Surprising transmissions
Unexpected ways in which viruses and other pathogens have spread + notes on zoonoses. (A companion letter to a story I wrote last year.)
However you found your way here, welcome! I’m Emily, and I write letters about how we seek and tell stories to make sense of a changing world and our place in it.
As the world continues to boil, I’ve been trying to find personal ways to understand the climate crisis—to better appreciate that it’s not something that is just happening to the world at large, but happening all around us, to us, and how every aspect of our human lives contributes to it. This has meant, for me, finding out more about how our natural and built environments work and how we interact with them; and what I’ve found particularly interesting is how pathogens (and zoonotic pathogens in particular) spread. I plug my newsletter as one that seeks out stories to make sense of our place in the world. For me, too, this includes making sense of, and reimagining, our place in nature.
I’ve been meaning to write a letter riffing off this story I wrote for South China Morning Post’s print magazine last year—about the increased risk of emerging zoonoses, through the historical lens of the deadly Nipah virus that broke out in a pig-farming village in Malaysia in the 1990s, owing to our intensification of animal agriculture. I found out so many things I didn’t know in the writing of the story, which has helped me better appreciate how human, animal, and planetary health are intimately intertwined; and the multitude of connections I keep discovering has recalibrated the lens through which I see a lot of things.
Now, with new developments on the H5N1 virus (it has spread for the first time from mammals—dairy cows—to humans), I finally got down to finishing this letter, which has been languishing half-done in drafts for a long time. Apparently, timeliness does give me the push I need 😅
H5N1 infection fuels renewed anxieties
A dairy farm worker in Texas recently contracted H5N1, a highly infectious strain of the zoonotic bird flu virus. Humans have been infected by this subtype of Influenza A before, in the U.S. and globally, via exposure to infected birds or contaminated environments. However, this is the first known case of the virus jumping from mammal to human: in this case, farmed cows, via close contact.
Cows were not previously thought to be vulnerable to H5N1. The virus was first detected in chickens in Scotland in 1959 but it took a more virulent turn around 2021. It has since spread globally and killed hundreds of thousands, if not millions, of wild birds and more than half a billion farmed birds (chickens, ducks, turkeys, geese), which died from infection or were culled by farmers to stop the spread of disease. It has also infected other mammals, possibly via the ingestion of infected birds or other contaminated food sources and water; but the recent infection of farmed cows—and also goats—is particularly concerning to humans, given our more regular proximity to them.
In the U.S., some 33 herds in eight states have so far reported cattle infections. Virus levels appear to be the highest in the cows’ milk itself; and the virus seems to be spreading between cows too (not just from birds to cows)—though it’s not clear yet precisely how—by air transmission, by exposure to contaminated surfaces such as milking machines that are not disinfected between cows, or via the excreta of farmed birds used in cattle feed. Moreover, factory farms, with genetically similar animals all living together in crowded conditions, are especially conducive to the rapid spread of viruses; and new batches of animals being bred continuously ensure that any pathogens managing to sneak their way in won’t fizzle out as easily as they would on a small farm.
Beyond the devastation to bird biodiversity, the cruelty of mass culling, and the risks to food security, why is all this troubling? Because cows are mammals, we’re mammals; the virus has also spread to wild skunks, bears, foxes, seals, even polar bears in Antarctica—in total, more than forty-eight species of mammals spanning 26 countries. At present, the virus doesn’t appear yet to pose a threat to public health, but as it better adapts to mammalian species, it can gain new mutations that enable it to spread more effectively to humans and between humans, arming it with “pandemic potential”. Experts are particularly worried about it spreading to farmed pigs, which, as in the case of Nipah, have been shown to be effective living vessels in which avian and human viruses can mix and become better at transmitting to people. This was what happened with swine flu, caused by the H1N1 virus (also a subtype of Influenza A). It emerged in 2009 in Mexico and the U.S. and has since killed at least 150,000 people globally.
Update, May 30, 2024:
There have since been two further cases of humans infected with H5N1 via dairy cows.
Surprising transmissions
I won’t go over the same ground in this letter that my published story covered. Instead, I would like to share some of the unexpected ways in which pathogens—of zoonotic origin or not—have spread, directly or indirectly, to humans: for example, some transmission pathways were unlocked by timely social and political factors you may not have thought would play a role. In putting this letter together, I’ve borrowed heavily from Sonia Shah, whose book, Pandemic: Tracking Contagions, from Cholera to Ebola and Beyond, really should be required reading for everyone in the wake of Covid-19 (though it was published before the pandemic). Even if you have no existing interest in the subject, you’ll find the book relates to so many aspects of how we live. I highly recommend it!
Let’s start with:
Chewy the bubble dog
Wisconsin, 2003. A pet prairie dog called Chewy was brought to a vet for sneezing and coughing. From a microbiologist, Shah gleaned his story:
The veterinarian decided to nebulize Chewy with oxygen, encasing him in a hamster ball, a hollow sphere made of plastic, while forcing a jet of oxygen into the ball through a vent.
Later, when Chewy was freed from the ball, people who came into the vet’s room also fell ill—it turned out, with monkeypox. Chewy was part of a species native to North America, but he had been held in a distribution centre along with other animals from Ghana, such as giant pouched rats, dormice, and rope squirrels, that had arrived in the U.S. as part of the global pet trade—and had contracted monkeypox from one or some of them.
So, as Chewy sneezed and coughed into his bubble, he filled it up with aerosolized monkeypox virus. When the ball was removed, the virus broke out and infected ten people who entered the room where Chewy was held. The vet had inadvertently created what the microbiologist Shah spoke to called a “poxvirus bomb”. Ultimately, Chewy and other prairie dogs held in the same distribution centre spread monkeypox to more than 70 people across six U.S. states.
Travelling surgical patients
We know tourists carry germs from one end of the globe to another—but also medical tourists in particular, due to the increasing number of people who travel where the same treatments are available for cheaper. Shah in her book:
A less noted but equally potent way air travel broadcasts pathogens is by extending the mobility of infected carriers who would have been far too fragile to travel via other means. Surgical patients, for example, played little role in the global spread of infectious pathogens in the past. People who’s recently undergone surgery were relatively immobile. Not so today. Surgical patients travel the world, carrying pathogens from operating rooms on one side of the globe to the other.
These surgical patients are particularly vulnerable to infectious pathogens:
Surgeons’ knives breach the protective layer of skin that separates the interior of the body from the exterior environment, allowing the army of microbes that live on the surface of the skin, in the air above the bed, and on the surgical instruments and other objects that pass over the open wound to slip in. Even the most elaborate sterilization practices often fail to stop them.
India is a popular destination for medical tourists, receiving hundreds of thousands who undergo surgical procedures there every year. Many of India’s bacterial pathogens are also resistant to antibiotics. In 2008, a bacteria containing an enzyme called NDM-1 (New Delhi metallo-beta-lactamase 1) was carried by medical tourists who had visited India back to their countries, spreading it around the world.
What’s worrisome: NDM-1 endows bacteria with the ability to resist fourteen classes of antibiotics, at the same time that human resistance to antimicrobials, including antibiotics, is becoming a serious impediment to global health—due in part to our overconsumption of them.
Foreclosed homes, stagnant pools
The 2008 housing crisis in South Florida, U.S., led to the foreclosure of 380,000 homes, many of them abandoned. This being Florida, many of the properties had pools. Shah in this NYT story:
Then the rains came. The pools filled with standing water. With nobody home to notice or to let inspectors in, those derelict swimming pools became giant mosquito hatcheries.
This led to an outbreak of dengue in Florida in 2009, infecting five per cent of the population of Key West.
(The same story, linked above, talks about “the unintended consequences of human activity that would seem to have little to do with the biology of microbes, from economics and housing policy to architecture”. Besides being attracted to these empty pools, mosquitoes have also been ferried across the world in the trade of used tyres—similar enough to “dark, wet tree holes” apparently—and “lucky bamboo” plants for home decor.)
Ship ballasts
In her book, Shah shows how cholera spread to the “New World” from India via new shipping routes established between American and European ports in the late nineteenth century. Some of these ships carried cholera via human or animal cargo on board, but some also transported them via the barnacles, molluscs, and other marine creatures that attached themselves to the ships’ wooden hulls.
The ships’ ballast, too, could carry cholera. Ballasts are materials such as sand, soil, and stone, that are used to fill a ship’s holds to keep it stable in the water, and they often contain thousands of species, such as crab, shrimp, jellyfish, sea anemones, sea grasses, etc. Shah in her book:
They would shovel it onboard and when they left shore and dump it miles later upon reaching their destinations, creating massive deposits full of alien invaders. Water ballast, used on iron ships, carried cholera even more efficiently.
Fast forward to the present: As the climate crisis worsens and more sea ice melts, new shipping routes are being opened in the Arctic and more ships (not just ice-breakers) will be able to cross previously inaccessible parts of the ocean—which could lead to higher numbers of microbes being transported around the world.
Discriminatory colonial policies
In 1845, a fungal pathogen called Phytophthora, Greek for “plant destroyer” and originating in Mexico, arrived in Ireland in a batch of potatoes.
Before the advent of steam travel, any potatoes infected would have gone bad during the long voyage, before they ever reached Ireland—and they would have been discarded. With the advent of steam travel, though, the infested potatoes could arrive intact; and once they were planted, the pathogens they carried could spread to surrounding plants and lead to mass crop failure.
That’s what happened this time. The Great Hunger set in between 1847 and 1851, leading to the deaths of one and a half million Irish. This, in turn, led to the great migration: between 1845 and 1855, nearly two million people emigrated from Ireland to America and Australia, and another 750,000 to Britain.
What made this famine possible? According to Shah, it was discriminatory British policies that forced Irish tenant farmers to grow crops for export and restricted the area of land they could work to feed their families. As a consequence:
The potato was the only crop they could afford, but their overreliance on it made them vulnerable to any pathogens that might prey on it.
Well-meaning United Nations peacekeepers
January, 2010. In the aftermath of a 7.0 magnitude earthquake that killed over 200,000 people and displaced over one million people in Haiti, a contingent of UN peacekeepers arrived from Kathmandu, Nepal—at the time struggling with a cholera epidemic.
They were stationed at a camp built by Nepalese soldiers in the mountains north of Port-au-Prince. Since there was no waste disposal system, they had to build their own—but, it turned out, one that did not take into account proper sanitary measures, sending raw sewage from the camp streaming into the Arbonite River.
Until then, Haiti had been cholera-free. But with the camp’s establishment, cholera began to infect the waters, which washed into the river delta where thousands of Haitian farmers grew rice. Unknowingly, they continued to use the water to irrigate their fields and to bathe, cook, and drink. By 2011, there were more cholera victims in Haiti than anywhere else in the world, clocking over 820,000 cases and nearly 10,000 deaths. By 2019, it seemed like the problem had gone away. But cholera made a comeback in 2022.
The UN was taken to court for this. Did it take responsibility? Yes and no.
Back to zoonoses: the big picture
Some facts of note, extracted from different sections of my SCMP story:
Of the millions of microbes that exist, roughly 1,400—which include viruses, bacteria, fungi, parasites—are pathogens that have the potential to harm humans, according to a United Nations Environment Programme (UNEP) report published in 2020. Of the 1,400 pathogen species, about 800 are zoonotic.
The Neolithic Revolution, at the end of the Ice Age some 10,000 years ago, marked a significant new chapter in the history of zoonoses. Humans, who had until then lived in nomadic hunter-gatherer communities, began to form settled agricultural communities to cultivate crops and domesticate wildlife, creating new spaces where humans and animals lived in close proximity, allowing pathogens to move from animals to humans more often. The larger human congregations also allowed diseases to spread more widely.
The bubonic plague, influenza, smallpox, measles, cholera: these are some of the oldest zoonoses the world has known. And measles, thought to have spilled over into humans from cattle in the 10th century, casts a long shadow. When Europeans arrived in the Americas in the 16th century, they brought measles and other infectious diseases with them, which are believed to have wiped out 95 per cent of the indigenous population, leading to the destruction of their ancient civilisations. Measles still causes epidemics globally, infecting about 30 million people and killing more than 2 million each year, according to the World Health Organisation.
It had taken hundreds of thousands of years for humanity to grow to 1 billion people, in 1800. Now, there are nearly 8 billion. This population explosion, followed by rapid urbanisation and growth of industry, has wrought drastic changes on our landscapes—particularly the deforestation that has encroached on and destroyed wildlife habitat. The UNEP report points to the increasing demand for meat, agricultural intensification, and the consumption and exploitation of wildlife as the most important human behaviours driving the emergence of zoonoses globally.
“The number-one driver of epidemics of zoonotic disease outbreaks is us—the things we are doing that influence the way we come into contact with wildlife and the way that our livestock comes into contact with wildlife,” says Jonathan Epstein, a veterinarian and disease ecologist at EcoHealth Alliance, a global non-profit organisation that works to safeguard animal and human health.
Earlier epidemics or pandemics happened two or three times per century, but we have had several major outbreaks in the past two decades, and 75 per cent of all emerging infectious diseases since 1970 have been zoonotic. The Covid-19 pandemic has reminded us of bird flu, SARS, MERS, Ebola, Zika: all infectious diseases originating from wildlife—spread directly to humans or indirectly through intermediate hosts such as farmed animals, vectors like mosquitoes or the consumption of contaminated meat.
It’s predicted that future pandemics are likely to be zoonotic in origin and caused by viruses. At present, nearly 1.67 million undiscovered viruses may exist in wildlife, of which some 631,000 to 827,000 have the potential to spill over into humans. According to the scientists, virus spillover will most likely emerge in forested tropical regions with dense populations and high wildlife biodiversity, and where there is a high rate of land-use change, such as in Asia and Africa.
Climate change, too, could drive more than 15,000 new cases of mammals transmitting viruses to other mammals over the next 50 years, as species move to cooler places and meet for the first time, opening up new pathways for new viruses to spread to humans. Warming temperatures could also amplify mosquito-borne diseases and spread them to new regions.
Read the rest of my piece here.
It starts with a tale of a bat that met the wrong pig, as the film Contagion put it…
Additional reading
How Humanity Unleashed a Flood of New Diseases by Ferris Jabr
Animals That Infect Humans Are Scary. It’s Worse When We Infect Them Back. by Sonia Shah
Mass Consumption Is What Ails Us by Sonia Shah
Think Exotic Animals Are to Blame for the Coronavirus? Think Again. by Sonia Shah
Our Cruel Treatment of Animals Led to the Coronavirus by David Benatar
Why Killer Viruses Are On The Rise by Michaeleen Doucleff and Jane Greenhalgh
Research with exotic viruses risks a deadly outbreak, scientists warn by David Willman and Joby Warrick
With this letter, I wanted simply to draw attention to the interconnectedness of all earthly things. I was in awe as I absorbed all these stories, from the sense that there are any number of things happening all at once around the world while we’re not looking, where we’re not even thinking of looking, and that calamity can strike when the right conditions finally coalesce over time, and it can strike anywhere, at anyone, without discrimination. As for zoonoses: spillovers from animals to humans probably happen more than we know, but we only find out about them when an outbreak is reported.
But it’s worth keeping in mind that only if a pathogen attains the capability for human-to-human transmission could it have “pandemic potential”, helped along by our ever more globalised ways of living. And I think it’s important to say that the takeaway from zoonotic spillovers shouldn’t be to see animals as dangers or pests. We’ve destroyed their natural habitats, we have to coexist with them. What we need to do is change our behaviour and our relationship with animals. Because there’s the rub, isn’t it? We’ve created these risks for ourselves through our own rapaciousness. As Tan Chong Tin, a Malaysian neurologist I spoke to for my published story, said, “Zoonotic diseases have always been with us. There’s nothing unusual about zoonosis. It’s human lifestyles that have changed.”
From KL,
E.