Zoonoses

Zoonoses are diseases caused in humans by bacteria, viruses, parasites, and fungi that have been transmitted from animals, reptiles, or birds to people.

Because many of the microorganisms that cause zoonotic disease are normal inhabitants of domestic animals and birds, people who are involved in agriculture and those who work in food processing plants can be at risk for infection. Prevention of such infections is actively studied. Understanding the host factors that contribute to immunity from disease and which factors promote the establishment of disease is essential if zoonotic infections are to be successfully prevented.

Humans can develop zoonotic diseases by different routes, depending on the microorganism. For example, some bacteria can cause infection following entry through a cut in the skin. Another common method of disease transmission is by the inhalation of bacteria, viruses, or fungi. A third route is via the ingestion of improperly cooked food or water contaminated with the fecal material.

A classic example of a zoonotic disease is yellow fever. The discovery of the disease and of its origin came during the construction of the Panama Canal during the first decade of the twentieth century. The construction of the canal took humans into previously unexplored regions of the Central American jungle. This brought the workers into contact with animals and microorganisms that they had not been in contact with before. One of the microbes produced the serious illness that came to be called yellow fever.

Another example occurred beginning in the 1970s. Then, the clearing of the Amazonian rain forest to provide agricultural land accelerated. Woodcutters became ill with infections caused by the Mayaro and Oropouche viruses. Another final example dates back only a decade ago. In the mid 1990s, a rapidly developing and often-fatal lung infection in the Southwestern United States was found to be caused by the Hanta virus that can be transmitted from rodents to humans. A final example is the viral zoonoses called West Nile virus. This virus, which can infect birds such as crows and blue jays, can be transmitted to humans by a mosquito that feeds on the bird and then subsequently feeds on a human.

There are a variety of zoonotic diseases that are caused by the movement of bacteria from an animal, bird, or insect host to humans. Examples include 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).

Fungi also produce zoonoses. An example is Aspergillosis, which is caused by Aspergillus fumigatus.

Two protozoan zoonoses have emerged in accelerating numbers in the past two decades. One protozoan is called Giardia. The debilitating diarrhea that is commonly dubbed "beaver fever" is caused by drinking water that is contaminated with Giardia lamblia. Cryptosporidium parvum causes an equally debilitating diarrheal malady called cryptosporidiosis.

Giardia and Cryptosporidium live naturally in many wild animals. The loss of natural habitat with increasing human development has brought the animals and their microbial cargo into more frequent contact with people, and human infections are the result.

Human encroachment is also fueling the emergence of fatal viral hemorrhagic fevers such as Ebola and Rift Valley fever. The zoonotic origin of these agents is not definitively known. However, the available evidence strongly supports the idea that primate populations harbor the viruses, and that periodically transmission of the viruses from primates to humans occurs.

Beginning in 1986, an outbreak of bovine spongiform encephalopathy (BSE) among cattle in the United Kingdom led to the eventual slaughter of over 150,000 domestic animals. Ominously, scientists have evidence that links the "mad cow" disease in the domestic animals with the brain degeneration in humans known as Creutzfeld-Jacob disease. Although not conclusively established, there is the possibility that the disease-causing agent of Creutzfeld-Jacob disease was passed to some of the people when they ate infected beef.

The increasing incidence of these and other zoonotic diseases may be in part due to the increased ease of global travel. Microorganisms are global residents, which can easily move along with people and cargo. New combinations of microorganisms and susceptible human populations have and continue to arise. As of July 2003, the virus responsible for severe acute respiratory syndrome (SARS) was thought to be zoonotic in origin, but further research is needed to clarify origin and transmission modes.