Immunity and Zoonotic Diseases

A zoonosis or zoonotic disease is a disease or infection which is caused by a pathogen (a virus, bacterium, fungus or parasite, for example) that originates in animals and when transmitted to a human can cause disease. Often, the disease does not make the animal himself sick but will sicken the human.  Sometimes the disease is directly transmitted from animals to humans, and other times the disease is transmitted from one animal species to an animal of another species and then to humans.

Ever since humans have inhabited the Earth, we have lived in coexistence with other organisms. Until fewer than 11,000 years ago, humans lived in small, nomadic groups where infectious diseases were probably unknown. Because these small bands of people were relatively isolated from one another, epidemic diseases were severely restricted to a particular local population; propagation and expansion of epidemics depend on frequent contact with other individuals who have not yet developed an adequate immune response. 

Medical anthropologists have discovered that the first period of infectious diseases occurred about 10,000 years ago when hunter-gatherers transitioned to a sedentary lifestyle due to the domestication of animals. The combination of sedentarization, increased population density and the cohabitation with animals led to the first major period of zoonotic diseases. Dr Michael Greger, the impassioned director of, said aptly during an illuminating video about pandemics, “When we brought animals into the barnyard, they brought their diseases with them.” In that same video, he describes how nearly all of the most widespread diseases – like measles, small pox, influenza and even the common cold – originally came from the animals we domesticated. In the wild, those very same animals kept their viruses to themselves; once we brought them out of their natural habitat, put them in ours and started living in proximity with them, their viruses had the ideal opportunity to “jump over” from them to us. 

It is often easy to see the zoonotic nature of certain diseases. For example, rabies is contracted from a bite or scratch of an infected animal – usually a dog; Lyme disease is transmitted to humans through the bite of infected ticks; malaria, dengue and chikungunya are all spread to humans by the bite of an infected mosquito; and Salmonella and E. coli are bacteria that humans contract by eating foods contaminated by animal (or human) feces. 

Recently we have seen an increase in severity of these diseases, but also an emergence of new diseases and a re-emergence of others like swine flu; bird flu/H1N1; Ebola; SARS (sudden acute respiratory syndrome); and coronavirus infections, the most recent of which is Covid-19. For the first time in human documented history, we are facing an intense increase in zoonoses.  This scary truth should make us all wonder what has changed to put us in such a threatening situation. 

According to microbiologists, the complex nature of the human-animal interface is continuously influenced by the effects of climate change, as well as natural and anthropogenic (human-made) factors.  As important and interesting it may be to detail how climate change and corresponding environmental transformations exacerbate zoonotic diseases, for the sake of brevity, we’ll turn our attention to how humans have unwittingly created “the perfect storm” for not just the origins of most zoonoses, but also their aggravation.

Dr. Greger describes three eras of human disease. The first was, as mentioned above, triggered by domestication – when we brought wild animals to barnyards who in turn brought diseases with them. The second era began about 150 years ago (in the West and much more recently in India) with the Industrial Revolution, leading to epidemics of chronic diseases (or “diseases of civilization”), like diabetes, obesity, cancer, heart disease, etc. The third era of human disease – the one we are currently living in – started in the 1980s and is due to drastic land use modifications and agricultural intensification.

What does this mean in terms of the risk of infectious diseases of animal origin? It means that for the past forty years or so, both in India and globally: 1) For many reasons (but primarily due to deforestation which itself is primarily due to livestock farming or growing crops for farmed animals), we have been encroaching on and destroying the habitats of wild animals, which not only leaves them exceedingly vulnerable to unnatural behavior and disease, but it also eliminates the “species distancing” that naturally occurs when their habitat stays whole and intact… leaving humans exceedingly vulnerable to being infected by their diseases. 2) The human population explosion has created an increased demand for food, especially animal products. This growing demand has been met with a parallel growth in intensive animal farming operations, where the animals are confined in horrific conditions that are not only inhumane but unhygienic.  An example close to home: India raises about 460 million hens annually to produce eggs and is currently the third largest producer of eggs in the world. More than 80 percent of eggs come from industrial animal farms, where egg-laying hens are intensively confined in barren, wire cages, called battery cages).

As filthy and appalling as these factory farms are, just about every other place where animals are bred, housed, slaughtered and packed is equally (if not more) sordid…and equally hazardous to humans. Most epidemiologists the world over agree that there is a clear link between the emergence of highly pathogenic avian influenza viruses and intensified poultry production systems. The level of confinement of these unfortunate animals is such that any virus in one will spread like wildfire to all the others (like the outbreak of bird flu in parts of Kerala and Karnataka in March 2020 that led to thousands of chickens being killed) but even more insidiously, these same viruses may “jump over” to those workers who are in direct contact with either the living or dead animals (or their secretions or excrements or blood etc) or to someone who unknowingly consumed an infected animal 

As well, the increase in the population of livestock bred for human consumption also leads to an increase in the contamination of potable water, soil, and air by their excreta.  India currently has a livestock population of about 536 million, up by about4.6% in seven years. The millions of tons of animal excreta don’t just disappear…they must go somewhere, of course, and that “somewhere” eventually ends up contaminating humans. For example, Salmonella and E. coli, present in the excreta of animals, are spread to humans via drinking water or soil contaminated by animal excreta. As stated in Animal-based versus Plant-based Agriculture, at least one-third of global diarrheal diseases are because of zoonotic causes. And though no one would argue that animal excreta alone is responsible for this dramatic report, we cannot help but deduce that there is a relationship between such a enormous livestock population in India (with a corresponding enormous amount of excreta) and the sad fact that diarrhea is the third leading cause of childhood mortality in India, and is responsible for 13% of all deaths/year in children under 5 years of age.

When regrettably an infectious disease is transmitted to a human, the extent of damage it inflicts on the body depends primarily on two things. This first is the frequency (in other words, how common the disease is in the population during a specific period of time) prevalence), severity and variety of the infection, and the second is the state of the immune system of the person infected. Though we have no control over the first factor, we have a great deal of control over our immune system. 

But first, what is the immune system? Dr.David C. Nieman says, “From birth, we are exposed to a continuous onslaught of bacteria, viruses, and other disease-causing organisms. Without an effective shield, each of us would soon succumb to infectious disease and cancer. In the battle with microbial invaders, we protect ourselves with a complex array of defensive measures collectively identified as the immune system.”

Our immune system has the capacity to protect our body against external attacks and to restore it to good health, by recognizing and eliminating a limitless variety of foreign invaders.   And while living in closer accord with nature is important for a solid immune system – by drinking clean water, breathing pure air and getting lots of sunlight, for example – the most important component of a healthy immune system is the food we eat. Why is that?

Simply speaking, to function optimally, our immune system depends on a continual supply of nutrients, namely the nutrients found in the food we eat. Most of these nutrients are best obtained from fruits, vegetables, whole grains, nuts, and seeds – in other words, plants. Eating a plant-based diet gives the immune system a boost, as the immune system relies on white blood cells that produce antibodies to combat bacteria, viruses, and other invaders. People on a plant-based diet have been shown to have more effective white blood cells when compared to those who eat animal products, due to a high intake of vitamins and minerals, phytochemicals and antioxidants found in plants and a low intake of animal fat. Plus, the fibre naturally present in most plants improves the health of the gut making it better able to absorb the nutrients ingested.

Indeed, not only do plants contain an abundance of immune-boosting nutrients, they do not contain the extremely harmful substances naturally found in all (even organic) animal-based foods, like cholesterol, saturated fats and hormones. These substances (and others…and also the fact that animal-based foods are highly acidic and low in fibre) are the major drivers of chronic diseases (like obesity, cardio-vascular disease, type-2 diabetes, various cancers, etc) which, aside from being life-threatening in and of themselves, compromise the immune system’s response to infectious diseases. 

The meat, eggs, and dairy coming from factory farmed animals (the vast majority of what we consume) contain not just the dangerous substances mentioned above, they also contain high amounts of steroids, hormones and antibiotics – chemical supplements given to the animals to increase their rate of growth, to increase their productivity (for dairy cows) and, in the case of antibiotics, to both promote growth in animals, to prevent them from dying from highly contagious contaminants created by their brutal and severe confinement, and to keep them alive in horrific living conditions that would otherwise kill them. As a matter of fact, the vast majority of antibiotics used in the world are given to farmed animals. This massive use of antibiotics in animals carries a danger both to animals and to humans who consume those animals. The name of this danger is antibiotic resistance. (Please note, we also use the term antimicrobial resistance or AMR in the case of farmed animals. To understand the difference, please click here. For the sake of this short article, though, we will continue to use the term “antibiotic resistance”)

Antibiotic resistance occurs when bacteria no longer respond to the antibiotics designed to kill them. That means the bacteria are not eradicated, continue to grow and eventually mutate into superbugs. When animals are infected with these antibiotic-resistant superbugs, they cannot be treated. Because most of these animals live in terribly confined spaces, when one animal is infected, the entire lot of them is, too. When a superbug is detected on a farm the animals are “culled” (a euphemism for “killed”), like the thousands of chickens in an Odisha poultry farm in January or the thousands in Kozhikode in March (both in 2020). 

The other great danger of the unregulated use of antibiotics in animals comes when we consume the products made from these animals (those animals that have lived long enough to produce milk, eggs or sent to the slaughterhouse for their flesh). Along with the cholesterol, saturated fat, steroids and hormones that were mentioned earlier, when we consume animal products we also consume the antibiotics that those animals ingested during their short, horrendous life. The resistant bacteria found more and more often in animals can easily travel to humans, creating untreatable, hard-to-contain infections. Studies have found that much of the meat on grocery store shelves today is contaminated with these bacteria, which cannot be killed with conventional antibiotics. That means that if you eat food tainted with these super-germs and become ill, many antibiotics that doctors rely on to treat infections will be less effective or even useless.

Antibiotic resistance is in no way a problem limited to developed countries. India is one of the top consumers of agricultural antibiotics worldwide, accounting for 3 percent of global consumption. In fact, India is considered to be one of a handful of global hotspots for antibiotic resistance in animals, because of a rising demand for meat, dairy and eggs.

Using animals as a source of food is clearly the main cause of zoonotic and chronic diseases, compromised immune systems and antibiotic resistance. Why continue endangering our health when a wholesome plant-based diet offers all the nutrients our body needs for a healthy immune system all the while reducing the risk of antibiotic resistance and a whole gamut of food-related chronic diseases?

-Dr. Rashmi Menon, Doctor of Homoeopathy and Holistic Medicine.


COVID-19: The Latest Zoonotic Disease Stemming from the Meat Industry:

Antibiotic Use in Food Animals: India Overview, published in 2018:

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