"Most and probably all of the distinctive infectious diseases of civilization have been transferred to human populations from animal herds." -William H. McNeill, Plagues and Peoples'"
Humanity's biblical "dominion over the fish of the sea and over the birds of heaven; and every living thing that moved upon the earth" has unleashed a veritable Pandora's ark full of humankind's greatest killers." Some diseases such as herpes and shingles seem to have always been with us, passed down the evolutionary chain. But most modern human infectious diseases were unknown to our hunter and gatherer ancestors. Early humans may have suffered sporadic cases of animal-borne diseases such as anthrax from wild sheep or tularemia ("rabbit skinner's disease") from wild rabbits," but the domestication of animals triggered what the director of Harvard's Center for Health and the Global Environment called the mass "spillover" of animal disease into human populations.
Archeological evidence suggests that small, nomadic groups hardly suffered from contagious disease. With the advent of agriculture, though, communities settled and grew in relatively fixed locations, increasing their close exposure to their own waste and reservoirs of disease. Populations that domesticated only plants became more exposed to the few diseases they already harbored, but it was the domestication of animals that brought people in contact with a whole new array of pathogenic germs.
Epidemic diseases tend to be harbored only by those animal species that herd or flock together in large numbers. This concentration allows for the evolution and maintenance of contagious pathogens capable of rapidly spreading through entire populations. Unfortunately, this same quality - the herd instinct - is what makes these animals particularly desirable for domestication. Domestication brought these animals once appreciated mainly from afar (along with their diseases) into close proximity and density with human settlements. As a zoonoses research team concluded, "The spread of microbes from animals to humans was then inevitable.
Tuberculosis, "the captain of all these men of death," is thought to have been acquired through the domestication of goats. In the 20th century, tuberculosis (TB) killed approximately 100 million people." Today, tuberculosis kills more people than ever before-millions every year. The World Health Organization declared tuberculosis a global health emergency in 1993 and estimates that between 2000 and 2020, nearly one billion people may be newly infected. What started out in goats now infects one-third of humanity.
Domesticated goats seemed to have beaten domesticated cattle to the punch. Between 1850 and 1950, bovine tuberculosis, acquired mostly by children drinking unpasteurized milk, was responsible for more than 800,000 human deaths in Great Britain alone. Interestingly, it can go both ways. The British Journal of Biomedical Science recounts that dozens of cases of bovine TB were traced back to a "curious farmworker practice of urinating on the hay, perhaps on the folklore premise that the salts in urine are beneficial to the cattle. Of course when it turns out the workers have genitourinary tuberculosis infections, it's not so beneficial.
Bovine tuberculosis continues to infect milk-drinking children to this day. In a study published in the American Academy of Pediatrics journal in 2000, doctors tested children with tuberculosis in San Diego and found that one-third of the tuberculosis wasn't human. One in three of the children was actually suffering from tuberculosis caught not from someone coughing on them, but, the researchers suspect, from drinking inadequately pasteurized milk from an infected cow. The investigators conclude, "These data demonstrate the dramatic impact of this underappreciated cause of zoonotic TB on U.S. chlldren."
Measles is thought to have come from domesticated cows, a mutant of the bovine rinderpest virus. The measles virus has so successfully adapted to humans that cattle can't get measles and we can't get rinderpest. Only with the prolonged intimate contact of domestication was the rinderpest virus able to mutate enough to make the jump. Though now considered a relatively benign disease, in roughly the last 150 years measles has been estimated to have killed about 200 million people worldwlde. These deaths can be traced to the taming of the first cattle a few hundred generations ago.
Smallpox also may have been caused by a mutant cattle virus. We domesticated pigs and got whooping cough, domesticated chickens and got typhoid fever, and domesticated ducks and got influenza. The list goes on. Leprosy came from water buffalo, the cold virus from cattle or horses. How often did wild horses have opportunity to sneeze into humanity's face until they were broken and bridled? Before then. the common cold was presumably common only to them.
New zoonotic infections from domesticated farm animals continue to be discovered. The 2005 Nobel Prize in Medicine was awarded to the scientists who discovered in 1982 that bacteria living in the human stomach, which they called Helicobacter pylori, caused stomach cancer and the vast majority of peptic ulcers worldwide. Roughly half of the world's population is now infected. This ulcer-causing bacterium is thought to have originated in sheep's milk, but is now spread person to person via oral secretions -saliva or vomit - or perhaps, like cholera, the fecal-oral route due to poor hand-washing following defecation. What has become probably the most common chronic infection afflicting humanity, according to the CDC, came about because humanity started to drink the milk of another species thousands of years ago.
A recent addition to the list of infectious farm animal bacteria is a cousin of H pylori, known as Heliobacter pullorum (from the Latin pullus for "chicken")," infecting a large proportion of chicken meat. H pullorum is thought to cause a diarrheal illness in people who contract it through the consumption of improperly cooked chicken fecal matter.
Yet another newly described fecal pathogen, hepatitis E, is one of the latest additions to the family of hepatitis viruses. It can cause fulminating liver infection in pregnant women, especially during the third trimester, with a mortality rate of up to 20%. Scientists began to suspect that this virus was zoonotic when they found it rampant in North America commercial pork operations. Direct evidence of cross-species transmission was obtained in 2003. Unlike a disease like trichinosis, which humans only get by eating improperly cooked pork, once a disease like hepatitis E crosses the species line, it can then be spread person to person. Between 1 and 2% of blood donors in the United States have been found to have been exposed to this virus.
"Wherever the European has trod, death seems to pursue the aboriginal." - Charles Darwin, 183677
UCLA professor Jared Diamond explored how pivotal the domestication of farm animals was in the course of human history and medicine in his Pulitzer Prize-winning book Guns, Germs, and Steel. In the chapter "Lethal Gift of Livestock," he argues convincingly that the diseases we contracted through the domestication of animals may have been critical for the European conquest of the Americas in which as many as 95% of the natives were decimated by plagues the Europeans brought with them. Natives had no prior exposure or immunity to diseases like tuberculosis, smallpox, and measles. Parallels were seen throughout the world with single missionaries unintentionally exterminating the entire target of their religious zeal with one of livestock's "lethal gifts."
Why didn't the reverse happen? Why didn't Native American diseases wipe out the landing Europeans? Because there essentially weren't any epidemic diseases. Medical historians have long conjectured that the reason there were so many plagues in Eurasia was that "crowd" diseases required large, densely-populated cities, unlike the presumed small tribal bands of the Americas. But that presumption turned out to be wrong. "New world" cities like Tenochtitlan were among the most populous in the world.
The reason the plagues never touched the Americas is that there were far fewer domesticated herd animals. There were buffalo, but no domesticated buffalo, so there was presumably no opportunity for measles to arise. No pigs, so no pertussis; no chickens, so no Typhoid Marys. While people died by the millions of killer scourges like tuberculosis in Europe, none were dying in the "new world" because no animals like goats existed to domesticate. The last ice age killed off most of the easily domesticated species in the western hemisphere, such as American camels and horses, leaving the indigenous population only animals like llamas and guinea pigs to raise for slaughter, neither of whom seem to carry much potential for epidemic human dlsease.
Once an animal is infected, the stress of transport can lead to increased shedding of the pathogen. In one study at Texas Tech University, for example, the average prevalence of Salmonella within feces and on the hides of cattle was 18% and 6%, respectively, before transport. But cram animals onto a vehicle and truck them just 30 to 40 minutes, and the levels of Salmonella found in feces jumped from 18% to 46%; the number of animals covered with Salmonella jumped from 6% to 89% upon arrival at the slaughter plant, where fecal contamination on the hide or within the intestines can end up in the meat. Similar results were found in pigs. The physiological stress of transport thus increases a healthy animal's susceptibility to disease. while at the same time enhancing a sick animal's ability to spread contagion.
No surprise, then, that the FAO blames "[t]ransport of animals over long distances as one cause of the growing threat of livestock epidemics ...." Dozens of outbreaks of foot and mouth disease, for example, have been tied to livestock movements or contaminated transport vehicles.
The FAO describes live animal transport as "ideally suited for spreading disease" given that animals may originate from different herds or flocks and are "confined together for long periods in a poorly ventilated stressful environment." Given the associated "serious animal and public health problems, the Federation of Veterinarians of Europe has called for the replacement of the long-distance transportation of animals for slaughter as much as possible by a "carcass-only trade.
Michael Greger "Bird Flu: A Virus of Our Own Hatching" (2006)
