whose goal is combating emerging infectious diseases, such as the ones we’ve talked about here today that could prove a threat as our environment changes. They receive money from the NIH, as well as other federal agencies, to study emerging diseases. To this end, they have ties with the Wuhan Institute of Virology (WIV), one of many institutes they work with to achieve their goals. A significant part of this research involves collecting viral samples from bats and performing experiments with them. Research into bat coronaviruses are funded by the NIH and conducted at WIV because SARS-CoV-2 is the third coronavirus to cause a significant pandemic in the past two decades. They work out of Wuhan because SARS-CoV-2 is the second coronavirus pandemic to originate from eastern China. We’ve been discussing why this is the case; eastern China has one of the highest population densities on Earth and is rapidly expanding and developing, building farms in bat territory. The location of this research isn’t suspicious, it’s good forecasting.
The WHO and several governments are currently investigating any potential links between WIV and the Covid-19 pandemic. It’s important to know the origin of pandemics like this for the sake of future prevention. And for a disaster of this scale, it’s important to examine every possibility. And there are...somewhat legitimate versions of the lab leak hypothesis, such as SARS-CoV-2 being found in a rural region of China by researchers at WIV, brought to the lab where it was leaked, and this is how it made it to a major metropolitan area. But even this version of a lab leak isn’t supported by evidence and if it were true, the leak would’ve only sped up the inevitable pandemic. And the vast majority of relevant experts agree that Covid-19 being a man-made virus is its least likely origin. Pandemics like this can and have happened naturally and there is nothing about this virus or its spread that couldn’t have originated naturally.
Now, it’s always possible that tomorrow, evidence will surface that SARS-CoV-2 was a man-made virus that escaped from a laboratory, in much the same way that evidence could surface tomorrow for literally anything. But in the philosophy of knowledge, there is a concept called the burden of proof, which describes which party in a dispute is obligated to provide proof for their position. It’s agreed that when one party is making a claim (Covid-19 was made in a lab) and one party is disputing it (no it wasn’t), it’s always the responsibility of the claim maker to prove their claim, especially if the claim is extraordinary. As Christopher Hitchens said, “What can be asserted without evidence can also be dismissed without evidence.” At time of writing, there is significantly more evidence of wild origin while any evidence of a lab leak is circumstantial. So until actual proof of a lab leak surfaces, anyone trying to claim that Covid was made in a lab has not earned your attention.
For More Details
https://www.climatecentral.org/news/zika-virus-climate-change-19970 https://www.snopes.com/fact-check/fauci-gain-function-covid/
*I should note that these were ‘humanized’ rats, genetically engineered to express the human protein ACE2. ACE2 is the protein that coronaviruses latch onto and use to enter human cells, so coronaviruses that could infect human cells could also infect these rat cells. That said, there are multiple steps for a virus to get to a position where it can infect a cell and there are numerous differences between surviving in a human body versus surviving in a rat body, which is why these viruses had to be heavily modified. Ultimately, a virus that could infect a humanized rat would not automatically be able to infect a human.
Happy Halloween!