Updated: Jun 5, 2021
You may have read our series on HIV? We mentioned that the virus, originally known as SIV (Simian Immunodeficiency Virus), came from chimpanzees in central Africa. The virus was thought to have jumped into humans by the consumption of infected bushmeat. This form of transmission is known as zoonoses.
Zoonoses: Disease that occurs naturally in wild animals that can be transmitted between animals and humans.
Zoonotic diseases may be transmitted from animals to humans through direct contact or indirectly by food, water, vectors or in the environment . Once a disease has jumped into humans, it may be capable of human-human transmission, allowing it to spread throughout a population.
Whilst some pathogens are already capable of human-human transmission, some are not. This is where the ability to evolve comes in handy, something which viruses are particularly good at! Some viruses can evolve within their animal hosts, gaining traits that allow for successful human-human transmission, in some cases resulting in devastating outbreaks 
This type of event is not a new occurrence - in fact it is a worryingly common cause of disease outbreak in humans, responsible for historic and present day outbreaks of disease.
Disease spilling over from animals, specifically from bats, is understood to be the origin of the current COVID-19 pandemic . In fact COVID-19 is the 3rd major outbreak known to occur during the 21st century caused by a coronavirus! The first was the Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) outbreak in November 2002, in which 774 deaths had occurred by July 2003 . In June 2012 Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV) was identified, since then there have been 858 known deaths with cases of the disease identified across 27 countries  .
The table below includes some examples of zoonotic disease and the believed/ confirmed origin.
So, what are the ways zoonotic diseases can jump into humans?
The world we have created, the lifestyles we lead and our behaviours have increased the contact between humans and animals.
The origin of many zoonoses is bats. They are hosts to more pathogens than any other animal. Humans destroying their natural habitats may have led them to migrate to areas where they are exposed to other animals. Animals which in turn are in closer proximity to humans, creating a route for transmission. 
Examples of behaviours and factors contributing to the spread of zoonoses:
E.g. In parts of Sub-Saharan Africa wild animals are killed for meat, this exposure to animals body fluids and blood allows for disease transmission.
Farming and meat production
E.g. Animals in close proximity to one another during mass farming and abattoirs allows for the rapid spread of disease . These animals are also in close proximity to farmers, allowing disease transmission.
E.g. Change in the climate causing animals or vectors of disease (e.g. mosquito) to migrate to new areas. In these areas they are exposed to other animals and humans and are able to transmit disease, causing its spread over new geographical regions.
E.g. Deforestation and changes to the environment means people are coming into contact with species they otherwise would not have, allowing for increased human- animal exposure and disease transmission.
E.g. Disease transmission from rodents that are attracted to areas of poor sanitation, contaminated food sources or surfaces etc.
Travel and food supply chains
E.g. The global movement of people and trade offers a route for disease to spread across borders. This route of transmission became particularly evident with the spread of H1N1 influenza, HIV and COVID-19.
Illegal wildlife trade
E.g. Rearing and selling of exotic animals in wet markets increases human exposure to animals that carry zoonotic pathogens.
What can we do to prevent further pandemics?
Dr Peter Daszak - the president of the Ecohealth Alliance reports that there are an estimated 1.7 million unidentified viruses that have the potential to infect humans . Emerging zoonotic diseases are an ever-present risk, but are there ways we can try to prevent further disease outbreaks?
Surveillance: Identify existing, new and potential risks. For example, identifying pathogens of concern that can transmit to humans, possible transmission routes and areas of high risk within the environment, whether this be urban, peri-urban or rural. This requires collaboration across sectors and also across countries.
Behavioural changes: A more sustainable approach is needed. The overconsumption of animal products, intense animal rearing practices, mistreatment of wild animals, and habitat destruction cannot continue without consequences, some of which we have already seen. Of course, there is no quick and simple solution to these issues, but a global response is required, with a One Health approach being an important place to start.
Education: Many are unaware of the risks - increasing people’s understanding of zoonoses may help reduce incidence and prevent unnecessary and potentially harmful interactions. For example, we cannot tell hunters to not hunt bushmeat when it is their only source of food, but they can be educated on how to minimise their risk of exposure to the virus in the blood and bodily fluids of animals.
Rewilding: Destruction of habitats through deforestation has shrunk habitats, connecting wildlife with human populations and forcing wildlife to migrate to new habitats, living amongst animal species they would not normally. These factors increase pathogen transmission amongst both animal species and humans. We must re-establish biodiversity within ecosystems through rewilding projects, to repair those that are damaged and give animals a place to thrive and live, in order to prevent our next pandemic.
It is important that we understand global inequality means that people may be at higher risks of exposure to zoonoses. For example, lack of access to sanitation or reliance on bushmeat for food or to sell and earn money. These are not ‘lifestyle choices’, many do not have the option to simply change their way of living because of the increased risk associated and/or unsustainability .
Antibiotic resistance is making it more difficult to control zoonoses . Antibiotics are often put in animal feed as a preventative measure to encourage growth and prevent the spread of disease amongst crowded conditions. Increased use of antibiotics allows drug-resistant strains of zoonotic pathogens to develop and spread rapidly within animals and then human populations as they enter human food supplies. Links between drug-resistant pathogens found in both animals and humans have been seen in pathogen species such as Escherichia coli, Staphylococcus aureus (MRSA) and Salmonella .
We can't know when and where the next infectious disease outbreak will happen or by what organism. But there is a good chance it will be as a result of animal to human transmission.
It is important we change our behaviours to minimise the risk of it happening on an uncontrollable scale.
Are previous and current pandemics enough to make us finally take responsibility for our actions, and more importantly change them?
- A & L
We highly recommend checking out this 3 part series podcast on BBC Sounds! Chris van Tulleken gathers experts in the field to talk about how pathogens jump from animals to humans, focusing on 3 particular examples: COVID-19, Bird Flu and HIV.
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 Chris van Tulleken, 2021, The Jump: Covid-19, BBC Sounds [podcast] Available at:https://www.bbc.co.uk/sounds/play/m000t47k
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