In 2007, five years after the outbreak of SARS-CoV-1, the first SARS virus to cause serious disease, a review of the available studies triggered by that epidemic warned that the possibility of the re-emergence of another serious SARS outbreak or the advent of a different novel virus is not an “if” but a “when. The need for preparedness should not be ignored (1).
Global pandemics are becoming ever more common. A 2008 study found that the number of new infectious diseases emerging every decade between 1940 and 2004 was quadrupled (2) and another study, from 2014, discovered that outbreaks of infectious disease per decade between 1980 and 2013 had more than tripled (3). New pathogens are constantly emerging from animal reservoirs to challenge human populations. It is estimated that 60% of known infectious diseases and up to 75% of new infectious diseases originate in animals (4).
Here is an overview of the epidemics caused by viral visitors of the 20th and 21st centuries.
1918 – Spanish flu – caused by an H1N1 influenza originating in birds. This is the most severe pandemic in recent history. It infected about one-third of the world’s population and caused at least 50 million deaths (5). Though its name would suggest so, this virus did not originate in Spain. Spain was neutral in World War I and, unlike most other countries, did not suppress news of this new illness. Scientists still do not know where the Spanish flu originated but the US, France, China and Britain have all been suggested as its possible birthplace (6).
1957/1958 – H2N2 influenza originating in birds – caused 1.1 million deaths worldwide (7)
1968 – H3N2 influenza originating in birds – caused 1 million deaths worldwide (8)
1980s – HIV (Human Immunodeficiency Virus) – originating in monkeys and apes but “jumped” into humans likely after a few people ate infected bushmeat (9).
2002/2003 – SARS-CoV-1 coronavirus – originating in bats, this virus transferred to humans through the intermediate host of farmed civet cats which were bred for human consumption – caused 774 deaths worldwide (10).
2009 – swine flu – caused by an H1N1 influenza originating in pigs – caused 284,000 deaths worldwide (11)
2012 – MERS coronavirus (Middle East Respiratory Syndrome) – originating in bats but passed into humans through camels. Three or four out of every ten patients contracting MERS die (12).
2014 – Ebola virus – originating in fruit bats and apes but spilled over into humans and now spread through direct contact or by eating infected meat. Ebola kills about half of the people who contract the disease (13).
2019 – We are now in the midst of Covid-19, the disease caused by SARS-CoV-2 coronavirus. As yet, its death rate is unknown because we also don’t know how many people are actually being infected. Given the low amount of testing that is going on, that data won’t be available until this pandemic is over and people can be tested for antibodies to the virus, proving that they were infected by it.
How do viruses cause infection?
Like any organism, viruses need to reproduce to survive. However, viruses are unable to accomplish this on their own. They need to find a host cell that they can compel to produce copies of themselves. They do this by attaching to the membrane of a cell in the organism that they have infected, injecting their genetic material (either DNA or RNA) into the cell and instructing that cell to make more viruses. If the virus is a DNA virus, the host cell can use the same replication process it employs to produce its own DNA. This is a slow, deliberate process that prevents copying mistakes. RNA viruses, on the other hand, can be replicated much more quickly and without any “proof reading”. This makes changes, or mutations, more likely (14).
SARS-CoV-2 virus is an RNA virus. Though this virus has indeed been mutating over the past few months, it appears to be doing so at a very slow pace and the new copies remain close to the original. Scientists believe that its mutations will not interfere with the creation of an effective COVID-19 vaccine (15).
How do hosts fight against viral infection?
Host cells are protected by the immune system of the body in which they exist. Every organism has an immune system that protects from outside threats. One of its most important jobs is to fight against invading pathogens. Over time, all cells of an infected host will gain defenses which will prevent the same type of virus cell from attaching to them in a future infection, so averting disease. This is how we develop immunity to a virus after overcoming an infection (16). Most often, this immunity will last a lifetime, but sometimes it may prevail for a much shorter amount of time. At this point, we do not know how long a person who has recovered from with Covid-19 will remain immune (17).
It is possible for the immune system to be thwarted. More rapid mutations during viral replication, for example, in the case of an RNA-type virus, make it difficult for the immune system of the host to keep up with the changes. This reduces the ability of the host to fight off the virus and develop strong immunity in case of future infection attempts by the same virus (14,18).
How do viruses spread from animals to humans?
Scientists have known for decades that viruses can make a “species jump” between animal species and from animals to humans (19). For instance, in 2004, a viral outbreak fatal to humans in Thailand and Vietnam was traced back to a highly pathogenic avian influenza virus, H5N1, that had caused disease outbreaks in poultry in Hong Kong in 1997 and in China and other east Asian countries from 2002 to early 2004 (20).
Close contact with any animal can allow a virus to be picked up by an animal of another species. Once within the new host, the circumstances in which a virus finds itself can increase the chance of mutations within the virus. For example, if more than one viral strain infects a person around the same time, the different forms can combine within the host and create a new type of virus. These recombined versions are often more able to spread to new hosts. In addition, the genomes of some viruses are made up of multiple parts, any one of which can mutate. For instance, the influenza virus exists in many distinct strains. During flu season, these diverse viral variations within a person can intermingle and produce new viruses. This explains why novel influenza variants can appear every year, making it difficult for each year’s flu vaccine to produce antibodies for all the strains it may encounter (18,21).
Bats have turned out to be the reservoir for quite a few viruses. Bats are numerous, being the second most common mammal in the world after rodents, but they also have a unique character that makes them an excellent host for viruses. Bats are the only mammals able to fly. This ability requires an immense amount of energy and increases the body temperature and metabolic rate of bats. Cell breakdown can result, with fragments of DNA being released within the bat’s body that are detected by the bat’s immune system as a potential foreign disease-producing threat. It appears that the immune system of bats has evolved to temper its reaction to these DNA fragments, responding effectively but weakly to the perceived danger. The end result is that bats are able to tolerate the presence of viruses in their bodies without becoming ill (22).
When viruses first show up in host animals such as bats, poultry or swine, they don’t generally spread directly from these animals to humans. An extra step seems to be required to intensify the mutation process before a virus attains the ability to transmit to humans. This is accomplished by passing through another mammal. For example, civet cats acted as an intermediate host for SARS-CoV-1 and camels did the same for MERS (23). SARS-CoV-2 appears to have utilized an odd, scaly ant-eating mammal from Asia and Africa called a pangolin. These creatures are harvested for their meat and for their scales that are used as ingredients in Chinese medicine. Research from 2019 discovered that the genome sequences of diseased pangolins possess coronaviruses that were 90% identical to the SARS-CoV-2 virus that causes Covid-19. In addition, the viruses found in pangolins carry a configuration on their outer surface for binding to cell receptors that is virtually the same as that of the human Covid-19 virus strain (24,25).
How Human Activities Affect Virus Spread
The risk of virus transmission between animals and humans increases as interactions between the two species become more frequent. Activities such as intensive farming of animals for meat, increases in long-distance animal transportation, upsurges in live animal markets and destruction of wildlife habitats increase opportunities for animal-human interactions, boost disease transmission into humans and enhance viral spread in general (26,27,28,29,30). A joint consultation of the World Health Organization (WHO), the Food and Agriculture Organization of the United Nations (FAO) and the World Organization for Animal Health (OIE) concluded that the major cause of diseases spreading to humans from animals was the increased demand for animal protein in the diet of humans (31).
Though humans have long included some meat in their diets, demand for meat began to rise in the 1970s and, by 2004, world meat production had nearly doubled (19). In the search for ways to keep the price of meat low, meat-producing farms quickly grew in size and became the economic standard for churning out enormous quantities of meat at low cost in a short amount of time. It is estimated that CAFOs (Concentrated Animal Feeding Operations), also known as factory farms, now supply more than 90% of the meat eaten globally (32).
Farming practices unwittingly aide in the transfer of viruses from animals to human beings. University of Guelph agriculture professor, David Waltner-Toews, known for his work on animal and human infectious diseases and founder of the Network for Ecosystem Sustainability and Health, explains that factory farms actually select for more dangerous pathogens. Such farms consist of huge numbers of animals such as cattle, pigs or poultry, contained in an overcrowded environment, furnishing prime conditions for the spread of a virus. Out in the wild, a virus will not survive if it kills off its host before it has time to enter another live host. In the dense population of an overfilled barn, there is no shortage of new hosts. Often these animals are genetically very similar, allowing disease spread to occur without encountering any genetic variants that might be less susceptible. The intermingling of animals from various geographical areas increases the number and variety of infectious agents in these farms, escalating the possibility that different viral strains will recombine to create new ones. Add to all this the inhumane living conditions of animals housed in this way, with no escape from their own waste products and suffering high levels of stress, and it is inevitable that the immune systems of these animals will become overwhelmed so that they are less able to fight off the infections with which they come into contact (33,23,18). A 2009 report determined that factory farms played a large part in the spread of swine flu from pigs into humans during the epidemic that broke out that year (33). In 2018, a research team looked back at historical “conversion events”, when a not-very-pathogenic avian (bird) flu strain became much more hazardous. They discovered that most of these events occurred in large commercial poultry systems in wealthier countries like Europe, Australia and the US who were transitioning from small farms to intensive meat production systems (34). Similar conditions exist in wet markets, marketplaces that sell live and dead animals and other perishable goods in an open area, which have been pinpointed as the place from which SARS-CoV-2 jumped into humans (35).
How Environmental Conditions Affect Viral Spread
Climate change can impact human infectious diseases through effects on the pathogen or the host. For instance, a warming climate can increase the length of the season in which effective transmission of a disease can take place. It can also boost the spread of infectious diseases into new geographical areas (36). A warmer world may have adverse effects on the immune system of the host. Animal studies have shown that higher ambient temperatures result in loss of appetite in mice that in turn weakens their immune system and reduces the ability of mice to fight off a virus. It is unknown as yet if these results will also be true in human beings (37).
In 2016, it was revealed that outbreaks of diseases spreading from animals to humans could be predicted based on environmental changes. Climate change, human population growth and land-use changes all bring people into more contact with disease-carrying animals (38). Climate change can also cause species loss resulting in lower species diversity which is known to increase the spread of infectious diseases (38). A 2013 study by Eco-Health Alliance, a non-profit organization that tracks infectious diseases globally, found that one in three emerging infectious diseases are linked to land-use changes such as deforestation (39).
Factory farms, with their thousands of animal inhabitants contained in very tight quarters, cause an abundance of environmental dangers all on their own. These include (40,41);
Pollution of air, water and soil
Increases in greenhouse gases due to
– the digestive processes of ruminant animals producing methane
– the waste from the large numbers of animals housed together
– the fertilizers and pesticides needed to produce feed for these animals
– land-use changes such as deforestation required both to produce feed and to house animals
Reducing the Incidence of Infectious Disease Epidemics
Minimize meat intake
It is clear that a worldwide decrease in the daily eating of meat would ease the conditions that lead to viral outbreaks. However, stopping the eating of meat completely is clearly not a viable option. We live in a world of choice and, no matter what the incentive might be, the sudden unavailability of a commonly enjoyed food such as meat would be fiercely protested. We know from history that barring substances does not work very well. For example, prohibition of alcohol was legislated in Canada in the late 19th and early 20th century. It did not stop the availability of alcohol but simply drove its production underground. Anyone who wanted alcohol could find it and the prohibition laws did not last long (42).
A more positive path to take for lessening the inclination of people to eat meat is the informing, educating and persuading of individuals to undertake changes in their own lives that will make a significant difference in the world in which they live. People need to understand the consequences of their food choices on collective human health, the environment, and animals. Reducing meat intake would result in far-reaching benefits – lowering the incidence of infectious diseases, cutting greenhouse gas emissions (45), diminishing pollution, decreasing animal cruelty and lessening the burden of human chronic diseases including heart disease, diabetes, and some meat-related cancers (43,44).
Research has shown that people who understand the reasons behind decreasing meat consumption are far more likely to do it. In 2017, a study was carried out that looked at the effect of swapping beans for beef on the greenhouse gas emission targets of the United States (45). As a part of the study, researchers looked into the possible willingness of US citizens to make dietary changes for altruistic reasons. They admitted that a national substitution of beans for beef would be socially demanding. Results of their on-line survey were promising. Over 12,000 consumers from twelve different countries participated and the results showed that, among those aware of the damaging impact of meat on the climate, 44% said they would likely reduce their meat consumption and 15% had already lowered it (46). In addition, in a public survey performed by the UK government, 85% of over 3000 participants answered that they were likely to or might change their diet for environmental improvements. 53% of participants stated that they were willing to give up red meat completely (47).
Increase the Availability of Meat Substitutes
Asking people to give up meat requires an acceptable alternative to take its place. This part of the solution already has a pretty good head start. There are many innovative foods available on the market today including meat substitutes, a wide variety of plant-based milks and egg replacements. Large meat-producing companies are starting to make “blended meats”, which combine meat with plant-based ingredients, (48,49) as well as completely meatless products (50). New companies have also popped up. For instance, Beyond Meat (beyondmeat.com) and Impossible Foods (impossiblefoods.com) are now making entirely plant-based meat substitutes that have become very popular.
“Cultured meat” is a completely new product that is expected to be available in 2021. According to culturedbeef.com, this product involves removing muscle cells from cattle and then “feeding and nurturing the cells so they multiply to create muscle”. The company claims that it “could have immense effects in reducing the environmental impact of our agriculture system, minimizing threats to public health, addressing issues of animal welfare, and providing food security”. A product of this kind certainly reduces the worry about possible transmission of viruses or other pathogens. In addition, it has been shown to have significant environmental benefits compared to meat. The production of cultured meat requires up to 45% less energy; reduces greenhouse gas emissions by 78 to 96% and water use by 82 to 96% (depending on the type of meat it is mimicking); and requires 99% less land than the production of meat (51).
Reduce intensive farming operations
In November 2019, the American Public Health Association (APHA) called for a moratorium on all new and expanding CAFOs. Their policy recommendation was developed in collaboration with John Hopkins Center for a Livable Future. APHA is asking for a change in the method of raising food animals to one that takes into account animal welfare, soil health, community health and the health of the planet. This is not the first time the APHA has issued such a call to action. Their first cautionary request came out way back in 2003 and they have repeated the appeal many times since then (41).
Another group, CIWF (Compassion in World Farming), is a registered charity founded in 1967 that works toward a more ethical and sustainable food supply. It is teaming up with some of the world’s largest food companies to end the practice of factory farming and replace it with a system that does not involve overcrowding of animals in filthy conditions but instead feeds the world in a humane way (33).
Factory farms jeopardize rural livelihoods. As the demand for meat lessens and the phasing out of large farms is gradually realized, it is hoped that a new and improved system of meat production will take their place. Smaller farms with a focus on producing high quality meats under caring conditions would fill the niche for those who desire to continue to enjoy real meat in their diets. These new farmers could replace quantity with quality and be rewarded with a decent return for their labour and product. This would be good news for the many small farmers who have been pushed out of business and off their land by CAFOs.
Reduce the demand for wild meat in wet markets
Banning these markets will not work any better than did banning alcohol. People have their reasons for eating wild meat, one of which is food insecurity. Economically challenged people are malnourished and wild meat is a local source of protein. Only through reducing the reasons that people rely on eating this type of meat will the wet markets lose their appeal. The community focus should be aimed at providing access to nourishing foods and improving job outlooks so that people are able to more easily support themselves (52).
Decreasing meat production will go a long way to diminishing the threat of climate change
Our food system is responsible for more than a quarter of all greenhouse gas emissions. Research has shown that following even conservative health guidelines for meat consumption could cut total food-related greenhouse gas production by almost 30%. Widespread adoption of a vegetarian diet could reduce greenhouse gas emissions by 63% and full-on adoption of veganism could reduce emissions by about 70% (53).
The 2017 study on swapping beans for beef mentioned earlier discovered that, if everyone in the US quit eating meat and ate beans instead, 70% of the required US greenhouse gas reductions would be accounted for. This substitution would also reduce the use of cropland (for grazing animals and growing animal feed) by 42% (45).
Thoughts for the Future
Many countries in the world have been working together over the last decade, crafting standards for detecting, reporting on and responding to infectious disease outbreaks (55). But it seems that little thought has been put into preventing such pandemics. We don’t know where the next viral epidemic will come from although the CDC (Centers for Disease Control and Prevention) in the US considers that H7N9, a bird flu virus, could be the most likely culprit (54). Unfortunately, H7N9 is much more virulent than Covid-19, killing about 39% of the people it infects. All of us who live on this planet need to come together to seriously examine the role that meat production is playing in disease spread and in other threats to ourselves and this planet on which we live.
The pandemic we are fighting right now has brought the world together, cooperating in the fight against Covid-19. Will we have the foresight to keep this positive change going by making the significant alterations required to prevent the next infectious disease outbreak?
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