Zoonoses

(biology) Diseases which are biologically adapted to and normally found in lower animals but which under some conditions also infect humans.

Infections of humans caused by the transmission of disease agents that naturally live in animals. People become infected when they unwittingly intrude into the life cycle of the disease agent and become unnatural hosts. Zoonotic helminthic diseases, caused by parasitic worms, involve many species of helminths, including nematodes (roundworms), trematodes (flukes), cestodes (tapeworms), and acanthocephalans (thorny-headed worms). Helminthic zoonoses may be contracted from domestic animals such as pets, from edible animals such as seafood, or from wild animals. Fortunately, most kinds of zoonotic helminthic infections are caused by rare human parasites.

The best-recognized example of a food-borne zoonotic helminthic disease is trichinosis, caused by the trinchina worm, Trichinella spiralis, a tiny nematode. People commonly become infected by eating inadequately prepared pork, but a sizable proportion of victims now contract the worms by eating the meat of wild carnivores, such as bear. Trichinosis is usually a mild disease, manifested by symptoms and signs of intestinal and muscular inflammation, but in heavy infections damage done by the larvae to the heart and central nervous system can be life threatening. Because of public awareness about properly cooking pork and federal regulations about feeding pigs, trichinosis has become uncommon in the United States. People who eat inadequately prepared marine fish may become infected with larval nematodes. Of the many potential (and rare) helminthic zoonoses from wild animals in the United States, Baylisascaris procyonis is particularly dangerous. The nematode is highly prevalent in raccoons, the definitive host. See also Cestoda; Medical parasitology; Nemata; Trematoda.

Zoonoses, or zoonotic diseases, are caused by infectious agents that are transmissible under natural circumstances from vertebrate animals to humans. Zoonoses may arise from wild or domestic animals or from products of animal origin. Zoonoses have been known since early hystorical times. There are biblical references to plague, a bacterial zoonosis mainly transmitted to humans by fleas; and some historians contend that a disease first described by Thucydides during the Plague of Athens (430–425 B.C.E.) was typhus, a louse-borne zoonosis (Zinsser). Certain zoonoses, such as yellow fever, malaria, and rabies, are well known to the general public, but a vast number of lesser-known zoonoses exist in limited cycles in different parts of the world. There are undoubtedly many zoonoses lurking in nature that have the potential to cause serious public health consequences if introduced into humans. This is, in fact, what may be our greatest concern about zoonoses—not the diseases that we know they are capable of causing, but the hidden potential of what diseases might arise in the future. Examples that foster our concern include the emergence of AIDS (acquired immunodeficiency syndrome) from nonhuman primates, which has developed into one of the most significant infectious disease threats in the world today, and the crossing of the species barrier of certain influenza virus strains that have led to large human pandemics. Diseases such as AIDS and influenza have their origins as zoonoses, but they subsequently adapted to human-to-human transmission.

There are a number of different types of microbial agents that cause zoonotic diseases, and various ways humans can become infected with these agents. This may best be explained by a few examples: (1) Lyme disease, a bacterial disease transmitted via the bite of an infected tick;(2) rabies, a viral disease acquired by the bite of an infected animal; (3) Ebola hemorrhagic fever, a viral disease spread by infected blood, tissues, secretions, or excretions; (4) hantaviral disease, a disease contracted by inhaling air contaminated with virus-infected excreta from rodents; (5) leptospirosis, a bacterial disease usually transmitted to humans through contact with urine from infected animals; (6) brucellosis, a bacterial disease contracted by ingestion of unpasteurized milk; and (7) cat-scratch disease, a disease contracted through bites or licks of infected cats.

Enteric bacteria such as Salmonella and Escherichia coli and parasites such as Cryptosporidium and Giardia are responsible for major food-borne and waterborne disease outbreaks around the world, and recently the nonmicrobial, transmissible agent of bovine spongiform encephalopathy (mad cow disease) appears to have crossed over to humans to produce a degenerative neurological disease known as variant Creutzfeldt-Jakob disease.

There has been a disturbing trend of reemergence of previously recognized zoonoses that were believed to be under control. This has been coupled with the emergence of new zoonotic diseases. Numerous factors may account for this, including: (1) alteration of the environment, affecting the size and distribution of certain animal species, vectors, and transmitters of infectious agents to humans; (2) increasing human populations causing an increased level of contact between humans and infected animals; (3) industrialization of foods of animal origin—that is, changes in food processing and consumer nutritional habits; (4) increasing movements of people, as well as an increased trade in animals and animal products; and (5) decreasing surveillance and control of some of the major zoonoses. Some supposedly "new zoonoses" have been around for a long time but have simply not been recognized. For example, several types of hantaviruses are transmitted by rodents such as deer mice and can cause the disease known as hantavirus pulmonary syndrome. This disease has likely been around for decades, if not centuries, but human cases were first documented only in 1993. In addition, global warming has the potential to broaden the geographic distribution and abundance of arthropods as well as the vertebrate hosts in which some zoonoses persist.

There is no single clinical picture that can be drawn of zoonoses, given the diverse group of microorganisms that are capable of causing zoonotic diseases. A partial list of symptoms may include some, but not all, of the following: fever (sometimes hemorrhagic), headache, rash, muscle aches, arthritis, respiratory distress (sometimes pneumonia), abdominal pain, vomiting, diarrhea, jaundice, cardiac abnormalities, and neurological involvement ranging from stiff neck to meningitis or encephalitis. The course of disease varies between different zoonotic pathogens but can be more severe in the very young or very old, or in individuals who are immunocompromised. Many zoonoses can be treated with antimicrobial drugs, but there are few drugs that can be used to successfully treat viral zoonoses. Treatment for a known or suspected exposure to a viral zoonosis such as rabies involves administration of immune globulin, whereas only supportive treatment can be offered for many other viral zoonoses.

Vaccines are available for the general public for a small number of zoonoses, such as Japanese encephalitis and yellow fever, and on a limited basis for individuals perceived to be at occupational or recreational risk. In addition, chemoprophylactic regimens such as antimalarial drugs are recommended for travellers to high disease-risk areas. The risk of contracting vector-borne diseases can be reduced by avoidance of areas infested by arthropods, use of insect repellents, and appropriate clothing (the less skin exposed the better). Occasionally it is possible to reduce zoonotic disease risks by decreasing the abundance of certain reservoir hosts such as rodents. Individuals should also not drink untreated water or unpasteurized milk. Areas containing potentially contaminated animal material such as rodent excreta should be cleaned using appropriate disinfectants. Patients with diseases such as Ebola virus should be kept in strict isolation. Diseases such as tularemia and leptospirosis may be contracted by handling infected animal tissue, so trappers should use gloves when handling dead animals.

The disease incidence and pattern of occurrence of zoonoses varies greatly between different regions within a country and between countries. In general, zoonoses do not occur in large numbers in the industrialized world. Because of this relative infrequency of occurrence, some zoonotic infections may be overlooked and underdiagnosed.

Certain individuals may be at greater risk for contracting zoonoses. These include people with occupational exposure, such as veterinarians, farmers, and slaughterhouse workers, or individuals who participate in outdoor recreational activities, such as hunters. The best defense against contracting zoonoses is education. Individuals should be aware of the respective zoonoses that may be circulating in their environment and the times of year of greatest risk for contracting these zoonoses. This type of information is generally available from public health departments and veterinarians, and can also be found on the Internet.

(SEE ALSO: Communicable Disease Control; Ecosystems; Epidemics; Epidemiology; Vector-Borne Diseases; Veterinary Public Health; and articles on diseases mentioned herein)

Bibliography

Lederberg, J.; Shope, R. E.; and Oaks, S. C. (1992). Emerging Infections. Microbial Threats to Health in the United States. Washington, DC: National Academy Press.

Meslin, F. X. (1997). "Global Aspects of Emerging and Potential Zoonoses: A WHO Perspective." Emerging Infectious Diseases, Vol. 3. Geneva: World Health Organization.

Zinsser, H. (1934). Rats, Lice, and History. Boston: Little, Brown.

— HARVEY ARTSOB

Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.

Some zoonotic diseases are identified as potential diseases (e.g., Tularemia) could be exploited by bioterrorists to cause death—including death or contamination of livestock—and widespread economic damage. As of May 2003, the best scientific evidence available suggested that the cornonavirus responsible for Severe Acute Respiratory Syndrome (SARS) was originally transmitted from animal hosts.

Zoonoses are relevant for humans because of their species-jumping ability. Because many of the causative microbial agents are resident in domestic animals and birds, agricultural workers and those in food processing plants are at risk. From a research standpoint, zoonotic diseases are interesting as they result from organisms that can live in a host innocuously while producing disease upon entry into a different host environment.

Humans can develop zoonotic diseases in different ways, depending upon the microorganism. Entry through a cut in the skin can occur with some bacteria. Inhalation of bacteria, viruses, and fungi is also a common method of transmission. As well, the ingestion of improperly cooked food or inadequately treated water that has been contaminated with the fecal material from animals or birds present another route of disease transmission.

A classic historical example of a zoonotic disease is yellow fever. The construction of the Panama Canal took humans into the previously unexplored regions of the Central American jungle.

A number of bacterial zoonotic diseases are known. A few examples are Tularemia, which is caused by Francisella tulerensis, Leptospirosis (Leptospiras spp.), Lyme disease (Borrelia burgdorferi), Chlaydiosis (Chlamydia psittaci), Salmonellosis (Salmonella spp.), Brucellosis (Brucella melitensis, suis, and abortus), Q-fever (Coxiella burnetti), and Campylobacteriosis (Campylobacter jejuni).

Zoonoses produced by fungi, and the organism responsible, include Aspergillosis (Aspergillus fumigatus). Well-known viral zoonoses include rabies and encephalitis. The microorganisms called Chlamydia cause a pneumonia-like disease called psittacosis.

Within the past two decades two protozoan zoonoses have definitely emerged. These are Giardia (also commonly known as "beaver fever"), which is caused by Giardia lamblia, and Cryptosporidium, which is caused by Cryptosporidium parvum. These protozoans reside in many vertebrates, particularly those associated with wilderness areas. The increasing encroachment of human habitations with wilderness is bringing the animals, and their resident microbial flora, into closer contact with people.

Similarly, human encroachment is thought to be the cause for the emergence of devastatingly fatal viral hemorrhagic fevers, such as Ebola and Rift Valley fever. While the origin of these agents is not definitively known, zoonotic transmission is virtually assumed.

Outbreaks of hoof and mouth disease among cattle and sheep in the United Kingdom (the latest being in 2001) has established an as yet unproven, but compelling, zoonotic link between these animals and humans, involving the disease causing entities known as prions. While the story is not fully resolved, the current evidence supports the transmission of the prion agent of mad cow disease to humans, where the similar brain degeneration disease is known as Creutzfeld-Jacob disease.

The increasing incidence of these and other zoonotic diseases has been linked to the increased ease of global travel. Microorganisms are more globally portable than ever before. This, combined with the innate ability of microbes to adapt to new environments, has created new combinations of microorganism and susceptible human populations.

Further Reading

Books

Chin, J. "Tularemia." Control of Communicable Diseases Manual. Washington, DC: American Public Health Association, 2000.

Electronic

World Health Organization. WHO Fact Sheets (May, 2003) <http://www.who.int/health-topics/zoonoses.htm> (May 12, 2003).