Zoonosis, also called zoonotic disease refers to diseases that can be passed from animals, whether wild or domesticated, to humans.
Although many diseases are species specific, meaning that they can only occur in one animal species, many other diseases can be spread between different animal species. These are infectious diseases, caused by bacteria, viruses, or other disease causing organisms that can live as well in humans as in other animals.
There are different methods of transmission for different diseases. In some cases, zoonotic diseases are transferred by direct contact with infected animals, much as being near an infected human can cause the spread of an infectious disease. Other diseases are spread by drinking water that contains the eggs of parasites. The eggs enter the water supply from the feces of infected animals. Still others are spread by eating the flesh of infected animals. Tapeworms are spread this way. Other diseases are spread by insect vectors. An insect, such as a flea or tick, feeds on an infected animal, then feeds on a human. In the process, the insect transfers the infecting organism.
The Centers for Disease Control (CDC) in Atlanta have said that most emerging diseases around the world are zoonotic. The director of the CDC has said that 11 of the last 12 emerging infections in the world with serious health consequences has probably arisen from animal sources. Wild animal trade occurs across countries and many people take in wild animals as domestic pets. However, pet shops and food markets are not properly testing for diseases and parasites that can cause harm to humans and other animals.
Some zoonotic diseases are well known, such as rats (plague ), deer tick (Lyme disease ). Others are not as well known. For example, elephants may develop tuberculosis, and spread it to humans.
Causes and symptoms
The following is a partial list of animals and the diseases that they may carry. Not all animal carriers are listed, nor are all the diseases that the various species may carry.
- Bats are important rabies carriers, and also carry several other viral diseases that can affect humans.
- Cats may carry the causative organisms for plague, anthrax, cowpox, tapeworm, and many bacterial infections.
- Dogs may carry plague, tapeworm, rabies, Rocky Mountain Spotted Fever, and Lyme disease.
- Horses may carry anthrax, rabies, and Salmonella infections.
- Cattle may carry the organisms that cause anthrax, European tick-borne encephalitis, rabies, tapeworm, Salmonella infections and many bacterial and viral diseases.
- Pigs are best known for carrying tapeworm, but may also carry a large number of other infections including anthrax, influenza, and rabies.
- Sheep and goats may carry rabies, European tick-borne encephalitis, Salmonella infections, and many bacterial and viral diseases.
- Rabbits may carry plague and Q-Fever.
- Birds may carry Campylobacteriosis, Chlamydia psittaci, Pasteurella multocida, Histoplasma capsulatum, Salmonellosis, and others.
Zoonotic diseases may be spread in different ways. Tapeworms can often spread to humans when people eat the infected meat of cattle, and swine. Other diseases are transferred by insect vectors, often blood-feeding insects that carry the cause of the disease from one animal to another.
Diagnosis of the disease is made in the usual manner, 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 cause is a fairly common infection. For example, tapeworms are usually species specific. Cattle, pigs, and fish all carry different species of tapeworms, although all can be transmitted to humans who eat undercooked meat containing live tapeworm eggs. Once the tapeworm has been identified, it is easy to tell which species the tapeworm came from.
Other zoonotic infections may be harder to identify. Sometimes the infection is fairly common among both humans and animals, and it is impossible to tell. Snakes may carry the bacteria Escherichia coli and Proteus vulgaris, but since these bacteria are already common among humans, it would be difficult to trace infections back to snakes.
Because of increased trade between nations, and changes in animal habitats, there are often new zoonotic diseases. These may be found in animals transported from one nation to another, bringing with them new diseases. In some cases, changes in the environment lead to changes in the migratory habits of animal species, bringing new infections.
Treatment is the established treatment for the specific infection.
Prevention of zoonotic infections may take 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 a suitable bait. In some places, the bait is dropped by airplane over the range of the potential rabies carrier. When the animal eats 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.
Many zoonotic diseases that are passed by eating the meat of infected animals can be prevented by proper cooking of the infected meat. Tapeworm infestations can be prevented by cooking, and Salmonella infections from chickens and eggs can be prevented by being sure that both the meat and the eggs are fully cooked.
Anthrax— A diease of wam blooded animals, particularly cattle and sheep, transmissable to humans. The disease causes severe skin and lung damage.
Bovine spongiform encephalopathy— A progressive, fatal disease of the nervous system of domestic animals. It is transmitted by eating infected food. Also known as Mad Cow disease.
Lyme disease— An acute disease which is usually marked by skin rash, fever, fatigue and chills. Left untreated, it may cause heart and nervous system damage.
Zoonotic— A disease which can be spread from animals to humans.
For other zoonotic diseases, programs are in place to eliminate the host, or the vector that spreads the disease. Plague is prevented by elimination of the rats—a common source of the infection—and of fleas that carry the disease from rats to humans. Efforts around the world to control bovine spongiform encephalitis, better known as Mad Cow disease, have focused on the destruction of infected cattle to prevent spread of the disease. Regulations on the makeup of the cattle feed to ensure safety and prevent the disease have helped curb its spread.
Other means of prevention simply rely on care. 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 bare legged, and wearing light-colored clothing so that the presence of the dark ticks can be readily seen.
"CDC Improves Monitoring of Outbreaks." American Medical News January 24, 2000.
"When man's best friend isn't." Mother Earth News June-July 2000.
"Zoonoses." Agricultural Research February 2000.
"Zoonotic Diseases." Medical Laboratory Observer March 2004: 12.
"The zoonotic threat: Curbing pet-to-people infections." Dog World October 1999.
American Association of Zoo Keepers (AAZK). Topeka Zoological Park 3601 SW 29th St., Ste. 133 Topeka, KS 66614-2054.
National Animal Disease Center Zoonotic Research Unit. 2300 Dayton Ave. PO Box 70 Ames, IA 50010.
By: Karen Kasmauski
Date: July 3, 2000
Source: Corbis Corporation.
About the Photographer: Karen Kasmauski has worked on twenty major stories for National Geographic since 1984, and is a photographer-in-residence. She is known for her photographs of complex social issues, which have been finalists for the National Magazine Awards. She also works on newspapers, and has published a book of her photographs, Hampton Roads, and contributed the images to Impact: Dispatches from the Front Lines of Global Health with text by Peter Jaret.
A zoonosis is an infectious disease such as rabies or plague that can be transferred from wild or domestic animals to humans. Humans can contract zoonoses, the plural of zoonosis, from mammals, birds, fish, amphibians, reptiles, and even insects. Depending on the disease, humans contract zoonotic diseases in different ways, including direct contact or close proximity with an animal, or by sharing an animal's water source.
Epidemiologists and other scientists who study diseases describe the transmission of zoonotic diseases in terms of hosts and vectors. A host organism is an animal that is infected with or carries a bacterial, viral, or parasitic disease. The disease can be transferred directly from the host to a person via a mechanism such as the exchange of fluid, or through a vector organism that is an intermediary between a human and the host. An example is the deer tick vector for the well-known illness, Lyme disease. The tick picks up the disease while feeding on an infected mouse or other small mammal. The tick carries the bacterial disease in its gut, and can pass it to a new host during a later feeding. Normally, Lyme disease is transmitted between small mammals, but a human can contract Lyme disease if bitten by the infected tick vector. Although deer do not become infected with Lyme disease, they are responsible for carrying and transporting ticks. There are some diseases which humans contract from animal vectors that are not classified as zoonoses; such as the transmission of malaria by mosquitoes. To complete their life cycle these non-zoonotic diseases require a human host. A true zoonotic disease is capable of being transferred to humans from animals, but does not need a human body in order to survive.
The Hantavirus Pulmonary Syndrome (HPS) is an often fatal zoonotic disease contracted by humans who breathe in the aerosolized virus transmitted by the urine, droppings, or saliva of infected rodents. This deadly pulmonary illness was unknown when it appeared in May of 1993, in the so-called "Four Corners" region of the United States, where the states of New Mexico, Utah, Arizona, and Colorado meet. Initially, it was unclear what was able to quickly kill healthy adults in this region. Virologists from the Center of Disease Control (CDC) used techniques that allow an analysis of a virus at the molecular level, to link the pulmonary illness to a previously unknown type of hantavirus, which was later named Sin Nombre (Spanish for "Without Name").
See primary source image.
Scientists are learning about HPS through the study of rodent populations, which often requires the trapping and collection of various mice species. Research in the Southwestern United States has linked the years having higher levels of precipitation with a larger population of rodents, as the moisture leads to a greater supply of food for rodents, as well as higher vegetation growth, which provides ample habitat and protection for the rodents. Due to the weather phenomenon El Niño in 1991 and 1993, there was much more rain in the Southwest. The population density of deer mice in New Mexico grew from one deer mouse per hectare to twenty to thirty per hectare during that time period. It is thought that this large population of mice led to the first outbreak of HPS in May 1993. Although rare, HPS has since been found throughout the United States. Rodent control around the household is the primary defense against the hantavirus.
There are some rare zoonotic diseases for which the host is still unknown. Ebola, for example, has several very deadly forms that periodically infect communities in Central Africa. Once a human is infected with Ebola, the disease can spread rapidly from human to human through the direct contact of bodily fluids. Research has shown that perhaps primates, elephants, or bats, could be responsible for carrying Ebola and transmitting it to humans. One hypothesis is that the disease spreads to a human who handles or ingests meat from an infected primate. Bats are considered possible vectors, since they do not die from Ebola, and could therefore be a host that maintains the disease in tropical forests. A much less dangerous form of Ebola is found in the Asia Pacific region.
One zoonotic disease, tularemia, which is similar to plague and found in rodents, is highly infectious. In its aerosolized form, it has been considered a possible biological weapon as far back as World War II, since it takes a very small amount of the bacteria to infect an individual. It is naturally spread by a type of biting flies, contaminated water, and through handling infected animal carcasses. These bacteria can survive in low-temperature water and soil. The United States is said to have maintained stocks of tularemia bacteria until the 1970s.
Preston, Richard. The Hot Zone: A Terrifying True Story. New York: Anchor, 1995.
Bunk, S. "Weathering Hantavirus: Ecological Monitoring Provides Predictive Model." The Scientist. (1999): 13(14)14.
"Tularemia." Journal of Environmental Health (April 2003): 65(8)31.
Centers for Disease Control and Prevention. "Ebola hemorrhagic fever." 〈http://www.cdc.gov/ncidod/dvrd/spb/mnpages/dispages/ebola.htm〉 (accessed January 16, 2006).
Centers for Disease Control and Prevention. "Lyme disease." 〈http://www.cdc.gov/ncidod/dvbid/lyme/index.htm〉 (accessed January 16, 2006).
Disease Control and Environmental Epidemiology Division, Colorado Department of Public Health and Environment. "Zoonotic Diseases." 〈http://www.cdphe.state.co.us/dc/zoonosis/zoonosis.asp〉 (accessed January 16, 2006).
World Health Organization. "Ebola." 〈http://www.who.int/mediacentre/factsheets/fs103/en/index.html〉 (accessed January 16, 2006).