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.

HARVEY ARTSOB

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