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.