The Human Role in Zoonosis: Why New Infectious Diseases will Keep Appearing

Zoonosis (noun) a disease which is transmitted to humans from animals; (adj.) zoonotic as in "COVID-19 is a zoonotic disease."

During the winter of 2019 a previously unknown acute respiratory disease apeared in China.  Now named COVID-19 it is caused by a virus pathogen named SARS-CoV-2.  It is normal that new diseases appear and then, just as mysteriously, disappear.  In 1485, 15,000 Londoners were killed by a new disease known as the English sweating sickness; it appeared again in 1502 killing Arthur, Prince of Wales.  It appeared one last time in 1551 before disappearing. 

A 1992 U.S. Institute of Medicine report defines an emerging infectious disease as one that has become more common in the last two decades or which threatens to become more common (e.g., dengue fever*), or one caused by a new infectious agent (e.g., SARS-CoV-2), or one that has gone undetected but which has been discovered circulating in the human population (e.g., hepatitis C*).

The majority (60 percent) of infectious diseases are zoonoses, a pathogen that has adapted to cross from a nonhuman animal to humans.  Among recent emerging infectious diseases, the percentage of zoonosis is even higher at 73 percent.  

While how a pathogen adapted to one host acquires the capacity to infect another is not fully understood, it is known to be most often the result of some kind of disruption in the ecosystem that results in novel or unusual physical contact between nonhuman animals (along with their pathogens) and humans. 

The historical record shows that humans have been creating new and unusual opportunities for pathogens that are hosted in nonhuman animals to exploit an ever-growing, and mobile species, Homo sapiens. (McNeill and McNeill, 1998)

Beginning between 5000 and 10,000 years ago, humans began to create permanent settlements based on the cultivation of grains and animal husbandry.  

While the new settlers could build fences and walls to protect themselves from large predators, they were oblivious to the fact that they now also shared the world with uncountable numbers of microorganisms, residing inside their domestic animals as well as in the unwelcome "domestic" animals like mice, rats, mosquitoes and flies.  Pathogens are just like large predators except that while "large predators devour victims from the outside in, pathogens do it from inside out." (Quammen, 2012)

Our distant ancestors were also unaware that these microorganisms-predators were extremely good at exploiting new opportunities.  Bacteria and viruses speed up the evolutionary timeline.  Viruses use the victim's cells to reproduce themselves and in doing so, they sometimes capture the host's own genetic material and incorporate it into the virus's genome.  Every time a virus infected cell bursts, shedding millions of new virus particles it is a "mutant viral swarm" which provides "crucial reservoirs of genetic diversity that can help a virus jump species and evolve." 

Our disease heritage (early zoonoses) from the 5000 to 10,000 years ago includes influenza, tuberculosis, leprosy, cholera, typhoid, smallpox, chicken pox, measles, malaria, schistosomiasis. (Arnold, 2020; McMichael, 2004; Woolhouse, 2012; Zimmer, 2018)

Beginning 500 years ago, European explorers opened new sailing routes to ships carrying not only conquistadors, merchants, colonists, and slaves, but also zoonotic infectious diseases such as malaria and yellow fever to the New World.  

We are now in the midst of yet another great ecological disruption.  A huge world population that steadily encroaches on and appropriates wild lands for itself creates even more disruption and opportunities for pathogens. (McMichael, 2004)

Factory farming of pigs creates huge pools of pathogen-filled waste that can seep into drinking water.  Destruction of habitat drives bats from their once isolated roosts to roosts close to humans such as to trees in an Australian horse paddock where bat feces fell into the paddock infecting the grazing horses along with several humans who tended the horses.  A dozen horses and one human died.  (Quammen, 2012)

COVID-19 is yet another example of zoonosis.  (Andersen, Rambaut, Lipkin, Holmes, & Garry, 2020)

Takeaways: 

1. Zoonotic diseases are part of the ecology.  While the appearance of a new zoonosis is not yet predictable, it is always a probability.  No one should ever be surprised when one appears and it is not too early to begin applying lessons learned during the current one to what may be on its way. 
2. Protecting ourselves from zoonotic diseases requires us to think ecologically since human activities play a major role in zoonosis.  The resurgence of zoonotic diseases is the result of changes in social conditions and behaviors including poverty, crowding and the weakening of public health infrastructure.  It is estimated that 6-7% of malaria is "attributable to the climate change that has occurred in the past quarter century." 
3. Zoonotic diseases have caused more deaths and disruption than wars.  The greatest threats to civilization are microbes not missiles.  (Gates, 2015)
4. As A.J. McMichael summarizes the problem:  "Humans and microbes are not 'at war'; rather, both parties are engaged in an amoral, self-interested, co-evolutionary struggle.  We need to understand better, and therefore anticipate, the dynamics of that process." (McMichael, 2004)

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*NOTE:
Dengue fever: the number of reported cases has grown fifteen-fold over the past two decades, from 500,000 cases in 2000 to 3,312,040 in 2015.
Hepatitus C: a viral infection that was not identified until 1989.

Resources:
Andersen, K. G., Rambaut, A., Lipkin, W. I., Holmes, E. C., & Garry, R. F. (2020). The proximal origin of SARS-CoV-2. Nature Medicine, 1-3. Retrieved from https://www.nature.com/articles/

Arnold, C. (2020). How Mutant Viral Swarms Spread Disease Quanta Magazine. Retrieved https://www.quantamagazine.org/how-mutant-swarms-spread-disease-20150825

Bill Gates (2014). "The next outbreak? We Are Not Ready." TED Talk, April 3, 2015. and "How we must respond to the coronavirus pandemic." TED Talk, March 24, 2020.

Jones, B. A., Grace, D., Kock, R., Alonso, S., Rushton, J., Said, M. Y., . . . Pfeiffer, D. U. (2013). Zoonosis emergence linked to agricultural intensification and environmental change. Proc Natl Acad Sci U S A Proceedings of the National Academy of Sciences of the United States of America, 110(21), 8399-8404. doi:10.1073/pnas.1208059110
 
McMichael, A. J. (2004). Environmental and social influences on emerging infectious diseases: past, present and future. The Royal Society. Retrieved from https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2004.1480
 
McNeill, W. H., & McNeill, W. (1998). Plagues and peoples. Anchor.
Quammen, D. (2012). Spillover: animal infections and the next human pandemic. WW Norton & Company.
van Doorn, H. R. (2014). Emerging infectious diseases. Medicine (Abingdon) Medicine (Abingdon, England : UK ed.), 42(1), 60-63. doi:10.1016/j.mpmed.2013.10.014
Woolhouse, M., Scott, F., Hudson, Z., Howey, R., & Chase-Topping, M. (2012). Human viruses: discovery and emergence. Philos Trans R Soc Lond B Biol Sci
Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 367(1604), 2864-2871. doi:10.1098/rstb.2011.0354
Zimmer, C. (2018). She Has Her Mother’s Laugh: The Powers, Perversions and Potenials of Heredity. New York: Dutton, an Imprint of Penguin.