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Zoonotic diseases: what are they and how do they spread?

Photo: Seb Alex / We Animals Media

Three out of every four emerging infectious diseases are zoonotic: they have an animal origin and have spread from animals to people. In the case of the recent outbreaks of COVID-19 and monkeypox (as it is still currently named), both of these diseases were transmitted to humans from animals. Understanding the risks of zoonotic diseases and the steps that you can take to avoid contracting one is necessary for keeping you and your loved ones safe.

What is a zoonotic disease?

According to the U.S. Centers for Disease Control (CDC), a zoonotic disease is “caused by germs that spread between animals and people.” Experts speculate that the COVID-19 pandemic was spread from animals to people in a market at which wild animals were sold. It is suspected that raccoon dogs, one of dozens of species of wild animal sold at the market, may have transmitted the disease to people. Monkeypox is another zoonotic disease that, contrary to its name, can spread from a variety of different species, including rats and squirrels, to people. Once a person has contracted the disease they can then spread it to other humans.

Preventing zoonotic diseases, also called zoonoses, is one reason why the animal companions we share our homes with get vaccines, dewormers, and other preventive care. Some of the diseases that these measures protect against, including rabies and many intestinal parasites, can be contracted by humans.

Zoonotic disease definition

The World Health Organization (WHO) defines a zoonotic disease as “an infectious disease that has jumped from a non-human animal to humans.” Though this definition has slight differences from the CDC’s definition above, they agree that zoonotic diseases impact both humans and animals. Zoonotic diseases can be caused by fungi, parasites, viruses, and bacteria.

Examples of zoonotic disease

The recent COVID-19 pandemic brought the world to a grinding halt. Entire countries shut down, some for weeks on end, in order to fight the transmission of the seemingly unstoppable virus. Though crushing in its effects for many, the pandemic provided a critical insight into how dangerous zoonotic diseases can be. Yet we continue to invite the spread of the next deadly zoonoses through the methods we use to raise farmed animals. Experts are well aware of the risks we run by continuing to raise most of the animals we farm in large-scale, industrial settings.

Perhaps the greatest risk that we take is continuing to raise chickens on factory farms, where tens of thousands of birds are shoved into sheds and diseases are able to quickly spread and mutate among animals that are already immunocompromised. In order to prevent a potentially even more devastating pandemic from emerging it is essential that we reduce the number of animals that we raise and change farming practices . That’s why we have called upon world leaders to create policies that eliminate the dangers of factory farming.

Examples of zoonotic diseases include:

  • COVID-19. COVID-19 has been responsible for the death of 6 million people around the world.
  • Zoonotic influenza (bird flu, swine flu). These strains of the flu are those which can be transmitted from animals to humans. The zoonotic influenzas that most of us are likely to be familiar with are swine influenza and avian influenza.
  • Rabies in the United States is most commonly found in wild animals such as bats, rats, and raccoons. Humans and companion animals typically contract the disease after being bitten or scratched by an infected animal.
  • Lyme disease. This disease is spread by ticks and is the most common vector-borne disease in the U.S.
  • West Nile virus. Most people who contract West Nile virus don’t feel ill, but there is no treatment or preventive vaccine available, even for those who do experience severe symptoms.
  • Plague killed millions of people in Europe during the Middle Ages and is still spread to humans by fleas in rural areas of the western United States, and far more commonly in Africa and Asia. If not treated promptly with antibiotics, plague can quickly cause serious illness or even death.
  • Psittacosis (Chlamydophila psittaci, Chlamydia psittaci). Proper handling and sanitation when handling birds is the best way to prevent the contraction of psittacosis. If one is infected the bacteria can cause pneumonia.
  • Trichinosis (Trichinella spiralis). Consuming raw or undercooked meat from an animal that has been infected is the most common way of contracting trichinosis. Symptoms of infection include facial swelling, fever, and chills.
  • Cat scratch disease (Bartonella henselae). Cat scratch disease (CSD) can be transmitted to humans from cats when they lick an existing wound or bite or scratch hard enough to break skin. Symptoms can include swollen lymph nodes, fever, and headache.
  • Rift Valley fever. Originally identified in the Rift Valley of Kenya in 1931, this disease causes severe illness in both animals and people. Infected animals run the risk of death or aborting fetuses. Though most cases in humans are mild, a severe case can lead to death. The majority of cases in humans are the result of contact with the blood or organs of an infected animal.
  • Intestinal parasites acquired from animals. Many of the parasites frequently found in animals can be transmitted to people. One way that this transmission can happen is by swallowing food or water that is contaminated by the stool of infected farmed animals. Contamination can take place when water sources or orchards are located near farms that are raising infected animals.
  • Bioterrorism diseases. Bioterrorism diseases are those that have been, or could be, used as a weapon. Examples of zoonotic diseases with bioterrorism potential are anthrax and plague.

What are the symptoms of zoonotic disease?

There are no universal symptoms of zoonotic diseases. Some diseases that can be transmitted to humans from animals have relatively mild symptoms that can be easily treated with proper medical attention while others cause much more severe suffering and death. Regardless of the type of zoonotic disease, there are a number of measures you can take to avoid contracting a zoonotic disease, including ensuring that your companion animals are kept up to date on all preventive care, washing your hands regularly, practicing proper food handling, and adopting a plant-based diet.

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

According to the CDC, those at greatest risk of falling ill from a zoonotic disease are people with weakened immune systems such as the elderly and children. The latter group is especially at risk because they are likely to touch their face and mouth more frequently than adults.

How do germs spread between animals and people?

Farming, ranching and animal husbandry

Perhaps one of the most pressing issues when it comes to the spread of zoonotic diseases is the way that animals are raised for food. Animal agriculture drives expansion into previously unspoiled areas, where potentially zoonotic diseases often circulate among wildlife. The expansion of animal farming exposes people and the animals we raise to these diseases from the wild animals who previously lived there. Though the mutations that allow a disease to jump to a new species host is rare, the sheer scale of animal agriculture means that practically speaking it is guaranteed to happen repeatedly.

The risk is further intensified by the sheer number of animals housed on factory farms and how closely together immunocompromised animals are housed. Such a housing system means that when one animal gets sick, the disease is able to spread quickly and mutate further.

Hunting and consuming wild animals

Several diseases have been traced back to the hunting and consumption of wild animals including the monkeypox virus and Ebola.


As large areas of land are deforested , species continue to face extinction. Those that can thrive despite the destruction of their homes—usually species such as bats and rats—are also those that are most likely to spread zoonotic diseases to humans.

Climate change

The majority of diseases that have the potential to be zoonotic presently only affect wild animals. However, climate change will continue to reduce and shift the ranges of wild animals, bringing them into contact with people. Of particular concern are bats, who account for a large proportion of viral transmission from animals to humans.

Contamination of food or water supply

Runoff from farms raising animals can spread into the drinking water supply or be used to water crops intended for human consumption, resulting in contamination. Research has suggested that water contamination is commonly a driver of large-scale breakouts of zoonotic diseases. In smaller, more routine breakouts that result in fewer cases, food contamination is the most common driver, followed by water contamination.1

Wildlife trade

Researchers suspect that COVID-19 was originally transmitted to humans at a market that housed a large number of wild animals that had been captured and were being sold. Though the specific species that transmitted the disease is not known, researchers suspect it may have been a raccoon dog.

Insect vectors

Despite their small stature, insects can carry a vast array of zoonotic diseases. Mosquitoes can carry and transmit West Nile virus, yellow fever, and malaria while ticks transmit Lyme disease.


Our companion animals can spread many zoonotic diseases to us. Most can be avoided with proper preventive care such as vaccines and a monthly parasite treatment.


Exhibitions that include animals as a form of entertainment, such as petting zoos, can be a hotbed for the transmission of zoonotic diseases. Most at risk of transmission are young children that are likely to pet the animals and then touch their faces.2

What can you do to protect yourself and your family from zoonotic diseases?

There are several steps that you can take to protect yourself and your loved ones from contracting zoonotic diseases.

  • Ensure all companion animals receive appropriate care. Making sure that all your companion animals are being cared for well is necessary to ensure they do not unwittingly bring any zoonoses into your home. This means making sure they are clean, are up to date on their preventive care, that their housing areas are clean, and that they are treated with respect to avoid any scratches or bites.
  • Support policies aimed at limiting the growth of factory farming. Industrial scale poultry and pig farming are widely seen as the types of animal farming with the greatest risk of zoonotic spillover. Supporting policies that limit the growth of factory farms and enable transition to forms of agriculture with lower zoonotic risk is one way we can act collectively to protect ourselves from future pandemics.
  • Stop eating meat and other animal products. Many types of zoonoses can be contracted by preparing or consuming the meat of an animal that had the disease. Leaving animals off our plates is one way to avoid contracting zoonoses in this manner.
  • Leave wildlife alone. When we see wild animals it can be tempting to interact with them by offering them food or attempting to get closer. Unless an animal is in active distress, the best thing to do for both you and them is to leave them alone.
  • Practice safe food handling. Properly washing the foods you buy prior to preparation and making sure they are prepared correctly is another effective way to get rid of any bacteria or parasitic hitchhikers.
  • Wash your hands. One of the best ways to prevent the spread of any disease is to wash your hands regularly to prevent the spread of bacteria.
  • Use insect repellent. To avoid zoonoses that are spread by insects, use insect repellent to prevent bites.
  • Don’t visit animal exhibitions. Visiting zoos, petting zoos, and other places where animals are treated as entertainment may put you at greater risk of coming into contact with zoonoses. Opting out of animals for entertainment is the best way to prevent this.


COVID-19, the global pandemic that stretched for three years and still claims victims daily, exemplifies how harmful and disruptive zoonotic diseases can be. Despite how recently we experienced this tragedy, we continue to place ourselves at risk of another global pandemic thanks to the way we raise farmed animals. In the U.S., 99 percent of farmed animals are raised in factory farms, which provide an ideal environment for the transmission and mutation of diseases that can then make the jump to human hosts. Any global effort to reduce pandemic risk must focus on reforming industrial animal agriculture. Deintensifying existing industrial farms while placing a moratorium on new factory farm construction is the public health measure that would most dramatically reduce the risk of the next pandemic virus.



Patrick R. Stephens et al., “Characteristics of the 100 Largest Modern Zoonotic Disease Outbreaks,” Philosophical Transactions of the Royal Society B: Biological Sciences 376, no. 1837 (November, 2021),


Cheyenne C. Conrad et al., “Farm Fairs and Petting Zoos: A Review of Animal Contact as a Source of Zoonotic Enteric Disease,” Foodborne Pathogens and Disease 14, no. 2 (February, 2017),