Zoonoses
Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.
As of 2005, the best scientific evidence available suggested that the cornonavirus responsible for severe acute respiratory syndrome (SARS) was originally transmitted from animal hosts. Also, the avian flu, which until 2004 was resident in poultry, has caused a number of human deaths and now appears able to be transmitted both from poultry to felines and, ominously in terms of a global epidemic, from human to human.
Zoonoses are relevant for humans because of their species-jumping ability. Because many germs that can transfer from species to species are found in domestic animals and birds, agricultural workers and those in food processing plants are at risk. From a research standpoint, zoonotic diseases are interesting as they result from organisms that can live in a host innocuously while producing disease upon entry into a different host environment.
Humans can develop zoonotic diseases in different ways, depending upon the microorganism. Entry through a cut in the skin can occur with some bacteria. Inhalation of bacteria, viruses, and fungi is also a common method of transmission. As well, the ingestion of improperly cooked food or inadequately treated water that has been contaminated with the fecal material from animals or birds presents another route of disease transmission.
A classic historical example of a zoonotic disease is yellow fever. The construction of the Panama Canal took humans into the previously unexplored regions of the Central American jungle, where mosquitoes ferried the yellow fever virus from monkeys to man. When mosquitoes fed upon an infected monkey (the disease's natural host), the virus passed into the mosquito (the vector), which in turn, infected humans with their bite. Only after mosquito prevention measures were employed about 1910, with techniques such as the use of mosquito netting to cover tents and water supplies, were efforts successful in carving the canal through the jungle.
A number of bacterial zoonotic diseases are known. A few examples are 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).
Zoonoses produced by fungi include Aspergillosis (Aspergillus fumigatus). Well-known viral zoonoses include rabies and encephalitis. The microorganisms called Chlamydia cause a pneumonia-like disease called psittacosis.
Within the past two decades two protozoan zoonoses have emerged. These are Giardia (also commonly known as "beaver fever"), which is caused by Giardia lamblia and Cryptosporidium, which is caused by Cryptosporidium parvum. These protozoans reside in many vertebrates, particularly those associated with wilderness areas. The increasing encroachment of human habitations with wilderness is bringing the animals, and their resident microbial flora, into closer contact with people.
Similarly, human encroachment is thought to be the cause for the emergence of devastatingly fatal viral hemorrhagic fevers, such as Ebola, Marburg, and Rift Valley fever. While the origin of these agents is not definitively known, zoonotic transmission is virtually assumed.
Outbreaks of "mad cow" disease (bovine spongiform encephalopathy or BSE) among cattle in the United Kingdom in the 1990s (the latest being in 2001) has established a probable zoonotic link between these animals and humans, involving the disease causing entities known as prions. While the story is not fully resolved, the current evidence supports the transmission of the prion agent of mad cow disease to humans, where the similar brain degeneration disease is known as variant Creutzfeld-Jacob disease.
The increasing incidence of these and other zoonotic diseases has been linked to the increased ease of global travel. Microorganisms are more globally portable than ever before. This, combined with the innate ability of microbes to adapt to new environments, has created new combinations of microorganism and susceptible human populations.
