Medical Microbiology
zoonoses zoonosen zoonoses
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Numerous infectious diseases are caused by agents that are directly or indirectly transmissible between different animal species and humans. Today, more than 200 diseases occurring in humans and animals are known to be transmitted mutually; they are caused by viruses, bacteria (including rickettsiae and chlamydiae), fungi, protozoa, and helminths, as well as arthropods. In 1958, an Expert Committee of the World Health Organization defined zoonoses as "diseases and infections which are naturally transmitted between vertebrates and humans". This definition is still valid.

Originally, zoonoses were regarded as animal diseases (in Greek, zoon means “animal”). In the 19th century, the meaning of the word changed. In 1855, in his Handbuch der Speciellen Pathologie und Therapie, R. Virchow included the chapter „Infectionen durch contagiöse Thiergifte“ („Infections Caused by Animal Poisons“) with the subtitle „Zoonosen“ („Zoonoses“). In the Etymologisches Wörterbuch der Veterinärmedizin und ihrer Hilfswissenschaften (Etymological Dictionary of Veterinary Medicine and its Auxilliary Sciences) by W. Probstmayer (1863), the word “zoonoses” received a double meaning for the first time: “Zoonosen sind erstens eigentliche Tierkrankheiten, zweitens Krankheiten der Menschen, welche auf dieselben vermittels eines Contagiums von Tieren übertragen werden” (i.e., zoonoses are [i] animal diseases and [ii] diseases of humans transmitted from animals by means of a vector or contact).

Today, no differentiation is made with regard to the direction of transmission, i.e., animal to human or human to animal. However, there were attempts to describe the direction of transmission by a more precise wording: the term “zooanthroponoses” referred to diseases transmitted from animals to humans, and the term “anthropo-zoonoses” referred to diseases transmitted from humans to animals. However, the latter play only a minor role in the epidemiology of zoonoses.

More recent epidemiologic knowledge has put into doubt the traditional associations of some infectious diseases with zoonoses. Diseases which do not require a vertebrate reservoir because of their occurrence in water, in soil, on plants, or in food or fodder, whence they are transmitted to vertebrates (including humans), are called sapronoses, saprozoonoses, or geonoses.

Classical infectious diseases like rabies, plague, and yellow fever, well-known for centuries, are zoonoses which have not been eradicated despite major efforts. Recently, new zoonotic entities, e.g., Lyme borreliosis, ehrlichiosis, infections with enterohemorrhagic Escherichia coli, cryptosporidiosis, and hantavirus pulmonary syndrome, have been detected.

The steadily increasing threat that zoonoses pose to humans has many causes which differ from country to country.
Overpopulation, wars, and progressive deterioration of living conditions cause migration of countless people into slums of large cities, with a subsequent breakdown of hygiene and public health care. The proximity of their dwellings to huge garbage dumping grounds and their dependence on water contaminated with sewage facilitate contact with rodents, stray animals, and their parasites.

Scarcity of food forces millions of humans to clear woodland for cultivation and to produce new settlements in areas where animal populations and parasites were formerly isolated from humans. Humans may participate unwittingly in unknown parasite-host cycles and become a new link in an infectious chain. In many of these cases, humans, as accidental hosts, are in no way adapted to the new pathogenic species, which may result in high mortality.

Artificial irrigation changes the ecology of whole countries. Artificial lakes and ponds attract animals and their parasites over vast distances and provide optimal breeding grounds, especially for mosquitoes.

Increasingly warm and moist winters in the Northern Hemisphere favor the propagation of parasites, especially ticks.

Stray animals, usually heavily infested with worms or ticks, are reservoirs of infectious agents and parasites, not only in countries of the Third World but also in developed countries.

Worldwide tourism, especially trekking tours to remote areas and so called adventure challenges (e.g., ?survival training? with camping in open areas and consumption of raw or insufficiently cooked food), has encouraged contact between humans from industrialized countries who grew up under nearly aseptic conditions, and agents and vectors that they have never encountered before.

Zoonotic agents of low virulence may cause fatal infections in immunosuppressed humans (e.g., persons infected with HIV).

In the urbanized western style world, pets, in particular, dogs and cats, are often considered substitutes for children. They live in immediate contact to humans, are sometimes caressed and kissed and allowed to sleep in the beds and to lick the face and wounds of their hosts. Thus, viruses, bacteria, fungi, and parasites may easily be transmitted.

A further potential source of infection is transport of breeding and slaughter animals over vast distances and across borders, often with insufficient inspection for disease control.

New disease agents may be introduced to a country by legal or, even worse, illegal importation of exotic animals for zoos, research purposes, or private homes.
Isolated animal organs (xenotransplants) and cultures of animal cells may contain dangerous zoonotic agents.

Furthermore, several zoonotic pathogens, e.g., Francisella tularensis, Yersinia pestis, Brucella spp., Bacillus anthracis, Coxiella burnetii, and hemorrhagic fever viruses, are considered possible bioterrorism weapons.

The problem of diseases transmitted between animals and humans has many aspects, especially since it is not uncommon for animals serving as reservoir or intermediate hosts to be clinically inapparent carriers and/or excreters of an agent.
According to WHO, 60% of known and 75% of emerging infectious diseases of humans are zoonoses.

Undoubtedly, currently unknown zoonoses will emerge in the future. New methods for direct or indirect detection of microorganisms contribute to the detection of new zoonoses. When human invasion of hitherto uninhabited areas results in voluntary or involuntary environmental changes, new and potentially dangerous zoonoses may become evident. Severe acute respiratory syndrome (SARS), caused by a newly emerged coronavirus, is the latest example of the threat of dangerous infections, although its possibly zoonotic background has not yet been clarified.

In the study of zoonoses, medical experts and veterinarians should cooperate closely to study the etiology, epidemiology, and frequently complex developmental cycles and modes of transmission of pathogens and their vectors as well as the clinical presentation, diagnosis, differential diagnosis, therapy, and prophylaxis of the attendant diseases. Our book is based on such cooperation according to the recently coined term “One World – One Health”