Zoonoses: Overview

Zoonoses (zoh-ah-NO-seez) are infections that humans contract from animals. The singular form is zoonosis (zoh-uh-NO-sus).

What Are Zoonoses?

Zoonoses are infections caused by parasites * , bacteria * , fungi * , or viruses * that pass from animals to humans. There are more than 140 diseases that can be transmitted from animals to humans, including plague * (transmitted from rats), avian influenza * (also called bird flu), tapeworm (transmitted from eating pork from infected pigs), canine influenza (transmitted from dogs), cat scratch disease (transmitted from cats), fish tank granuloma (transmitted from fish), prion diseases (transmitted by eating meat from infected cows), monkey virus (transmitted from infected monkeys), psittacosis (sih-tuh-KO-sis; transmitted from infected birds), rabies (transmitted from infected dogs, raccoons, wolves, and other animals), and Salmonella (sal-muh-NEH-luh; transmitted from infected turtles). Most people contract zoonotic (zoh-uh-NAH-tik) infections from pets, farm animals, and other animals with which they are in contact. Places where such contact can occur include petting zoos, nature parks, circuses, and pet stores as well as such special events as state or county fairs, carnivals, and rodeos.

In some cases, there is an intermediate substance, usually food or water, that conveys the disease organism from the animal to humans. Many food-borne illnesses result from consuming eggs, meat, or milk from infected animals, or from drinking water contaminated by animal urine or fecal matter. In some cases, the intermediate substance is fur from an infected animal or cloth made from its wool. Anthrax, a potentially fatal disease, is caused by bacterial spores carried on animal hair that can enter the body through breaks in the skin, by ingestion (swallowing), or by inhalation. Wild animals and insects can be the source of disease, too, particularly for diseases spread by the bite of a tick, mosquito, or fly. Such animals as wild rodents, raccoons, and bats also can carry diseases that may be harmful to humans.

Zoonoses can cause either minor or serious illnesses. In some cases, the organisms involved may infect humans, but the people do not become ill. Other zoonoses can be very dangerous to people, especially anyone with an immune system weakened by age or illness. In addition, some people are at increased risk of contracting a zoonosis because of their occupation: hunters and fishers, farmers and ranchers, zookeepers, foresters, veterinarians, laboratory technicians, and field biologists.

Are Zoonoses Contagious?

Most of these infections do not spread directly from person to person or do so only in rare instances. Usually they spread from animals to humans in the following ways:

* or urine, either through the mouth (perhaps by touching a contaminated object and then touching the mouth) or by breathing in dust particles from dried feces
  • Bite or scratch of an infected animal
  • Consumption of the meat or other food products from an infected animal, particularly undercooked meat
  • What Are Examples of Zoonoses?

    Cat scratch. The bacterium Bartonella henselae is spread by a scratch to the skin. After an incubation period, a red papule arises on the site of the scratch.

    Cat scratch. The bacterium Bartonella henselae is spread by a scratch to the skin. After an incubation period, a red papule arises on the site of the scratch.
    Dr. Kenneth Greer/Getty Images

    Anthrax is a potentially fatal illness caused by contact with the spores of a bacterium called Bacillus anthracis. The spores are extremely difficult to kill by heat, drying, or most disinfectants, which is why they have been used as agents of bioterrorism. Anthrax spores can enter the human body by breathing, by eating meat from an infected animal, or through breaks in the skin during direct contact with the animal. Once inside the body, the spores multiply and release toxins that can cause the death of tissues and other severe symptoms. Anthrax that affects the skin typically produces black-colored areas of dead skin tissue. If inhaled, the spores produce flulike symptoms, heavy sweating, and chest pains. If swallowed, anthrax spores produce nausea and bloody vomiting, diarrhea, abdominal cramping, fever and chills, and painful swallowing. About 25 percent of people who develop anthrax from swallowing the spores die if untreated. Anthrax can, however, be treated with penicillin or other antibiotics when diagnosed in time.

    Cat scratch disease * , extreme tiredness, headaches, chills, and fever. Children are more likely than adults to develop cat scratch disease, particularly if they have not yet been taught to play gently with cats and kittens. Doctors may prescribe antibiotics to treat the infection.

    A sampling of infections contracted from animals.

    A sampling of infections contracted from animals.
    Illustration by Frank Forney © 2016 Cengage Learning®.
    Hantavirus * pain, and chills. In severe cases a person may experience shortness of breath and the lungs may fill with fluid. No cure was available as of 2016 for hantavirus infection, but people who have HPS typically are hospitalized in an intensive care unit, where they can receive oxygen and other types of supportive care. A vaccine against hantavirus was under development but was not available for use as of 2016.

    Reverse Zoonosis: When Humans Infect Animals

    Zoonoses are not just diseases that animals can give to humans; they also include diseases that humans can transmit to animals, in a process called reverse zoonosis or bidirectional zoonosis. Humans can transmit such serious diseases as methicillin-resistant Staphylococcus aureus infection, tuberculosis, influenza, giardiasis, cryptosporidiosis, ringworm, and pneumonia to household pets as well as to livestock or even wild animals.

    Reverse zoonosis has become a worldwide problem since the 1990s because of increased ease and speed of air travel and tourism; the worldwide transportation and sale of meat and meat products; and the growing popularity of exotic birds and reptiles as household pets in the United States and other developed countries.

    Lyme disease

    Borrelia burgdorferi (buh-REEL-e-uh burg-DOR-feree) bacteria inside an infected tick can cause Lyme (LIME) disease in humans after a tick attaches to the skin and feeds on a person's blood. Ticks pick up the bacterium by feeding on the blood of infected deer and mice, which serve as reservoirs for the organism. Lyme disease can produce a number of symptoms, including extreme tiredness, muscle aches, and swollen painful joints. Patients often describe the symptoms as being flulike and pay a visit to the doctor's office because no one else they know has the flu. At the site of the tick bite, some (but not all) people develop a bull's-eye rash, a red rash surrounded by rings that resembles a bull's-eye target. A person with Lyme disease usually is treated with antibiotics.


    Plague is a bacterial infection caused by Yersinia pestis (yer-SIN-e-uh PES-tis). People can contract plague through the bite of a flea that has become infected through contact with an infected rodent, such as a rat. The disease causes symptoms such as fever, headache, extreme weakness, abdominal pain, and swollen lymph nodes. In some cases the infection spreads through the blood and can infect the lungs. If this happens, plague can spread from person to person through coughing or sneezing. Plague was the cause of epidemics * in Europe and Asia during the Middle Ages, and it is still seen in the 21st century in many developing countries. It occurs in many developed countries too, including the United States, although not as many cases occur. Most cases in the United States occur in the Four Corners region of the Southwest (Arizona, Colorado, New Mexico, and Utah), California, Oregon, and Nevada. The disease can be fatal if it is not treated with antibiotics. Like anthrax spores, the plague bacterium has been used as a weapon of biological warfare.

    Psittacosis Rabbit fever (tularemia)

    Rabbit fever, or tularemia, is a disease that can be spread to people by rabbits, hares, and rodents. Tularemia is caused by the bacterium Francisella tularensis. There are several ways in which the disease can be transmitted, including being bitten by infected ticks, mosquitoes, horseflies, and deer flies; inhaling (breathing in) infected dirt or plant material; direct contact with infected animals through a break in the skin; drinking contaminated water; or, more rarely, eating meat from an infected animal.

    Signs of rabbit fever usually develop three to five days after exposure and vary depending on how the bacteria enter the person's body. The person may have fever, chills, headache, joint stiffness, muscle pains, shortness of breath, sweating, weight loss, a red spot on the skin (usually at the site of the bite) that becomes sore, and in some cases, pink eye (conjunctivitis). Some people may develop pneumonia. Although rabbit fever is rarely fatal, it has been used as an agent of bioterrorism because it can severely incapacitate people and cause public panic. Rabbit fever is usually treated with antibiotics.


    A virus that is carried in the saliva of infected animals can cause rabies when transmitted through a bite or, less commonly, through contact with saliva. Rabies in the United States is most often associated with raccoons, followed by bats and skunks, but any bite produced by an animal, whether domestic, stray, or wild, should be reported immediately to a local animal control agency. Symptoms include fever, difficulty swallowing, delirium * , seizures * , and coma * . If treatment does not begin very soon after the bite, death can result. Treatment includes intensive care in a hospital. A series of vaccinations * started at the time of a bite from a possibly infected animal can prevent the person from developing the disease.

    Human fatalities from rabies in the United States are rare, about one or two per year as of 2016, compared to over a hundred per year in 1900. Widespread vaccination of domestic pets like cats and dogs is the major reason why most cases of rabies in the United States are now diagnosed in wild animals, mostly bats, raccoons, and foxes. Worldwide, about 26,000 people die of rabies each year, most of them in Africa and Asia.


    Eating contaminated meat or having contact with the feces of an infected cat can put a person at risk of toxoplasmosis (toxo-plaz-MO-sis). This zoonosis is caused by a parasite called Toxoplasma gondii, and can produce such symptoms as swollen lymph nodes, muscle aches, headaches, and sore throat in a healthy person, and life-threatening brain infections in people with weakened immune systems, especially those who have HIV * /AIDS * Trichinosis

    If people eat meat (especially raw or undercooked pork products or the meat of wild carnivorous animals, including bears and feral hogs) infected with the eggs of Trichinella (trih-kih-NEH-luh) worms, the people can contract trichinosis (trih-kih-NO-sis), also known as trichinellosis. People cannot, however, transmit the disease directly to other people. Trichinosis is a disease that produces such symptoms as diarrhea, vomiting, and abdominal pain. It can cause nerve and muscle damage and heart and lung problems; in severe cases, people can die from trichinosis. Trichinosis is treated with antiparasitic drugs that kill the worms and allow the body to expel them. Patients with muscle pain are given steroid medications to ease the discomfort.

    How Do Doctors Diagnose and Treat Zoonoses?

    The diagnosis of a zoonosis begins with a careful patient history. The doctor will ask about the patient's occupation; pets in the household, including birds or small rodents (like gerbils or hamsters), as well as cats and dogs; any recent trips to zoos, fairs, circuses, or other events where live animals are displayed; any recent trips outside the United States; any history of immune disorders, allergies, or skin problems; and any recent incidents involving wild or stray animals near the patient's house.

    The doctor will then ask about the patient's specific symptoms and their location in the body as well as their time of onset and severity. The doctor will usually take the patient's temperature and blood pressure; listen to the patient's breathing, and palpate (press gently on) the patient's abdomen to determine whether the patient has a fever, is having trouble breathing, has a swollen abdomen or enlarged liver, or other internal symptoms. If the patient reports an animal bite or sting, the doctor will examine the bite mark or wound. In some cases the doctor may take a blood or tissue sample to test for the presence of a suspected bacterium or parasite.

    Patients who have suffered deep or repeated dog bites or have been bitten by cats are sent to the emergency department of a hospital at once because of the danger of severe infection. People bitten by a raccoon, bat, or other wild animal that may be rabid are usually given one dose of human rabies immunoglobulin (im-yoo-no-GLOB-yoo-lin) as soon as possible after the bite and a series of four injections of rabies vaccine over the next 14 days.

    Most zoonoses can be treated with a course of antibiotics along with aspirin or acetaminophen to relieve pain and bring down fever.

    Can Zoonoses Be Prevented?

    See also Antibiotic Resistance • Bacterial Infections • Bioterrorism Agents: Overview • Bites and Stings • Cyclosporiasis and Cryptosporidiosis • Food Poisoning • Giardiasis • Hantavirus Pulmonary Syndrome • Influenza • Intestinal Infections • Lyme Disease • Parasitic Diseases: Overview • Plague • Prion Diseases • Rabies • Rocky Mountain Spotted Fever • Roundworm Infection • Salmonellosis • Tapeworm • Tick-Borne Illnesses: Overview • Toxoplasmosis • Travel-Related Infections: Overview • Trichinosis • Tularemia • West Nile Fever • Yellow Fever


    Books and Articles

    Bauerfeind, Rolf, et al. Zoonoses: Infectious Diseases Transmissible from Animals to Humans. 4th ed. Washington, DC: American Society for Microbiology Press, 2016.

    Despommier, Dickson D. People, Parasites, and Plowshares: Learning from Our Body's Most Terrifying Invaders. New York: Columbia University Press, 2013.

    Sing, Andreas, ed. Zoonoses: Infections Affecting Humans and Animals: Focus on Public Health Aspects. New York: Springer, 2014.


    Centers for Disease Control and Prevention. “Diseases That Can Be Spread from Pets to People.” http://www.cdc.gov/healthypets/diseases/index.html (accessed April 23, 2016).

    Centers for Disease Control and Prevention. “Healthy Pets Healthy People.” http://www.cdc.gov/healthypets (accessed April 23, 2016).

    Centers for Disease Control and Prevention. “Psittacosis.” http://www.cdc.gov/pneumonia/atypical/psittacosis.html (accessed April 23, 2016).

    Centers for Disease Control and Prevention. “Tularemia.” http://www.cdc.gov/tularemia/index.html (accessed April 23, 2016).

    MedlinePlus. “Cat Scratch Disease.” U.S. National Library of Medicine, National Institutes of Health. https://www.nlm.nih.gov/medlineplus/catscratchdisease.html (accessed April 23, 2016).

    MedlinePlus. “Psittacosis.” U.S. National Library of Medicine, National Institutes of Health. https://www.nlm.nih.gov/medlineplus/ency/article/000088.htm (accessed April 23, 2016).

    MedlinePlus. “Tularemia.” U.S. National Library of Medicine, National Institutes of Health. https://www.nlm.nih.gov/medlineplus/ency/article/000856.htm (accessed April 23, 2016).


    Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-CDC-INFO, 800-232-4636. Website: http://www.cdc.gov (accessed April 23, 2016).

    Centers for Disease Control and Prevention, Division of VectorBorne Diseases. 3156 Rampart Rd., Fort Collins, CO 80521. Toll-free: 800-CDC-INFO, 800-232-4636. Website: http://www.cdc.gov/ncezid/dvbd/ (accessed April 23, 2016).

    National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-CDC-INFO, 800-2324636. Website: http://www.cdc.gov/ncezid/ (accessed April 23, 2016).

    * parasites (PAIR-uh-sites) are organisms such as protozoa (one-celled animals), worms, or insects that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. Parasites live at the expense of the host and may cause illness.

    * bacteria (bak-TEER-ee-a) are single-celled microorganisms, which typically reproduce by cell division. Some, but not all, types of bacteria can cause disease in humans. Many types can live in the body without causing harm.

    * fungi (FUN-eye) are a group of organisms that reproduce by spores; they include yeasts, molds, mushrooms, and mildew.

    * viruses (VY-rus-sez) are tiny infectious agents that can cause infectious diseases. Viruses can reproduce only within the cells they infect.

    * plague (PLAYG) is a serious bacterial infection that is spread to humans by infected rodents and their fleas.

    * influenza (IN-floo-EN-za) is a contagious viral infection that attacks the respiratory tract, including the nose, throat, and lungs. Also known as the flu.

    * feces (FEE-seez) is the waste products excreted from the gastrointestinal tract.

    * lymph nodes (LIMF) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

    * abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

    * epidemics (eh-pih-DEH-miks) are outbreaks of diseases, especially infectious diseases, in which the number of cases suddenly becomes far greater than usual. Usually, epidemics that involve worldwide outbreaks are called pandemics.

    * delirium (dih-LEER-e-um) is a condition in which a person is confused, is unable to think clearly, and has a reduced level of consciousness.

    * seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

    * coma (KO-ma) is an unconscious state, like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.

    * vaccinations (vak-sih-NAYshunz), also called immunizations, are the giving of doses of vaccines, which are preparations of killed or weakened germs, or a part of a germ or product it produces, to prevent or lessen the severity of the disease that can result if a person is exposed to the germ itself.

    * HIV, or human immunodeficiency virus (HYOO-mun ih-myoono-dih-FIH-shen-see), is the virus that causes AIDS (acquired immunodeficiency syndrome).

    * AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).

      This information is not a tool for self-diagnosis or a substitute for professional care.