Zoonotic Diseases: How Diseases Spread from Animals to Humans

As humans navigate the growing risk of zoonotic disease, it's important to understand where these diseases come from, and how reversing our dependance on meat, dairy, and other animal products can help make the world a safer place.

What is a zoonotic disease?  

In simple terms, a zoonotic disease is a disease that transfers from an animal population into a human population. According to the CDC, zoonotic diseases, also known as zoonoses, are caused by germs such as bacteria, viruses, parasites, and fungi. There are many different types of zoonotic diseases that currently cause human illnesses ranging from mild to life-threatening in severity.

In fact, it's estimated that six out of ten known infectious diseases are spread between animals and humans—and zoonoses are only becoming more frequent.

What causes zoonoses? 

Humans around the globe live in close proximity with both wild and domesticated animals, thanks in large part to the reliance of our food system on industrial animal agriculture. As we navigate our daily lives sharing space with animals, health experts say it's important to understand the different ways that zoonotic diseases can spread.

Direct contact

Diseases can jump from animals to humans who make direct contact with them, by touching an infected animal, for example, or getting bit by one. Diseases usually spread through direct contact with an animal's bodily fluids, such as urine, blood, saliva, feces, or mucus.

Indirect contact

Humans can also become infected while they're in an animal's habitat or living quarters. For example, zoonoses can spread to a human while they're cleaning out an aquarium or chicken coop, or when they're handling a pet's food and water dishes. On industrial chicken farms, birds live in squalid conditions, with the floor drenched in urine and feces.

Vector-borne

In epidemiology, a "vector" usually refers to insects, arachnids, and other small organisms that spread an infection from one host to another. Vector-borne diseases are particularly concerning to people who spend lots of time outside in very buggy areas. An infected tick, for instance, can attach to a human and transmit Lymes disease. It's also why mosquito bites are not just annoying, they can also potentially cause serious illnesses like Zika virus, malaria, and yellow fever, among others.

Foodborne

Foodborne diseases, or food poisoning, can be caused by eating raw or undercooked animal products. Eggs and chicken are among the most common sources of food poisoning. Salmonella and campylobacter—which can come from raw eggs and chicken—are common zoonotic diseases. Unnervingly, chickens are one of the most widely farmed land animals in the US and around the world. The US raised over 9 billion chickens for meat in 2020 alone.

Waterborne

Consuming water that has been contaminated with harmful bacteria can cause illness in humans. Lakes, rivers, and streams that have been contaminated with animal waste, for example, might have elevated levels of E. Coli bacteria—which is found in the fecal matter of warm-blooded animals. E. Coli contamination is just one reason why factory farms can have devastating impacts on neighboring bodies of water. If ingested by humans, water with E. Coli can cause anything from minor stomach discomfort to serious health problems, or even death.

How do zoonotic diseases spread between animals and people? 

There are many ways for zoonotic diseases to spread between animals and people. However, public health researchers have found a common thread through many of the primary drivers of zoonotic diseases. Namely, factory farming and the consumption of animals around the world is sharply driving up the risks of the next big outbreak.

Farming and ranching

By a conservative estimate, the agricultural industry has been responsible for at least half of all new zoonotic diseases since 1940. Epidemiologists say that percentage is probably higher. Nearly every aspect of intensive animal farming contributes to conditions that are ideal for the spreading of disease—overly stressed animals, living in often sordid conditions, crowded into tight spaces with one another, and in regular contact with human workers. All together, intensive animal farming creates a "perfect storm" for the spreading of zoonotic disease.

Wildlife trade

In addition to animal agriculture, both the legal and illegal wildlife trade—which refers to the global commerce of non-domesticated animals and plants—are serious drivers of zoonotic disease. A 2021 study by The Nature Conservancy found that 25% of mammals in the wildlife trade host 75% of all known zoonotic diseases. Because the complex process of transporting these animals results in "upwards of one billion" direct and indirect contacts among wildlife, animals, and domestic animals, the study's lead author Dr. Shivaprakash Nagaraju concluded, "wildlife trade is [conceivably] the key risk factor driving the global spread of zoonotic and emerging infectious diseases."

Animal captivity

There are many ethical and moral problems with holding animals in captivity and exploiting them for human entertainment. On top of that, animals in captivity also pose public health risks. Petting zoos and other animal exhibits where humans are able to directly touch captive animals are particularly concerning. Between the years 1990 and 2000, more than 25 zoonotic outbreaks were linked to animal exhibits.

Insect vectors

There are more than a billion insects for every human on planet Earth. While these tiny beings are essential to our survival, some of them can also act as the carriers of deadly disease. Ticks, fleas, and female mosquitoes (male mosquitoes don't bite) are common insects that can cause a variety of illnesses, most of which are now treatable. One of the worst pandemics in history however, the Black Plague, has been traced back to infected fleas, who entered human populations on the backs of diseased rats.

Deforestation and habitat destruction

Deforestation and habitat destruction—primarily driven by the agriculture industry—poses serious environmental risks. When wild animals see their habitats destroyed, they have no choice but to uproot and search for a new home. As they make these journeys, they're more likely to "bump into" other animals, increasing the chances for a once-contained disease to spillover into another population.

Climate change

For similar reasons as habitat destruction, scientists say that climate change is already increasing the chances for the next zoonotic pandemic. Animal agriculture contributes more global greenhouse gas emissions than the transportation sector at large—including the emissions from cars and planes. The extreme heat at the equator that results from global warming has wild animals and plant species racing toward the poles for more hospitable climates. This has prompted once-remote animal species to travel great distances, likely coming into contact with humans and other animals, and therefore potentially spreading disease along the way.

Contaminated food and water

Food or water that's been contaminated by a bacterium, virus, or parasite leads to tens of millions of annual food poisoning cases and more than 1,000 deaths in the United States alone. Norovirus, or stomach flu, is the leading cause of food-borne-illness-related death in the US. After that, salmonella, C. perfringens, and campylobacter are the next most fatal pathogens. In 2021, campylobacteriosis was the most highly reported zoonosis in the European Union, with over 127,000 cases, a more than 2% increase than in 2020. The European Food Safety Authority (EFSA) found that chickens and chicken meat may have directly accounted for roughly 20-30% of human cases.

How are zoonoses classified? 

Public health experts classify zoonoses by their root cause. In other words, the disease's pathogen, which could be a bacterium or a virus, or something else.

Bacterial zoonoses

Bacterial zoonoses are diseases caused by bacteria, single-cell microorganisms that are found almost everywhere on Earth and inside the human body. Most bacteria are harmless or even helpful, and relatively few cause disease.

Viral zoonoses

Viral zoonoses are diseases caused by viruses, infectious microbes made up of DNA or RNA surrounded by a "protein coat." Viruses can infect not only humans and other animals, but also plants, fungi, and bacteria.

Parasitic zoonoses

Parasitic zoonoses are diseases caused by parasites, organisms that attach themselves onto or inside of a host's body. Parasitic diseases can spread from animals to humans through the consumption of raw or undercooked meat, or by consuming food or water that's been contaminated by an infected animal's stool.

Fungal zoonoses

Fungal zoonoses are diseases caused by fungi, spore-producing organisms such as molds, yeasts, and mushrooms. Fungi can cause topical infections in people (such skin rashes and brittle nails) or inside the body (such as infections of the lungs or bloodstream).

Rickettsial zoonoses

Rickettsial zoonoses are diseases caused by an usual type of bacteria that can live only inside the cells of other organisms. Rickettsial infections are usually transmitted to humans through the bites from vectors such as ticks, lice, fleas, and mites.

Chlamydial zoonoses

Chlamydial zoonoses are diseases caused by the family of bacteria called chlamydiae, which are one of the most common infectious agents in humans. One type of the bacteria is frequently transmitted in people as a sexually transmitted.) infection—that infection is often referred to colloquially as simply "chlamydia." Another version, however, affects domesticated birds and can spread to humans when handling their birds or cleaning out their cages.

Mycoplasma zoonoses

Mycoplasma zoonoses are diseases caused by an atypical family of bacteria that are harder to kill through antibiotics. Mycoplasma pneumoniae can infect humans' respiratory system—the condition is often called "walking pneumonia—and spreads through small droplets from coughs and sneezes. There are also reported cases of this sort of bacteria spreading from animals to humans—like one case when an entire class of elementary school students became sick from the hamster they kept in their classroom.

Protozoal zoonoses

Protozoal zoonoses are diseases caused by protozoal parasites and often spread from companion animals to their owners. Many humans are protected from these types of diseases by strong immune systems. However, immunocompromised pet owners are at a much higher risk.

Acellular non-viral pathogenic zoonoses

Acellular non-viral pathogenic zoonoses refer to diseases caused by very unusual and not-well-understood pathogens, such as prion, which is a misfolded protein. Prion is believed to be the cause of neurological disorders such as mad cow disease, and similar diseases affecting the brains of humans.

Zoonotic disease examples

Avian influenza

Avian influenza, or bird flu, is a viral disease that especially affects wild waterfowl and domesticated poultry. Since 2022, over 58,000,000 birds have been affected by an outbreak of bird flu. It has infected humans in rare cases, but given an ongoing, severe outbreak in both wild and domestic bird populations, public health experts are concerned about potential spillover events coming in the near future.

Salmonellosis

Salmonellosis is caused by an infection from salmonella bacteria, which live in the digestive tracts of birds and other animals. Humans increase their risk of infection if they don't practice good hand-washing habits, and if they consume raw eggs, unpasteurized dairy products, or undercooked meat.

Psittacosis

Humans can get psittacosis from infected birds, such as parrots and cockatiels, or domesticated poultry such as chickens and turkeys. In people, the illness is usually quite mild and relatively brief.

Rabies

Rabies is a viral disease that can affect any mammal, but is most associated with bats, raccoons, skunks, foxes, and dogs. Rabies is usually transmitted to humans through the bite of an infected animal. The infection affects the central nervous system and once symptoms are present, is nearly always fatal.

Cat scratch disease

Cat scratch disease is a bacterial infection that is, of course, caused by cats. It can spread to humans when a cat licks an open wound, or if they scratch a person deep enough to break the skin. The infection can cause redness around the site of the wound, swollen glands, as well as flu-like symptoms. Symptoms are usually mild, but in rare cases, the infection can become serious if its spreads to other organs.

Malaria

Malaria is a disease caused by a parasite and spread to humans through mosquito bites. Left untreated, malaria can be fatal, especially in children under five. The disease is most commonly found in countries near the equator, especially in sub-Saharan Africa and Oceania.

Zika virus

Another vector-borne disease, mosquito bites are the most common cause of the Zika virus. However, it can also be passed )from a pregnant mother to her child or through sexual contact. The virus' symptoms usually include mild rash and illness. However, it can cause serious birth defects in newborns.

Lyme disease

Lyme disease is the most common vector-borne zoonose in the US. In humans, it's caused by infected black-legged tick bites. Left untreated, Lyme disease can result in prolonged pain, stiffness, and swelling, as well as memory problems and difficulty concentrating.

Bubonic plague

Responsible for the most infamous pandemic in history—known as the Black Plague or Black Death—the bubonic plague still crops up in cases throughout the world. These days, however, outbreaks of this bacteria-borne zoonotic disease can be controlled with antibiotics.

Swine flu

Also known as the H1N1 flu, swine flu is caused by an influenza virus that began infecting humans in 2009, in addition to pigs and birds. Pigs with swine flu may develop symptoms ranging anywhere from fever, coughing, and sneezing to depression and a lack of appetite. Humans can get sick from being within close proximity to pigs on farms or at county fairs.

COVID-19

Scientists agree that the COVID-19 pandemic was started by an animal, however, there's no definitive conclusion about what that animal was. According to the World Health Organization, the leading theory is that COVID-19 virus originated in bats and spread to humans via another animal.

Who is at risk for zoonotic diseases? 

As the far-reaching effects of COVID-19 have recently taught us, no one is immune from getting a zoonotic disease. However, there are various risk factors that can put certain people at higher risk of infection than others.

Who is at a higher risk of serious illness from zoonotic diseases? 

According to the CDC, children under five, adults over 65, people with weakened immune systems, and pregnant women are at heightened risk for serious illness from zoonotic diseases.

What can you do to protect yourself from zoonotic diseases? 

Whether they're buzzing around us outdoors, wandering through our backyards, or even living inside our homes, animals are all around us. This means there's always a chance of a bacterium, virus, or other kind of pathogen to jump from an infected animal to one of us. 

The good news is the CDC has laid out a practical list of ways to protect ourselves from preventable zoonotic diseases:

• Wash your hands after touching animals or animal products. Soap and running water for 20 seconds is one of the most effective ways to stop the spread of germs. If soap and water aren't available, hand sanitizer that contains at least 60% alcohol is the next best thing.
• When outdoors avoid tick, flea, and mosquito bites by wearing bug spray and long-sleeved clothing, and by paying attention to CDC health warnings in your area.
• If you have companion animals in your home, get educated about what kinds of disease typically affect your animal, and how to keep them healthy.
• Be wary of animal exhibits and petting zoos.
• This is one might be obvious, but avoid getting bitten or scratched by an animal.

However, the rise of industrial animal agriculture over the course of the twentieth century has transformed our food system into one in which thousands, and sometimes millions, of animals are packed into dense, unclean living quarters—creating conditions ripe for disease. While it's important to take individual precautions to avoid contracting zoonoses, the threat of zoonoses remains high unless we act collectively to change the role that animals play in our lives.

Preventing zoonotic disease globally

To stem the accelerated rise in emerging zoonotic diseases, public health experts are in agreement that the world needs to address its primary root cause: animal consumption. A 2022 research article in the journal Science Advances explains that it's not merely a matter of changing the way we raise animals for meat. Intensive agriculture is the main cause of deforestation—which drives the global spread of disease. The industry can reduce rates of deforestation by "intensification," but this process involves confining animals—and their waste—together into tight spaces. Not only is this inhumane, but it too greases the wheels for the spread of disease.

Slowing the spread of zoonotic diseases, the research concludes, will require reducing global reliance on animal products as a source of food, while accelerating forest conservation efforts worldwide.

Ending intensive animal agriculture

There's a lot of work to be done if we are going to end intensive animal agriculture and make the world a safer place for animals, and ourselves. 

Fortunately, The Humane League is rising to the challenge, and so can you.

At The Humane League, we work hard to hold corporations and elected officials accountable for how their actions affect the wellbeing of animals and the state of public health at large. There are plenty of ways to get involved in that effort.

Join THL's Fast Action Network, which gives you a way to take part in various impactful online activities, such as signing petitions, emailing decision-makers, or applying pressure via social media.