How Sheep’s Blood Helped Disprove This Wacky Nineteenth-Century Theory of Illness

Scientists didn’t understand that bacteria caused disease, but then enter Louis Pasteur

Pasteur took blood samples from a cow, a sheep and a horse who had died of anthrax. Wikimedia Commons

Long before it was put into letters to major news outlets, anthrax was a major problem for livestock farmers.

Whole herds died in so-called "cursed fields," and human lives as well as the food supply were threatened. But until people started to understand bacteria and how it spread, the future wasn't looking good.

Back in the 19th century, before it was definitively known that bacteria and viruses caused illness by being spread from host to host, many people subscribed to the theory of "spontaneous generation"—that living organisms could be generated from non-living matter. By the late 19th century, writes Encyclopedia Britannica, scientists had ceased to believe that, for instance, cheese could spontaneously generate mice, but spontaneous generation at the microbial level was still accepted.

That meant that when scientists studied diseases like anthrax and even found anthrax bacteria in an infected animal’s bloodstream, they didn’t understand that the bacteria had anything to do with the disease, and assumed that it was either a symptom or an unrelated phenomenon. Meanwhile, bacterial diseases spread unchecked. 

Enter French scientist Louis Pasteur, already famous for his work with preserving milk and other foodstuffs through pasteurization. His work on developing an anthrax vaccine helped scientists understand how people (and animals) got sick. On this day in 1877,  Pasteur went to a slaughterhouse at Chartres, France, to get blood samples from the corpses of animals who died of anthrax. It was the beginning of an interest in the disease that would lead to the first vaccine for anthrax and more proof of the germ theory of disease.

Animals that eat plants are more susceptible to anthrax than humans or other species, writes the World Health Organization, although humans can and do get anthrax by coming into contact with infected animals or eating their meat. The herbivores frequently get the infection by eating grass or other plants that have been contaminated with Bacillus anthracis, a bacterium which can live for decades without a host. 

Animal doctors and scientists couldn’t agree on the cause of anthrax, just like they couldn’t reach scientific consensus on the cause of any other infection. They could see a bacterium in the blood of beasts that died of anthrax—but scientists who supported spontaneous generation maintained that the bacteria was unrelated to the disease.

Pasteur was working on a hunch: not long before he started his work, a microbiologist named Robert Koch had isolated a bacteria that he theorized caused anthrax. Koch built on work Pasteur had previously done, and in turn Pasteur (who was famously competitive) built on Koch's work with anthrax. They both believed bacteria caused disease, but that theory was controversial at the time, and anthrax was at the fore of the controversy. “Scientists all over Europe raced to prove or disprove its tenets using different diseases, most notably anthrax,” write molecular biologists Erika R. Sams, Marvin Whiteley and Keith H. Turner.

Chartres, a French city, had suffered a number of anthrax outbreaks in animals, writes medical historian Steven Lehrer. Pasteur started there. “At a local slaughterhouse, [Pasteur] obtained anthrax-infested blood from the carcasses of a horse, a sheep and a cow,” Lehrer writes. That blood formed part of the basis of a paper he published in a French science journal about a month later—the beginning of research into anthrax that would be important to developing the first vaccine for anthrax and debunking the spontaneous generation theory of disease once and for all.

But despite his advances, Pasteur never fully understood germs, and nor did many of his contemporaries. He instead believed that germs caused diseases by essentially sucking out nutrients. However, Pasteur's incomplete knowledge of germ theory still allowed him to develop vaccines for anthrax and rabies, saving many lives in the process.

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