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Zoonotic Diseases: The Next Global Plague?

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An in-depth overview of an evolving human health threat

An overwhelming amount of nutritional research implicates the consumption of meat, dairy and eggs as a key cause of cancer, heart disease, diabetes, and other serious illnesses. Industrialized factory farming is also one of the main drivers of environmental degradation, from pollution to global warming, pushing humanity’s very survival toward the brink of extinction. Yet there is another potentially deadly danger linked to the eating and production of animal products: zoonotic diseases that can be transmitted from domesticated farm animals to people.

According to the Centers for Disease Control and Prevention (CDC), an agency of the U.S. Department of Health and Human Services, food borne illnesses are responsible for about 325,000 hospitalizations and 5,000 deaths every year in the U.S. The CDC estimates that at least one-third of these illnesses and fatalities are directly attributable to the consumption of meat. However, this does not account for cases of food-borne illness caused by eating tainted dairy or eggs, the consumption of plant foods contaminated by bacteria originating from animals, or direct contact with infected animals.

The biological symmetries between humans and other animals enable some viruses to leap the species barrier when humans eat contaminated animal flesh, or by infecting farmers working in close proximity to livestock (who in turn become contagious and pass viruses to people with whom they come into contact). Some of these contagions are the result of complex evolutionary processes which make them so novel that the human immune system has not yet developed defenses against them. In other instances, bacteria growing inside the intestines of animals can afflict whoever ingests contaminated food with symptoms ranging from indigestion to fatal organ failure.

However they are transmitted, zoonotic diseases are clearly a grave threat to public health that some virologists say could potentially kill hundreds of millions of people worldwide should any of them grow into pandemics. Fortunately, there are many preventative measures we can take today to minimize the impact of this medical menace in years to come. With this in mind, below is an overview of the most common and emerging zoonotic diseases.

Methicillin-resistant Staphylococcus aureus (MRSA)

  • Prevalence: Over 90,000 confirmed cases and 18,000 deaths in the U.S. per year
  • Symptoms: Large staph infections (usually on the face, behind the knees, under the arms, and/or on the buttocks) that can turn into abscesses and lethally infect the bloodstream, bones, joints, lungs, and heart
  • Transmission: Direct contact between humans and infected pigs; possibly communicable through consumption of contaminated pork products

MRSA (pronounced “mersa”) now claims more human victims in the U.S. than the AIDS virus, and poses the greatest danger to people with compromised immune systems. Scientists have established that MRSA is transmitted to people through direct physical contact with pigs. However, fears that the disease may have entered the human food chain arose in 2008 when three patients in Scotland who had no contact with pigs were diagnosed with a variant strain known as ST398.

A team of independent researchers from the University of Iowa found MRSA present in 70 percent of the 209 hogs tested on 10 different farms in two states. Despite this evidence, the U.S. Department of Agriculture has no plans to test pigs or pork products for infection. Meanwhile, the pork industry categorically opposes testing, claiming that taking such precautions is “unnecessary to protect public health”.

Variant Creutzfeldt-Jakob disease (vCJD, or Mad Cow Disease)

  • Prevalence: 117 confirmed deaths worldwide since 1990
  • Symptoms: Loss of muscle control, changes in personality, impaired memory, and degenerating vision rapidly leading to severe dementia and death
  • Transmission: Consumption of beef products infected with prions (aberrant protein agents) from the brain or spinal cord tissue of cattle

For many years, farmers inexpensively fattened cattle to slaughter weight by feeding them a high-protein diet made from the detritus on the slaughterhouse floor — including the bones, entrails, brains, and spinal cords of other cattle, which is where the virtually indestructible prions that cause bovine spongiform encephalopathy (BSE) reside. BSE was nicknamed “Mad Cow Disease” because the prions damage bovine’s brains, causing them to exhibit symptoms of “madness” such as uncoordinated movement, loss of balance, and radical temperamental alterations. People who contract vCJD (the human equivalent of BSE) display similar behavioral abnormalities as the prions rapidly destroy their brains.

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The disease typically gestates for a period of years in the body, but usually kills its victims within weeks or months of the onset of symptoms. The vast majority of cases in both humans and cattle have occurred in the United Kingdom, but in the U.S., three BSE-infected cattle have been found, and three humans are know to have contracted vCJD. Notably, the CDC does not require doctors and hospitals to report when patients are diagnosed with the disease (so the number of affected could be considerably higher), and the USDA scaled back its detection efforts by about 90 percent in 2006 based on its claim that there is only “a very, very low level of BSE in the United States”.

H1N1 (Swine Flu)

  • Prevalence: In the 2009-2010 outbreak, over 16,455 confirmed human deaths worldwide, including 2,009 deaths in the U.S.
  • Symptoms: Fever, cough, sore throat, runny or stuffy nose, muscle stiffness, headache, chills, fatigue, diarrhea, and vomiting
  • Transmission: Humans contract H1N1 through direct contact with infected pigs (e.g., in hog barns on farms), then spread the virus to other people; there have been no documented cases of infection through consumption of pork products

H1N1 (commonly known as “swine flu”) gets its abbreviated name from the hemagglutinin (type 1 of 16) that binds the recombinant human-pig-bird virus to the host cell (the first of 9 types of neuraminidase). The recent H1N1 outbreak is descended from an influenza pandemic that killed more than 50 million people worldwide in 1918 and 1919, and recurred several times throughout the 20th century in different locations. Indeed, the evolutionary trajectory of the virus has been tracked by research identically matching 80 percent of the newest H1N1 strain’s gene sequences to a superbug that first emerged in 1998 from a Smithfield pig processing plant in North Carolina (the top pork-producing state in the U.S.) and ravaged the North American pig population.

While the virus had originally spread from pigs to farm employees working in close proximity to contaminated animals, the first case of a human infecting a hog herd occurred in Alberta, Canada. Soon after this discovery, the Egyptian government ordered the slaughter of the entire nation’s pig population – about 400,000 animals – ostensibly to stem the spread of H1N1 to humans. As the United Nations criticized the move as “a real mistake” that would do nothing to protect the populace, animal rights groups maintained that factory farms (rather than pigs) were responsible for the virus, and culling herds would be cruel and counterproductive.

H5N1 (Avian Flu)

  • Prevalence: 486 confirmed cases and 287 human fatalities worldwide; no reported human cases in the U.S.
  • Symptoms: Viral pneumonia, respiratory distress, and multiple organ failure
  • Transmission: Most cases are the result of direct physical contact with infected birds on farms and in live markets, or surfaces contaminated with their feces; passage of the virus from person-to-person remains relatively rare

Like other avian flu viruses, H5N1 is naturally present in the intestines of wild birds and rarely causes them to be sick, but this disease is devastating to domestic chickens, turkeys, ducks, and geese unfortunate enough to contract it. Transmitted via saliva, mucus or feces, H5N1 is highly communicable among avian populations, and can very quickly devastate whole flocks (with an average mortality rate near 90 percent) and expand to others. Notably, while this virus does not spread as easily to humans, it is almost as deadly, killing more than half the people it has infected.

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H5N1 was first found in the human population in 1997, and while most of the cases have occurred in Indonesia, the virus has also struck African and European countries. Given the relatively high fatality rate associated with H5N1, scientists are concerned about this strain mutating into a form that can pass more easily from person-to-person and cause a global pandemic that could claim hundreds of millions of lives. Laboratory research suggests that medications currently on the market could be used to effectively treat people for avian influenza, but experts also warn that the virus could quickly develop resistance to these drugs.


  • Prevalence: Kills several thousand people worldwide (approximately 580 in the U.S.) every year
  • Symptoms: Fever, abdominal cramps and diarrhea (which can spread infection to the bloodstream and become fatal)
  • Transmission: Salmonella typically infects people who consume foods contaminated with animal feces (usually meat, milk or eggs, but salmonella-tainted manure from farms can also poison fruits and vegetables)

With more than 40,000 cases diagnosed each year in the U.S., salmonellosis is among the most common intestinal infections affecting the human population. Infants, the elderly and persons with compromised immune systems are the most vulnerable to salmonella bacteria, which comes from the digestive tract of birds, reptiles and mammals. Studies have found that about 25 percent of chickens sold in the U.S. are contaminated with salmonella.

Persons infected with salmonella typically exhibit symptoms within 6 to 48 hours of ingesting the bacterium, and recovery usually takes about a week. Antibiotics are often not an effective treatment for salmonellosis because many strains have become antibiotic-resistant, and because these compounds can slow the process of intestinal shedding necessary for recuperation. Cooking food thoroughly and washing all surfaces that come into contact with raw meat can help prevent the spread of infection.

Escherichia coli 0157:H7

  • Prevalence: Causes approximately 73,480 illnesses, 2,168 hospitalizations and 61 deaths a year in the U.S.
  • Symptoms: Abdominal cramps, diarrhea, fever, and vomiting; approximately 10 percent of subjects (mostly children and the elderly) develop haemolytic uraemic syndrome (HUS), a disease that causes acute renal failure in as many as 5 percent of those who contract it
  • Transmission: Most infections are the result of physical contact with infected people or animals, or the consumption of contaminated water or food (most commonly milk or cheese that has not been pasteurized); can also be passed from person to person

One of the most common bacteria living in the digestive tract of humans and other animals, E. coli is usually harmless to its hosts. However, some strains make people sick, and one particular variety that has developed in the intestines of cattle (E. coli 0157:H7) has the power to kill. Research indicates that as much as 3 percent of the U.S. cattle herd may harbor this deadly contagion.

E. coli spreads very easily, as ingesting only a tiny amount of bacteria (as little as 10 microbes) can cause serious illness and even death. Food contaminated with E. coli looks, smells and tastes the same as nontoxic food, so it is impossible to tell using just one’s senses whether meat, dairy and other products (such as alfalfa sprouts and other vegetables) are safe to eat. Cooking food thoroughly and washing all surfaces that come into contact with raw meat can help prevent the spread of infection.


  • Prevalence: Approximately 2.4 million cases and 124 deaths in the U.S. every year
  • Symptoms: Diarrhea, abdominal pain, nausea, vomiting and fever; can become life-threatening to persons with compromised immune systems if it infects the bloodstream
  • Transmission: Most cases are caused by eating raw or undercooked poultry, or cross-contamination of other foods by infected flesh; outbreaks are also associated with the consumption of unpasteurized milk or infected water

By far the most common cause of food-borne illness, campylobacter affects almost 1 percent of the U.S. population every year. This bacterium naturally resides in the intestinal tracts of mammals and birds, doing them no harm. However, ingesting it can cause humans to suffer severe sickness and even death.

Studies indicate that more than one-half of raw chicken carcasses sold in the U.S. contain campylobacter. However, it is a relatively fragile bacterium that can be killed by drying, exposure to oxygen, excessive heat (i.e., cooking), and the pasteurization process. Cooking food thoroughly and washing all surfaces that come into contact with raw poultry can help prevent the spread of infection.

Factory Farms: Breeding Grounds for Zoonotic Diseases

Meat-borne diseases have plagued humankind since our pre-historical hunting and gathering days, but a relatively recent (mid-20th century) agricultural innovation – factory farming – has created a “perfect storm” of conditions for the rampant development and rapid dissemination of new pathogens among the human population. Some of the profit-driven techniques steering the world toward impending disaster include:

  • Overcrowding – Factory farms typically cram thousands of animals together in a single massive building where they spend most of their lives. Population density is increased through the use of intensive constriction devices, such as battery cages (for egg-laying hens) and gestation crates (for pregnant sows). Keeping animals packed together in regimented rows saves space and makes large farms easier to manage, but also greatly amplifies the spread of contagions through constant close contact.
  • Environmental deprivation – The stress caused by intensive confinement, lack of exercise, unnatural diet, absence of sunlight, taking children away from mothers, and other traumatic factors impairs animals’ immune systems to the point where their bodies become dangerously susceptible to disease. The living conditions at Concentrated Animal Feeding Operations (CAFO) are so bad that many animals would not survive without the antibiotics farmers constantly feed them. In fact, the Union of Concerned Scientists estimates that about 70 percent of the antibiotics administered in the U.S. are given to farm animals, an abusive practice that is making these drugs less effective in the treatment of human diseases.
  • Substandard sanitation – The average CAFO generates millions of gallons of waste a year, and animals therefore typically wind up standing and lying in their own decaying feces. As the waste putrefies on the ground, it also fouls the air with ammonia, methane, hydrogen sulfide, and other toxic gasses that diminish immunity and breed respiratory infections. Not surprisingly, microbes from animal feces are the primary source of most foodborne pathogens, often contaminating meat during the slaughter process.
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Downed animals – those too sick or injured to stand or walk – are more likely to carry dangerous viruses or bacteria that can be transmitted to humans who come into contact with them or eat their flesh. In March 2009, the U.S. government announced a new rule banning the slaughter of all downed cattle, a move that will help protect the public from zoonotic diseases and prevent much unnecessary animal suffering. However, it is still legal to cruelly force other species of animals to the kill floor for processing by prodding them with electric shocks, dragging them by chains, or pushing them with forklifts.

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As long as producers can make even a small amount of profit from the flesh and blood of downed animals, there will be an incentive to abuse them without regard for their suffering and endanger people’s health by processing potentially diseased animals into food. You can take a stand for human health and farm animals by going vegan and contacting Secretary of Agriculture Tom Vilsack urging him to extend the ban on slaughtering downed cattle to all animals used for food.


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