What is a Pathogen?
A pathogen is a biological agent that causes disease or illness. A zoonotic pathogen refers to a pathogen naturally transmitted from animals to humans.
All animals including pets, livestock, wildlife and humans, are possible hosts of potential human pathogens. This web site will focus on pathogens originating from livestock and poultry that might be transported to humans via air and water connections with animal manures. Other transport pathways such as by food or by hand-to-mouth contact are not addressed.
Zoonotic Pathogens
There are three general classes of zoonotic pathogens:
- protozoan parasites
- bacteria
- viruses
Chip Simmons, Research Professor at the University of North Carolina Chapel Hill School of Public Health, introduces these three classes. “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. … In contrast to this, viruses and parasites are only capable of reproducing in an infected individual. Because of this, there will be no growth of these organisms in the environment...”
Zoonotic bacteria that can cause human disease include Salmonella spp. and certain strains of E. coli. Cryptosporidium parvum is an example of zoonotic protozoa. For viruses, the vast majority is not zoonotic and is very host-specific—they only infect their host animal species. Detailed discussion of protozoan parasites, bacteria, and viruses can be found on pages 5, 12, and 18, respectively, of the USDA NRCS technical note
Waterborne Pathogens in Agricultural Watersheds
Sheridan Haack, PhD, Research Hydrologist/Microbiologist for US Geological Survey indicated that there have been several records of outbreaks of human illness, and deaths, associated with contamination of water with livestock manure (additional information found in What the Experts Say...) She noted that a recent summary of 66 drinking water outbreaks from affluent nations (Hrudey and Hrudey, 2004 – see Research Summaries, identified 12 outbreaks in which livestock manure was the probable pathogen source. These included:
- An outbreak at the 1999 Washington County Fair, New York (E. coli O157:H7; of 781 confirmed cases, 71 people were hospitalized, and 2 died);
- An outbreak in Walkerton, Ontario, Canada in 2000 (E. coli O157:H7 and Campylobacter jejuni; 2,300 people were ill, 65 were hospitalized and 7 died).
Not all illness outbreaks are livestock related. For example, animal manure was initially suggested as the source of the largest drinking water outbreak in U.S. history – the Cryptosporidium outbreak in Milwaukee, WI in 1993. Several years later following advances in microbiology and genetics, human sewage was identified as the likely contributor.
Additional information on health issues and pathogens is found in Sheridan Haack’s webcast presentation. Real Player is needed to view the video segment. Power Point slides and other resources related to the webcast presentation can be found at the webcast archive.
A listing of possible zoonotic pathogens can be found on pages 6 – 10 of an EPA literature review.
Page Managers: Rick Koelsch, University of Nebraska, and Janice Ward, US Geological Survey
Reviewed by: Dan Shelton and Jeanette Thurston-Enriquez, USDA ARS, and Sheridan Haack, USGS.
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