Importance of zoonosis and anthropozoonosis

Zoonosis, comes from the Greek words, zoo: animal and nosos: disease. Its meaning in veterinary infectology and epidemiology, as well as in human public health, refers to infectious-contagious diseases transmitted by animals to the human being, either by direct contact, through contaminated foods or through various types of vectors.

In the face of the globalization of infectious diseases, a good example is the H5N1 avian influenza virus. (Gloszilla Studio | Shutterstock)
In the face of the globalization of infectious diseases, a good example is the H5N1 avian influenza virus. (Gloszilla Studio | Shutterstock)

Zoonosis, comes from the Greek words, zoo: animal and nosos: disease. Its meaning in veterinary infectology and epidemiology, as well as in human public health, refers to infectious-contagious diseases transmitted by animals to the human being, either by direct contact, through contaminated foods or through various types of vectors.

There are numerous written, archaeological and paleontological testimonies demonstrating the presence of diseases that affected mainly domestic animals and primitive man, since the most remote old times.

It is reasonable to think about and admit that various pathogenic agents have colonized and been established with a morbid vocation in higher mammals, during the long and patient evolutionary process of the species.

The study of lesions found in fossils in archaeological areas of human settlements and in tombs of ancient civilizations allow modern microbes hunters to diagnose with relative precision and safety many of those infectious and parasitic processes.

In addition, there are written testimonies of some references of animal and human disorders. The interpretation of clinical descriptions through signs and symptoms is hardly certain, especially when it comes to literature texts describing epidemics and epizootics. Thus, medical science historians do not always agree on their diagnosis over time.

Terms such as “epidemics,” “pests,” “malignant fevers,” “miasmas,” “contagious emanations,” etc., are too general, if they are not accompanied by more precise clinical descriptions.

Anthropozoonosis

The current certainty of the existence of infectious-contagious diseases common to animals and humans neither imply nor mean that we were previously aware of this reciprocal contagion, that is, of anthropozoonosis. Anthropozoonosis is defined as infectious diseases that are naturally transmitted from humans to vertebrate animals.

The experience acquired through the diachronistic-historical analysis of epizootiology and the epidemiology of many infectious entities shows that the behavior of microbial populations − prions, viruses, bacteria, fungi and pathogenic parasites and their hosts − frequently change, so signs and symptoms of clinical manifestations and the evolution of outbreaks could have changed over time.

Pathogens of zoonotic origin

According to the World Organization for Animal Health (OIE), of the 1,415 pathogens affecting humans, 61 percent are of zoonotic origin and therefore have a direct relationship to veterinary public health activities.

Let us remember Pasteur's phrase in the cursed fields of Poullit-le Fort in 1881, during the first immunization of animals with a live attenuated bacterin to live bacteria immunization against anthrax: “The germ is nothing, the circumstances are everything.” And the research of the other giant of German bacteriology, Robert Koch, with the Vibrio Cholera in Calcutta, India in 1883, which showed that dirt, filth, overcrowding and overpopulation trigger the bacillus Vibrius colericus.

Manifestations and consequences on the population of Europe and the Americas of the cases of human influenza, exanthematic typhus and treponematoses were radically different from how they were known before the discovery of the American continent.

These diseases, as a result of the movements of human and animal populations between the Old and New Continent, profoundly altered the biocenotic balances mainly of the Americas and, to a lesser extent of Europe. New species and strains of pathogenic microbes and completely “naive” hosts were introduced, i.e. immunologically susceptible, both in relation to animal and human infectious diseases.

Avian influenza and unified health

Finally, given the irreversible, accelerated and massive process of globalization experienced by humanity at the beginning of the 21st century, and in the face of the globalization phenomenon of infectious diseases, what a better example, than the H5N1 virus of avian influenza of Asian origin.

The moment will come when we will not talk more about animal health or human health, treating them as distinct and separate entities or disciplines, but we will talk about a “unified health”: One World, One Health!

Other diseases and overpopulation

Of all the contagious diseases that humankind currently suffers, perhaps the oldest of them is malaria. The hemoprotozoa that parasitize chimpanzee and gorilla hematocytes are practically indistinguishable from those that affect humans. Thus, it is assumed that humans acquired malaria parasites at the evolutionary time of the separation of humans from the upper primates.

Epidemic diseases such as bubonic plague, cholera, smallpox, yellow fever and influenza are apparently germs of late arrival in the human body, resulting from overcrowding in urban areas.

Germs that have caused big pandemics require overpopulation in urban areas to reach an epidemic state. Malaria, on the contrary, has an endemic status. While humans remained as a non-sedentary hunter-gatherer, they remained free of communicable diseases, except malaria.

When humans became sedentarized because of agricultural and pastoral activities, they organized themselves into tribal systems, and began living in villages, towns and cities. In this way, it became easy prey to infectious diseases.

The path to zoonosis

The other widespread concept is to consider that most infectious diseases suffered by humanity are of zoonotic origin. Then, they reached the second evolutionary phase, when the pathogen manages to adapt and achieve such virulence capacity that can be horizontally transmitted, loosing the animal origin reservoir in its life cycle.

From the infectology point of view, health care has been changing over time among different civilizations. Infectious diseases that affected primitive humans in settlements of little demographic concentration were initially parasitic, fungal and bacterial.

As humans organized into increasingly complex industrialized societies and inhabited urban areas with a higher population concentration since the 19th century, contagious diseases have, above all, become viral-type conditions.

Return of diseases?

With the development of public health, the discovery of antimicrobials such as sulphonamides and antibiotics, and with the development of preventive vaccines, human life expectancy considerably increased as of the second half of the 20th century.

The demographic explosion and the aging of urban societies have led to a turning point in preventive and mass therapeutic medicine, with new pathologies, such as heart disease and cancer, in addition to the challenging problems that geriatrics poses.

During the preceding centuries, devastating pandemics served as great regulators of human population and as arbiters of the forms of society.

While it is true that large pests, such as bubonic plague, smallpox, and cholera, have given way, even though smallpox has been declared by the World Health Organization as missing, pathogens that cause these contagious diseases can return. If they have once emerged, they can sprout again.

Four possible scenarios facing humanity

  1. The emergence of new strains of previously known pathogens or new highly virulent pathogenic agents, provided with impregnable defensive and evasion mechanisms. Such is the current case of retroviruses responsible for acquired immunodeficiency syndrome (AIDS).
  2. An atomic catastrophe, which leads to the annulment and collapse of all our current infectious disease control systems, such as sewage treatment and conduction systems, mass production of vaccines and insecticides. Such a catastrophe could lead to the reintroduction of typhus, cholera or smallpox germs.
  3. Possibility of the emergence of new pathogens by mutation or genetic recombination of diseases that are currently under good control, showing new modalities of extreme virulence, of great horizontal transmission capacity or of antibiotics resistance.
  4. Revival of pandemic diseases of antiquity, due to immunosuppression processes.

Main diseases transmitted by birds to humans

The most important known avizoonoses are:

  • Psittacosis or chlamydia (Chlamydia psittaci)
  • Salmonellosis (S. typhimurium, S. Enteritidis, S. arizonae, etc.)
  • Campylobacteriosis (Campylobacter jejuni)
  • Avian influenza (subtypes H5N1 and H7N9)
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