We are all living in a zoo: Animals as hosts
We interact with animals and the environment in many ways. The more we are exposed to each other, the more likely these diseases are transmitted.
Animals as hosts for infection
You’ve learned that humans can transmit pathogens to other humans via a variety of transmission modes. You’ve also learned that humans can contract infections from pathogens they encounter from animals, either directly or via a vector. These are zoonoses, or zoonotic infections.
The reverse can occur as well; this is known as reverse zoonosis. According to the Centers for Disease Control and Prevention (CDC), scientists estimate that 6 out of 10 infections are zoonotic (CDC, 2017). It is also estimated that globally there are 2.5 billion cases resulting in 2.7 million deaths each year (Grace et al., 2012). Wild animals, domestic livestock, and even our pets can act as reservoirs or hosts for countless diseases. Examples include Rabies, Anthrax, and Toxoplasmosis.
Interestingly, there have been instances where a zoonotic disease has actually provided unexpected benefits for humans. Cowpox, a disease that can pass from cows to humans, is one such example. Edward Jenner observed that milkmaids previously infected with cowpox were immune to the much deadlier smallpox. Cowpox is antigenically similar enough to smallpox for the human immune system to recognise, providing cross protection and subsequent immunity. Because of this discovery, a vaccine was created for smallpox, completely eradicating it after thousands of years plaguing the human race.
Of course, pathogens do not necessarily need animals as a reservoir. Environmental reservoirs exist as well. These can be living things other than animals, such as plants, or non-living things such as soil and water. Infectious diseases that reside in an environmental reservoir include Tetanus, Botulism and Legionnaire’s Disease. Some classes of infectious agents can even use other classes as hosts and reservoirs. Examples include bacteriophages: viruses that infect bacteria, and mycoviruses that infect fungi.
One Health approach
So, not only do we need to be mindful of sick humans, we need to be careful around animals and the environment. We live in the environment, and rely on animals for food, work and companionship. That’s why we shouldn’t just look after ourselves; we must also look out for the health of everything. This is known as the One Health approach. One Health recognises that our health is intrinsically connected to the health of animals as well as the environment, and so aims to improve the health of all three.
Unfortunately, as the human population grows, it gets even harder to avoid infectious diseases. In order to support our growing population, we are expanding into new geographical areas, demolishing habitats and requiring more agriculture, all of which increase potential exposure to new or existing pathogens. Add the destructive and intrusive nature of humans to the rapid natural evolution of infectious agents as well as sudden mutations that increase virulence, and it’s no wonder we have so many emerging and re-emerging infectious diseases today. Severe acute respiratory syndrome (SARS) is thought to have mutated from strains of coronavirus that colonise cave bats (Hu et al., 2017). Human Immunodeficiency Virus (HIV), which causes the currently pandemic Acquired Immune Deficiency Syndrome (AIDS), is theorised to have evolved from Simian immunodeficiency virus (SIDS) which infects non-human primates (Keele et al., 2006; Van Heuverswyn et al., 2007). Even Measles, a human specific virus, may have actually evolved from Rinderpest, a recently eradicated disease mostly affecting cattle (Furuse, Suzuki, & Oshitani, 2010).
Hendra virus is an example of an emerging zoonotic disease caused by human ‘progress’. In Australia, the destruction of fruit bat (flying fox) habitat has forced them to roost and eat more often in ‘human’ territory. Fruit bats are reservoirs for Henipaviruses, shedding them through bodily fluids. These viruses infect horses grazing under trees where bats roost and eat. Infected horses can transmit the virus to other horses, as well as to humans, resulting in illness and death for both. Since there is no evidence of direct transmission from fruit bats to humans, the basic premise is that if we keep our horses healthy, we keep ourselves healthy. The same goes for everything else: if we keep our animals and our environment healthy, we keep ourselves healthy. That is why the One Health approach is so important.
Can you think of ways to break the chain of infection for Hendra virus? You can use the chain of infection interactive we looked at in Week 1 to identify the links in the chain for Hendra virus and how you can break it. If you are feeling a bit creative, you could draw the chain of infection for Hendra, save it on the web or google drive and share a link.
Centers for Disease Control and Prevention (CDC). (2017). One Health. Retrieved from https://www.cdc.gov/onehealth/basics/zoonotic-diseases.html
Furuse, Y., Suzuki, A., & Oshitani, H. (2010). Origin of measles virus: Divergence from Rinderpest virus between the 11th and 12th centuries. Virology Journal, 7 (1), 52.
Ghabrial, S. A., & Suzuki, N. (2009). Viruses of plant pathogenic fungi. Annual Review of Phytopathology, 47, 353-84.
Grace, D., Mutua, F., Ochungo, P., Kruska, R., & Jones, K. (2012). Mapping of poverty and likely zoonoses hotspots. Nairobi, Kenya: International Livestock Research Institute. Retrieved from https://cgspace.cgiar.org/handle/10568/21161
Hu, B., Zeng, L.P., Yang, X.L., Ge, X.Y., Zhang, W., Li, B.,…Shi, Z.L. (2017). Discovery of a rich gene pool of bat SARS-related coronaviruses provides new insights into the origin of SARS coronavirus. PLOS Pathogens, 13(11), e1006698.
Keele, B. F., Van Heuverswyn, F., Li, Y., Bailes, E., Takehisa, J., Santiago, M . L., …& Hahn, B. H. (2006). Chimpanzee reservoirs of pandemic and nonpandemic HIV-1. Science, 313(5786), 523-6.
Van Heuverswyn, F., Li, Y., Bailes, E., Neel, C., Lafay, B., Keele, B. G., …& Peeters, M. (2007). Genetic diversity and phylogeographic clustering of SIVcpzPtt in wild chimpanzees in Cameroon. Virology, 360(1), 155-71.
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