A dairy worker in Texas contracts H5N1 bird flu after contact with infected cows, and suffers eye inflammation. Weeks later, a dairy worker in Michigan begins to cough and then tests positive for the virus. A ferret in a cage (ferrets are often used as study proxies for humans) becomes infected with the virus by airborne transmission from a sick ferret in a nearby cage. These data and other recent cases of H5N1 suggest that the virus might be evolving to spread more easily to — and among — people.
One implication is that while U.S. health authorities say the risk to the general public remains low, that risk could increase quickly. Another implication, less obvious but worth pondering, is that our collective appetite for on-demand inexpensive meat and dairy is leading us toward another catastrophic pandemic, not just pink eye and coughing in a few people.
It is fair to criticize government bodies, such as the U.S. Department of Agriculture, the Centers for Disease Control and Prevention, and state departments of health and of agriculture, for their laxity, tardiness, lack of transparency and inefficacy in dealing with the dangers of H5N1 on dairy farms. For instance, why isn’t blood testing for signs of the virus among dairy workers now mandatory in all U.S. dairy operations? Why isn’t there widespread use of protective equipment? Why haven’t there been earlier and broader requirements for the testing of cows?
But we should reserve some of the blame for ourselves. Americans are eager customers of the products that industrial-scale animal husbandry provides: milk, eggs, beef, chicken and pork. They arrive on our supermarket shelves wrapped in plastic or in cardboard cartons from vast factory farms perfectly suited to serve as petri dishes for the evolution of novel pathogens — novel to humans, anyway. We have surrounded ourselves with chattel animals, raised and milked and fattened and slaughtered and plucked and butchered in staggering numbers. It’s no surprise that sometimes they give us their viruses.
One contributing factor to the looming threat of H5N1 is that it has spread among poultry flocks. Quantity of hosts correlates with the quantity of opportunities, and there are, by one authoritative estimate, about 34 billion chickens alive on Earth at a given moment. Most of those are in big commercial operations. What makes such scales dangerous is not the inhumanity involved (that’s a separate issue) but the abundance and concentration of animals. Evolution is a numbers game like roulette, though with higher stakes, and for a virus, even in a single host, the numbers are often huge.
One particle of a flu virus replicating in an animal might produce 100 billion more flu particles in a few days. Those offspring will contain many random mutations, which are raw material for evolution. The more spins of that roulette wheel, the greater cumulative chance that the pearly ball will land on a number that breaks the bank.
There are reasons H5N1 probably won’t evolve from an avian virus transmitted in feces into a human virus of the airways, capable of killing millions of people. It would need a combination of long-shot mutations: changes in how it copies itself and in what sorts of cells in what parts of a host’s body it infects and whether it can remain lethal while floating through indoor air. Combining all those changes into one incarnation of the virus is highly unlikely. But odds against any unlikely event go down as the number of chances goes up. That’s how evolution over the ages has given us mammals that fly (bats), birds that swim (penguins), insects that live within elaborate social systems (ants) and the duck-billed platypus.
One area where those improbable mutations might be brought together is in the udders of dairy cows. Cow udders are hot spots for copious replication of the virus, and some new research (albeit not yet peer-reviewed) suggests that udders may contain both cells with receptors hospitable to bird flu viruses and other cells with receptors hospitable to human flu viruses. If a bird flu and a human flu happened to infect the same udder cell at the same time, they could swap sections of their genomes and emerge as a hybrid capable of causing a pandemic.
If. Probably won’t. Could. Nothing is certain about how an influenza virus will evolve until it happens.
The point here is not to demonize cows or malign their udders. The point, on the contrary, is to remind us how humans treat cows, how we gather them in dense herds to achieve industrial-scale economies in supplying the dairy and meat products we crave. The United States is home to about 9.4 million dairy cows, most of those (again, like the chickens) on large operations with more than a thousand animals. If H5N1 continues circulating among them, there will be many spins of the wheel.
When humans began domesticating cattle about 10,000 years ago it was a big step toward our becoming one of the most successfully organized and “civilized” species. It was also the beginning of a broad new pathway for the cross species sharing of infectious diseases. The measles virus, which killed many millions of people in the centuries before modern vaccination, seems to have entered the human population almost 3,000 years ago in the form of the rinderpest virus, a virulent pathogen of cattle (unless rinderpest was derived from measles, and they got their infection from us).
Everything comes from somewhere. Recent viruses among humans (such as Nipah or the MERS virus) generally come from old viruses among nonhuman animals, remaking themselves quickly thanks to their inherent evolutionary changeability, and seizing opportunities to spread more widely and abundantly.
By the choices we make as consumers, seeking economies of scale and low costs for the meat and dairy we want, produced in situations where thousands and millions of animals are crammed together in feedlots and barns, we create risks for ourselves and opportunities for the next dangerous virus. The roulette wheel turns, the pearly ball rolls. Ladies and gentlemen, place your bets.
