Zoonoses
Zoonoses
Disease History, Characteristics, and Transmission
Introduction
A zoonosis (pronounced ZOO-oh-NO-sis) is a disease that can be transmitted from animals to humans under natural conditions. Some zoonoses, after being transmitted from animals to humans, can be transmitted from human to human. There are hundreds of zoonoses, including some of the most deadly diseases known. For example, avian influenza (flu) pandemics begin as zooneses transmitted from birds, and malaria is transmitted by mosquitoes and kills about a million people annually. New zoonoses have been emerging in recent years with increased frequency due partly to the incursion of increasing human populations into animal habitats. The World Health Organization (WHO) said in 2007 that 75% of new communicable human diseases that have emerged over the last 10 years are zoonoses caused by pathogens found in animals or animal products. Zoonoses are found in all societies, but are most common and most deadly in poorer countries.
Disease History, Characteristics, and Transmission
Many zoonoses have afflicted human beings throughout their history. Before the development of agriculture in approximately 8,000 BC, people lived in small nomadic groups. Because their diet included wild game, they were prone to parasitic zoonotic diseases, such as hookworm. However, members of such groups who survived to adulthood tended to be healthy. Because of the relative mutual isolation of these groups, epidemic disease was rare.
After the domestication of plants and food animals, people began to live in larger groups and in closer contact with a variety of animals, including sheep, pigs, cattle, goats, and chickens. This made the transmission of diseases from animals to humans more likely, as well as the transmission of diseases from person to person. Tuberculosis, measles, smallpox, and influenza first entered human populations through agricultural contact with animals and animal products, and new influenza strains still are transmitted to human populations from domesticated pig and bird populations in Asia.
The development of global trade routes allowed the spread of epidemic diseases that had lost all connection with their original animal hosts, as well as of diseases that remained zoonotic, such as bubonic plague. Bubonic plague, which is caused by the bacterium Yersinia pestis, first appeared in Europe in 542 AD, but it only devastated the European population after 1346, when fleainfested furs were brought from China over recently opened trade routes. In a few years this zoonosis killed approximately one-third of the population of Europe and about 75 million people worldwide. Bubonic plague is maintained in rodent populations and transmitted to humans by flea and rodent bites.
Since the development of powered transportation in the nineteenth century, a whole new set of opportunities has arisen for zoonoses to travel the world. Migrant workers, political and economic refugees, tourists, and military personnel move quickly and in large numbers from one part of the world to another. Animals and animal products are moved globally by airplane and ship, sometimes inadvertently, as when rats and insect eggs are transported along with cargo. The tiger mosquito may have hitchhiked from Asia to the United States in worn tires shipped from the United States to Japan for retreading and then returned to the United States. Some experts warn that these mosquitoes may contribute to increased zoonotic transmission of North American viruses, including the LaCrosse virus, as they spread. The West Nile virus, which produced a notorious outbreak in 1999 and is transmitted to humans through mosquito bites, was apparently brought to the United States from Israel on an airplane, either by a stowaway mosquito or an infected human traveler.
Zoonoses can be transmitted to humans by bites, scratches, saliva, dander (skin flakes, similar to dandruff), droppings, and blood. Certain zoonoses, such as trichinellosis, are transmitted when humans eat the flesh of certain animals. Dog and bat bites can transmit rabies, cat bites can transmit cat-scratch fever, monkey bites can transmit hepatitis B, and rat bites can transmit leptospirosis, bubonic plague, salmonellosis, and rat-bite fever.
WORDS TO KNOW
EMERGING INFECTIOUS DISEASE: New infectious diseases such as SARS and West Nile virus, as well as previously known diseases such as malaria, tuberculosis, and bacterial pneumonias that are appearing in forms that are resistant to drug treatments, are termed emerging infectious diseases.
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
RE-EMERGING INFECTIOUS DISEASE: Re-emerging infectious diseases are illnesses such as malaria, diphtheria, tuberculosis, and polio that were once nearly absent from the world but are starting to cause greater numbers of infections once again. These illnesses are reappearing for many reasons. Malaria and other mosquito-borne illnesses increase when mosquito-control measures decrease. Other diseases are spreading because people have stopped being vaccinated, as happened with diphtheria after the collapse of the Soviet Union. A few diseases are reemerging because drugs to treat them have become less available or drug-resistant strains have developed.
RESERVOIR: The animal or organism in which the virus or parasite normally resides.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
Animals serve as the natural reservoir of some zoonotic diseases, and the vector of others. A reservoir is a long-term host organism that maintains an infective agent, usually a virus, bacteria, or parasite. The reservoir is usually not affected by the agent or is without symptoms, and can then pass on the infectious agent either by direct contact, or through a vector. A vector transfers an infectious agent from an infected host to an uninfected animal or human. Vectors are often arthropods, a group of invertebrate animals including insects. For example, birds are a natural reservoir of West Nile virus, and the disease is transmitted to humans by a mosquito vector.
Some experts reserve the term “zoonosis” for a disease that is transmitted to humans from vertebrate animals, that is, animals with spinal columns, such as cattle, dogs, cats, and rats. By this definition, diseases spread to humans by insect bites are not zoonoses, but disease or death cause by the bites of venomous snakes and fishes are. Snake bites kill over 30,000 people per year worldwide, mostly in Asia.
Scope and Distribution
The geographical distribution of any given zoonosis depends on the geographical distribution of its host animal or animals. All parts of the world are affected by zoonotic diseases. While most zoonoses are most common in less developed countries in Asia, Africa, and South America, a few, such as Lyme disease and trichinellosis, are more common in North American and Europe. Millions of people get sick each year from food-borne bacterial zoonoses, such as E. coli, salmonellosis, campylobacteriosis, anthrax, and brucellosis. Cystericercosis, a zoonosis caused by a parasite that lives in pigs, can cause headache and epilepsy and infects approximately one out of every 1,000 inhabitants of Latin America. Viral zoonoses include rabies, which kills about 55,000 people every year worldwide (mostly from dog bites). Avian influenza, a viral zoonosis that derives its name from the fact that the animal reservoir for the virus is birds, could infect a large part of the human population if it combined with other viruses or mutated in such a way as to become transmissible directly between humans. Experts fear that tens or hundreds of millions of people could die in an pandemic outbreak of such a virus, as occurred in 1918, when between 50 and 100 million people died during the Spanish influenza pandemic.
Treatment and Prevention
Treatment of zoonoses varies by infection type. Zoonoses can be caused by viruses, bacteria, parasites, and fungi. Bacterial infections can usually be combated with antibiotics; antifungals, antiparasitics, and antivirals may be used against other infections, but options are generally more limited for fighting non-bacterial infections. For example, there is no available treatment for West Nile virus, which can cause fatal encephalitis in some victims. Treatment of West Nile is supportive, that is, directed at relieving symptoms and helping the patient's own immune system fight off the infection, if it can.
Prevention of zoonoses is primarily aimed at reducing the incidence of the zoonosis in the animal population from which it is transmitted to humans. For example, destroying rabid dogs decreases the incidence of rabies in humans. Spraying DDT to keep down mosquito populations that can transmit malaria and other diseases has a similar effect. Veterinary medicine and human medicine must cooperate in many of these measures designed to reduce zoonotic disease.
Humans can also decrease the likelihood of contracting a zoonotic disease by avoiding contact with infected animals and their feces and infected food products. Food products may be rendered safe by excluding infected animals from the food supply, as in the case of anthrax or trichinellosis, or by improved food preparation (e.g., pasteurizing, thorough cooking, slaughter-house sanitation).
Impacts and Issues
Over 200 zoonoses are known, and more are emerging all the time. As long as an expanding human population continues to encroach on animal habitats, the likelihood that more zoonotic diseases will appear remains high. As noted earlier, 75% of recently emerging or re-emerging diseases are zoonoses. These include unconventional diseases such as prion diseases (transmissible spongiform encephalopathies), which are caused not by viruses or living organisms but by deformed proteins that, when ingested, cause similar protein deformation in the animal or human that has eaten the meat. These deformed proteins can accumulate in the central nervous system and cause mental degeneration and death. However, an emergent pathogen need not be unknown or unusual. A disease is considered emerging if it appears in an area where it has not previously been reported, as, for example, West Nile virus appearing in the United States.
See AlsoAnimal Importation; Arthropod-borne Disease; Bovine Spongiform Encephalopathy (“Mad Cow” Disease); Brucellosis; Cat Scratch Disease; Dengue and Dengue Hemorrhagic Fever; Ebola; Emerging Infectious Diseases; Giardiasis; Hantavirus; Lyme Disease; Malaria; Prion Disease; Rat-Bite Fever; Ringworm; Trichinellosis.
BIBLIOGRAPHY
Books
Krauss, Hartmut. Zoonoses: Infectious Diseases Transmissible from Animals to Humans. Washington, DC: ASM Press, 2003.
Torrey, E. Fuller, and Robert H. Yolken. The Beasts of the Earth: Animals, Humans, and Disease. Piscataway, NJ: Rutgers University Press, 2005.
Periodicals
Chomel, Bruno B., et al. “Wildlife, Exotic Pets, and Emerging Zoonoses.” Emerging Infectious Diseases 13 (January 2007): 6–11. Also available online at: <http://www.cdc.gov/ncidod/eid/13/1/6.htm> (accessed May 11, 2007).
Meslin, F.-X. “Global Aspects of Emerging and Potential Zoonoses: a WHO Perspective.” Emerging Infectious Diseases 3 (April-June 1997): 223–228. Also available online at: <http://www.cdc.gov/ncidod/eid/vol3no2/meslin.htm> (accessed May 11, 2007).
O'Brien, Sarah J. “Foodborne Zoonoses.” British Medical Journal 331 (November 26, 2005): 1217–1218. Also available online at: <http://www.bmj.com/cgi/content/full/331/7527/1217> (accessed May 11, 2007).
Web Sites
San Diego Natural History Museum. “Zoonoses: Animal-borne Diseases.” <http://www.sdnhm.org/fieldguide/zoonoses/index.html> (accessed February 27, 2007).
World Health Organization. “Report of the WHO/FAO/OIE Joint Consultation on Emerging Zoonotic Diseases.” 2004. <http://whqlibdoc.who.int/hq/2004/WHO_CDS_CPE_ZFK_2004.9.pdf> (accessed February 28, 2007).