Zoonosis is any infectious disease that can be transmitted from animals, both wild and domestic, to humans. The word is derived from Greek words zoon (animal) and nosos (disease). Many serious diseases fall under this category.

The emerging interdisciplinary field of conservation medicine, which integrates human and veterinary medicine, and environmental sciences, is largely concerned with zoonoses.

A partial list of agents that can carry zoonoses includes:

• Bats
• Cats
• Cattle
• Dogs
• Goats
• Horses
• Pigs
• Primates
• Rabbits and hares
• Rodents

Zoonoses can be listed according to the infective agent:

• arthropods
• bacteria
• protozoa
• viruses
• worms

Some of the most important zoonoses are:

• Anthrax
• Brucellosis
• Borna virus infection
• Bubonic plague
• Campylobacteriosis
• Cutaneous larva migrans
• Ebola fever
• Lassa fever
• Leptospirosis
• Listeriosis
• Marburg virus infection
• Ornithosis
• Q-Fever
• Rabies
• Salmonellosis
• SARS (possibly; civet cats may spread the disease, or may catch the disease from humans -- see SARS article.)
• Toxoplasmosis
• Trichinosis
• Typhus and other Rickettsial diseases

Other zoonoses might be

• Foot and mouth disease (The human Hand, foot and mouth disease is a different virus)
• Bovine spongiform encephalopathy (more commonly BSE or mad cow disease)
• Glanders

This list is by no means complete. The influenza virus is an interesting example of these: it continually recombines genes between strains found in humans, ducks, and pigs, producing new strains with changed characteristics, and occasionally, as in 1918, killing millions worldwide.

The plural of zoonosis is zoonoses, from which an alternative singular zoonose is derived by back-formation.

The simplest definition of zoonosis is a disease that can be transmitted from animals to humans. A slightly more technical definition is a disease that normally exists in animals, but can infect humans. On other occasions it may be used to mean a disease that can complete its life cycle without a human host. None of these are wrong, although the first is overly simplistic.

Zoonotic diseases are important for a number of reasons:

Most of human prehistory has been spent as small bands of hunter-gatherers; these bands were rarely larger than 50 individuals, and were not in contact with other bands very often. Because of this, epidemic or pandemic diseases, which depend on a constant influx of humans that haven't developed an immune response, tended to burn out after their first run through a population. To survive, a biological pathogen had to be either a chronic infection, staying alive in the host for long periods of time, or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, for many 'human' diseases, the human is actually an accidental victim and a dead-end host . (This is the case with rabies, anthrax, tularemia, West Nile virus, and many others). Thus much of human development has been in relation to zoonotic, not epidemic, diseases.

Many modern diseases, even epidemic diseases, started out as zoonotic diseases . It is hard to be certain which diseases jumped from other animals to humans, but there is good evidence that measles, smallpox, influenza, and diphtheria came to us this way. AIDS, the common cold, and tuberculosis may also have started in other species.

In modern days, zoonoses are of practical interest because they tend to appear suddenly and be particularly virulent - the West Nile virus appeared in the United States in 1999 in the New York City area, and moved through the country in the summer of 2002, causing much distress. The plague was a zoonosis, as are salmonella, tetanus, Rocky Mountain spotted fever, and Lyme disease.

The major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans or domestic animals and wildlife (Daszak et al., 2001). This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity due to anthropologic or environmental disturbances. An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999, when intensive pig farming intruded into the natural habitat of fruit bats carrying the virus. Unidentified spillover events caused infection of the pig population which acted as an amplifier host, eventually transmitting the virus to farmers and resulting in 105 human deaths (Field et al., 2001).

Similarly, in recent times avian influenza and West Nile virus have spilled over into human populations probably due to interactions between the carrier host and domestic animals. Highly mobile animals such as bats and birds may present a greater risk of zoonotic transmission than other animals due to the ease with which they can move into areas of human habitation.

Diseases like malaria, schistosomiasis, river blindness, and elephantiasis are not zoonotic, even though they are borne by insects or other animal vectors, because they depend on the human host for part of their life-cycle.

References

• Field, H., Young, P., Yob, J. M., Mills, J., Hall, L., Mackenzie, J. (2001). The natural history of Hendra and Nipah viruses. Microbes and Infection 3, 307–314.

• Daszak, P., Cunningham, A.A., Hyatt, A.D. (2001). Anthropogenic environmental change and the emergence of infectious diseases in wildlife. Acta Trop. 78(2), 103–116.