Author of Response: Chip Simmons, PhD, Assistant Research Professor at the University of North Carolina-Chapel Hill, School of Public Health, osimmons@email.unc.edu

Response: Bacteria and viruses represent two of the major classes of pathogens, along with a third class, the parasites. Zoonotic pathogens are those that can infect both humans and animals. There are zoonotic pathogens from each class that are associated with animal agriculture and that can potentially pose risks to human health. Bacteria are single cell organisms capable of replicating in the environment, given proper growth conditions. All bacteria are not harmful, and many are actually advantageous.

Reproduction in the environment is of particular importance for potential bacterial pathogens due to the microbial concentration necessary to cause a clinical infection. Bacteria may be at low concentrations in the environment and therefore not capable of causing an illness. However, given the proper growth conditions, the bacteria may reproduce to high concentrations that may be capable of causing infections in humans. In contrast to this, viruses and parasites are capable of reproducing only in an infected individual. Because of this, there will be no growth of these organisms in the environment, only reduction due to adverse environmental conditions.

Viruses and parasites are generally infectious at lower doses than are the bacteria. However, this is highly dependent on the particular type and strain of organism. Examples of zoonotic bacteria that can cause human disease are the Salmonella spp. and certain strains of E. coli. Examples of zoonotic viruses that can cause human disease are reoviruses and possibly rotaviruses, adenoviruses, and Hepatitis E viruses. An example of a zoonotic parasite is Cryptosporidium parvum.

Additional Reading:
Cleaveland, S., M.K. Laurenson, and L.H. Taylor. 2001. Diseases of Humans and Their Domestic Mammals: Pathogen Characteristics, Host Range and the Risk of Emergence. Philosophical Transactions of the Royal Society B: Biological Sciences. Vol. 356(1411):991-999.

Pell, A.N. 1997. Manure and Microbes: Public and Animal Health Problem? Journal of Dairy Science. Vol. 80(10):2673-2681.

Taylor L.H., S.M. Latham, and M.E. Woolhouse. 2001. Risk Factors for Human Disease Emergence. Philosophical Transactions of the Royal Society B: Biological Sciences. Vol. 356(1411):983-989.