Zoonoses, also called zoonotic diseases, are diseases or infections that can be transmitted under normal conditions from animals to humans.


Many modern diseases are thought to have started as zoonotic diseases when humans first began to record history. Biblical references to a plague are believed to have been caused by bacterial zoonosis transmitted from fleas to humans. The Plague of Athens, in 430 BCE is thought to have been caused by a bacterium in the family Rickettsiae often found in ticks and lice. History continues to hold the secret as to when many human diseases were first acquired from animals. Nevertheless, students of medical history believe that many diseases such as influenza, smallpox, and measles began as zoonotic diseases.

Many emerging diseases such as West Nile virus, Ebola virus disease, Zika virus disease, Rift Valley fever, and Marburg virus disease are zoonotic diseases that pose a serious threat in the twenty-first century. Other zoonotic diseases such as anthrax and plague, although well understood, are of concern because they can, under the right circumstances, be used as weapons of bioterrorism.

Zoonoses can be transmitted from animals to humans in a variety of ways, and some diseases can be passed in more than one way. Methods of transmission include:

Certain changing conditions have increased the chance that new zoonotic diseases will emerge and known zoonotic diseases will spread to new geographic areas. These include:


The risk of contracting a zoonotic disease is highest among people who have a great deal of contact with animals and those who spend a lot of time outdoors where they may be exposed to vectors such as mosquitoes, ticks, and rodents. Although anyone can contract a zoonotic disease, children under five years of age, adults over age 65, and people with weakened immune systems are most likely to become seriously ill.

Common diseases and disorders

The following is a partial list of reservoir hosts and the diseases that they may carry. As hosts, they do not exhibit symptoms of the diseases they carry. They provide a means of disease transmission, as well as an environment that sustains the disease while it is not being transmitted. Not all animal carriers are listed, nor are all the diseases that the various species may carry.

A disease of warm blooded animals, particularly cattle and sheep, transmissible to humans. The disease causes severe skin and lung damage.
A disease caused by protozoa of the genus Babesia characterized by a malaria-like fever, anemia, vomiting, muscle pain, and enlargement of the spleen. Babesiosis, like Lyme disease, is carried by a tick.
Bovine spongiform encephalopathy—
Also known as mad cow disease, a progressive, fatal disease of the nervous system of domestic animals that is transmitted by eating infected food.
Lyme disease—
An acute recurrent inflammatory disease involving one or a few joints, transmitted by a tick-borne virus. The condition was originally described in the community of Lyme, Connecticut, but has also been reported in many other parts of the United States and other countries. Knees and other large joints are most commonly involved with local inflammation and swelling.
Disease caused by the presence of protozoan parasites of the genus Plasmodium in the red blood cells, transmitted by the bite of anopheline mosquitoes, and characterized by severe and recurring attacks of chills and fever.
A disease marked by high fever, chills, and muscle pain. It is most common in Australia and often goes unrecognized in North America, Europe, and parts of Africa.
Reservoir host—
An animal source of an infectious disease to humans that provides a means of transmission, as well as housing the disease in a sustainable environment.
An animal carrier that transfers an infectious organism from one host to another. The vector that transmits Lyme disease from wildlife to humans is the deer tick or black-legged tick.

Causes and symptoms

Zoonotic diseases occur when an animal disease is transferred either directly or indirectly to a human. Most often blood-feeding insects carry the infective agent from one animal to another.


Definitive diagnosis of each disease is made by identifying the infecting organism. Each disease has established symptoms and tests. Identifying the carrier may be easy or may be more difficult when the causative agent is a common. Sometimes the infection is common among both humans and animals, and it is impossible to tell the difference. Snakes, for example, may carry the bacteria Escherichia coli and Proteus vulgaris, but since these bacteria are already common among humans, it becomes difficult to trace infections back to snakes.


The treatment of zoonotic infections depends on the specific disease. Many bacterial zoonoses such as plague can be effectively treated with antibiotics if they are diagnosed early. Viral zoonoses are more difficult to treat. Zoonoses such as Ebola virus disease and chikungunya have no specific treatment and are treated with supportive care.

Public health role and response

The Global Early Warning System for Major Animal Diseases, including Zoonoses (GLEWS) is an early warning system for outbreaks of animal diseases. The World Health Organization (WHO), the Food and Agricultural Organization of the United Nations (FAO), and the World Organization for Animal Health (OIE) created GLEWS. It is used to help alert the international community in the prevention and control of threats from animal diseases, including zoonoses.



The prognosis for any zoonosis depends on the causative organism; however, for the most part, the prognosis for full recovery is good if treatment is promptly given with appropriate medicine. such as Ebola virus disease, which causes a high rate of mortality.


Prevention of zoonotic infections takes different forms, depending on the nature of the carrier and the infection. Some zoonotic infections can be avoided by immunizing the animals that carry the disease. Pets and other domestic animals should have rabies vaccinations, and wild animals are immunized with an oral vaccine that is encased in suitable bait. In some places, the bait is dropped by airplane over the range of the potential rabies carrier. When the animals eat the bait, they also ingest the oral vaccine, thereby protecting them from rabies and reducing the risk of spread of the disease. This method has been used to protect foxes, coyotes, and other wild animals.

Zoonotic diseases that are transmitted by eating the meat of infected animals or drinking infected water can often be prevented by proper cooking of the infected meat and adequate water purification.

Other means of prevention simply rely on consistently adjusting human behaviors. People living in areas where Lyme disease is common are warned to take precautions against the bite of the deer tick, which transfers the disease. These precautions include not walking in tall grass, not walking outside with bare legs, and wearing light-colored clothing so that the presence of the dark ticks can be readily seen. Use of mosquito repellant and sleeping nets in areas when mosquito transmission of disease is common helps to prevent infections as does hand washing after handling pets.

See also Anthrax ; Avian flu ; Ebola virus disease ; Escherichia coli ; Hantavirus infections ; Lyme disease ; Smallpox ; Tick-borne diseases ; Vector control ; Zika virus disease .



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National Center for Emerging Zoonotic Diseases, 1600 Clifton Rd., Atlanta, GA, 30329-4027, (800) 232-4636; TTY 888-232-6348, https://www.cdc.gov/ncezid/index.html .

United States Centers for Disease Control and Prevention (CDC), 1600 Clifton Rd., Atlanta, GA, 30329-4027, (404) 639-3534, (800) CDC-INFO (800-232-4636); TTY: 888-232-6348, http://www.cdc.gov .

World Health Organization (WHO), Avenue Appia 20, 1211 Geneva 27, Switzerland, +22 41 791 21 11, Fax: +22 41 791 31 11, info@who.int, http://www.who.int .

Samuel D. Uretsky, PharmD
Revised by Tish Davidson, AM

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