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Spillover

We have been slow to learn the role of virus ecology in human health.

J.S. Koen was a veterinarian and inspector for the U.S. Bureau of Animal Industry in Fort Dodge, Iowa. He became unpopular during the Spanish flu outbreak in 1918 when he saw “flu-like” symptoms in pigs.

“The similarity of the epidemic among people and the epidemic in pigs was so close, the reports so frequent that an outbreak in the family would be followed immediately by an outbreak among the hogs, and vice versa, as to present a most striking coincidence if not suggesting a close relationship between the two conditions. It looked like the flu, and until proven it was not the flu, I shall stand by that diagnosis.”

J.S. Koen, veterinarian and inspector, U.S. Bureau of Animal Industry

Pig farmers weren’t impressed.

Koen published “A Practical Method for Field Diagnosis of Swine Disease” in the American Journal of Veterinarian Medicine in 1919 based on those observations. Today, scientific articles cite him as the first to observe zoonotic or animal-borne disease.

A few years later, Richard Shope of the Rockefeller Institute of Comparative Anatomy made a discovery. He found pigs had the same flu antibodies that people alive for the Spanish flu had. But people born after the outbreak lacked the antibodies.

Ten years later, other researchers with the U.S. Bureau of Animal Industry also proved Koen’s theory. They transferred mucous samples from isolated sick pigs to healthy pigs, which became ill.

Pigs have similar organ systems to humans. Forensic scientists use pig bodies to study decomposition. Gastroenterologists see the same digestive diseases in humans that veterinarians see in pigs. Many scientists estimate 70% of our immune system resides in our guts.

By extension, humans share many of the same infectious diseases as pigs.

The Spanish flu mystery lives to this day. Pigs shared the disease, but infection went both ways. We both suffered outbreaks. Neither of us was a natural reservoir for the disease.

Virologist Rob Webster took the investigation one step further in the 1970s.

He and a team studied the genetics of viruses in many wildlife species. They found flu viruses everywhere. Birds, in particular, carried and passed influenza viruses with little to no disease. There, the viruses were at home. Birds were the viruses’ natural niche.

Additionally, modern pig and human flu strain genetics trace to viruses carried in several waterfowl species. Likely, the Spanish flu mutated enough to jump from its home to pigs and then to humans, or humans and then to pigs. Scientists call this jump a spillover.

Emory University disease ecologist Thomas Gillespie discusses the origins of Ebola, how it starts and spreads, and why this outbreak may just be the beginning of a much larger problem.

Spillover events happen all the time. Two-thirds of infectious diseases in humans come from animals.

Some are self-contained such as rabies. Here, a single infected bite from a raccoon to a person would infect only one person, assuming the person wasn’t vaccinated and could get infected at all. Each bite counts as a spillover event.

Others like major flu outbreaks might originate from one or several spillover events. Each may go on to have widespread human-to-human transmission.

We can point to the Middle East respiratory syndrome (MERS) as a disease with many spillover events. Geneticists traced different virus mutations from separate transmission events from camels to humans.

Yet severe acute respiratory syndrome (SARS) traces one genetic event from civets to humans.

And so, the age of disease ecology began.

Viruses belong to a web of interactions just like anything else. In their natural setting, they cause little problem for their hosts. Once disturbed, and with the right accident of mutations, viruses spill over to new hosts.

Sometimes the new hosts have no symptoms. But other times, they reject the foreign invasion and become sick.


References

Runstadler, Jonathan. “The Spillover Effect.” Tufts Now, Tufts University (15 March 2018). https://now.tufts.edu/articles/spillover-effect, accessed 18 April 2020.

Hollenbeck, James E. “An avian connection as a catalyst to the 1918-1918 influenza pandemic.” International Journal of Medical Sciences, vol. 2, 2 (15 Mary 2005): 87-90. DOI: 10.7150/ijms.2.87, accessed 20 April 2020.

“Coronaviruses.” National Institute of Allergy and Infectious Diseases. https://www.niaid.nih.gov/diseases-conditions/coronaviruses, accessed15 April 2020.

Andersen, Kristian G et al. “The proximal origin of SARS-CoV-2.” Nature Medicine, vol. 26 (17 March 2020): 450-2. DOI: 10.1038/s41591-020-0820-9, accessed 15 April 2020.

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