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For the longest time, we have always discussed how infectious diseases get passed onto humans due to various reasons such as encroachment into wildlife spaces, wet markets, etc. We’re just one species out of the millions that are out there and albeit, maybe at the moment it seems like we’re the superior being, does that actually mean that we’re the only ones that matter? That’s the hard-hitting question that even the COVID-19 pandemic has brought in.

With many questions arising about whether or not the SARS-CoV-2 virus can pass onto other species such as our domesticated pets, as days are passing, more evidence is coming to light. For example, gorillas in a zoo were found to be infected, while on the other hand, a domesticated cat has also tested positive for the same. So, how does this happen? This is where the phenomenon of Reverse Zoonosis comes into the picture.

What is reverse zoonosis?

Zoonosis refers to the transmission of infectious agents from animals and for the longest time, the focus has been on just that. But now it is becoming more evident that infectious diseases are not just limited to the ones that affect humans but even those that affect animals. It’s important for us to monitor these because at the end of it, there are many reasons as to how this is applicable to humans, which will be discussed further in the article.

We have come to a point where we are finding viral diseases in species that were not susceptible to them previously. This continuing cycle of infection, spillover and re-infection only increases our problems as years pass by. Due to such phenomena arising over the past few decades, authorities have recently begun pushing the ‘One health initiative’ which involves understanding the emergence of infections while considering humans, animal and their environment as well. But how could this help? This helps us map out and control the infection right at the source itself. It also helps in real time monitoring of infections and that in turn helps us predict the emergence of any problematic strains and mutants!

P. S.: To better understand the emergence of zoonotic infections, I suggest you watch the Netflix series 'Pandemic'. It gives an accurate picture of what real time monitoring looks like at ground level. 
Image result for pandemic netflix

How does reverse zoonosis occur?

Generally, for any kind of spillover event to occur, there are certain barriers that need to be crossed over for it to be considered an actuality. First, there needs to be a great amount of contact between the infected species and the recipient species. This allows the infectious agent to replicate and develop mutations that can actually help it pass on from one host to another! This spillover event helps us determine whether this is something we should actually worry about or not. When I say worry, what I mean is that we can understand whether this mutation that they have developed gives them survival instincts to survive within the new host.

Reverse zoonosis events of SARS-CoV-2 from humans to animals (Source: Ayudhya & Kulken, 2020)
Reverse zoonosis events of SARS-CoV-2 from humans to animals (Source: Ayudhya & Kulken, 2020)

Another key issue that we need to think about is that most of these events don’t particularly arise from one species only. More often than not, we have always considered the source of the infections to come from one species only, such as in the COVID-19 pandemic. Are bats the only source of the viral infection? Most definitely not! Pangolins have been found to be a huge reservoir of the same and other phylogenetic studies are still trying to understand the true source of the same. For more on this, you can read the previous article here. There are many reports that indicate that humans have already managed to pass on the virus to species such as minks, cats, gorillas, etc.

Why is this so worrisome?

As a species, we have always benefitted from having close ties with animals especially those that are domesticated such as dogs, cats, horses, etc. In terms of improving our physical health from the standpoint of immunity and happiness, they are some of the best companions to do so. The benefits are not just limited to us but extend to those species as well. While there have been side effects of domestication such as selective breeding leading to a narrowing gene pool and even the development of lifestyle-related diseases such as obesity, there is an even bigger problem at hand. In a recent study of pet food, many strains of antibiotic-resistant bacteria were including those with rates of fatalities such as Escherichia coli O157, Campylobacter spp. and the like.

Reverse zoonotic transmission events from the year 1988 to 2012 (Source: Messenger, Barnes & Gray, 2014)
Reverse zoonotic transmission events from the year 1988 to 2012 (Source: Messenger, Barnes & Gray, 2014)

This means that they could be acquiring these strains and in turn helping them mutate further to a point where they could become even more dangerous with time. As these species harbor such resistant strains, eventually they can get passed on to humans and wreak more havoc! This entire cycle would eventually destroy any species that come into contact with them especially when existing interventions don’t match up to the problem. Despite this, current clinical guidelines do not require physicians to ask you about the presence of pets or whether you have come into contact with other animals in the past, even in the case that you have an infection. This is also one of the reasons why most zoonotic infections get missed.

What evidence exists to prove this concept?

I’ve been rambling on about how bad things could get or are rather getting, but what evidence do we have to support these arguments? We have just one too many. In a global report published in the year 2014, it was found that humans had passed on 7 fungal, 21 bacterial, and 12 viral pathogens over to other species. Surprisingly, wildlife seems to be the most affected which could be due to human encroachment into such spaces. Now in 2020, the SARS-CoV-2 strain is itself a reverse zoonosis event after it was discovered in cats and gorillas.

Potential events of zoonotic disease transmissions (Source: US Government Accountability Office)
Potential events of zoonotic disease transmissions (Source: US Government Accountability Office)

From a health perspective, it is quite dangerous but when considering its impact on agriculture, the effects are insurmountable. For example, in India, there has been evidence indicating that tuberculosis strains have passed onto cattle resulting in their death. So much so that it has even been found in elephants that are domesticated as well. Even in relatively untouched places such as Antarctica, not only have we managed to transmit many enteric infections to seals and seabirds, we’ve also managed to transmit COVID-19! When such events occur, it only brings into question that what more can be done to prevent this? This is one of the reasons why the One health approach has become more popular with time because of its ability to account for all three aspects – humans, animals, and the environment.

“However, the emerging or invasive pathogens that arrive at highly sensitive populations, such as the Antarctic and Sub-Antarctic fauna- could have severe consequences and cause the local collapse and extinction of some populations”.

Marta Cerdà-Cuéllar, researcher at the IRTA-CReSA

What is the takeaway?

In this scenario, the main points to takeaway is that humans need to stop thinking only about their own well being. We often miss the point that we’re just one species whereas there are millions of other ones out there. our actions also impact other species. Maybe we could use this opportunity to understand that impact and try to subvert any future threats especially to endangered species.

References

Lainé, N. (2018). Elephant tuberculosis as a reverse zoonosis. Medicine Anthropology Theory5(3).

Cerdà-Cuéllar, M., Moré, E., Ayats, T., Aguilera, M., Muñoz-González, S., Antilles, N., … & González-Solís, J. (2019). Do humans spread zoonotic enteric bacteria in Antarctica?. Science of the Total Environment654, 190-196.

Nelson, M. I., & Vincent, A. L. (2015). Reverse zoonosis of influenza to swine: new perspectives on the human–animal interface. Trends in microbiology23(3), 142-153.

Sooksawasdi Na Ayudhya, S., & Kuiken, T. (2020). Reverse Zoonosis of COVID-19: Lessons From the 2009 Influenza Pandemic. Veterinary Pathology, 0300985820979843.

Overgaauw, P. A., Vinke, C. M., van Hagen, M. A., & Lipman, L. J. (2020). A one health perspective on the human–companion animal relationship with emphasis on zoonotic aspects. International journal of environmental research and public health17(11), 3789.

Messenger, A. M., Barnes, A. N., & Gray, G. C. (2014). Reverse zoonotic disease transmission (zooanthroponosis): a systematic review of seldom-documented human biological threats to animals. PloS one9(2), e89055.

Reverse zoonosis: human pathogens are ruining wildlife (2018, December 11). Retrieved from https://www.healtheuropa.eu/reverse-zoonosis-human-pathogens/89348/

Ranganathan, P. & Raja, N. (2020, June 28). Reverse Zoonosis – When Humans Pass Diseases on to Animals. Retrieved from https://science.thewire.in/environment/reverse-zoonosis-when-humans-pass-diseases-on-to-animals/

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