Author of Response: Sheridan Kidd Haack, Research Hydrologist/Microbiologist, U.S. Geological Survey
It is difficult, given our current system of determining and documenting the causes of human illness, to specify how many cases of human sickness are due to pathogen contamination of water resulting from agricultural livestock sources. Not all waterborne disease outbreaks are recognized, investigated, or even reported to health agencies. In many cases, the pathogen causing the outbreak is not determined, and even when the pathogen is determined, by the time the outbreak environment is investigated, the source of contamination may no longer be present. For example, the largest drinking water outbreak in U.S. history—the Cryptosporidium outbreak in Milwaukee, Wisconsin, in 1993—was first recognized by absenteeism at schools and work and shortages of antidiarrheal medications at pharmacies. Confirmation that Cryptosporidium had entered the water supply was determined after the fact by examination of ice prepared with the contaminated water. Animal manure washed into the water supply by rain was first implicated as the Cryptosporidium source. Several years later, advances in microbiology and genetics permitted distinction between animal and human sources of Cryptosporidium. Preserved samples of human stools yielded only the human type, indicating human sewage was the likely contributor.
There are several records of outbreaks of human illness and deaths associated with contamination of water with livestock manure. A recent summary of 66 drinking water outbreaks from affluent nations (Hrudey and Hrudey, 2004) 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 two 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 seven died).
- In addition, the largest outbreak of leptospirosis in the United States occurred among triathlon participants who swam in a lake in Illinois in 1998 (Morgan et al., 2002). Of 876 athletes, 98 became ill, and three were hospitalized, one with acute renal failure. No specific animal reservoir was identified; however, wild animals generally did not test positive for Leptospira, while some horses, sheep, and cattle from the watershed were positive.
The Centers for Disease Control (CDC) maintains a record of the cause of all drinking water and recreational water outbreaks reported since 1971 (CDC, 2004). In the period 1993-2002, zoonotic agents* (Cryptosporidium species, Giardia intestinalis, and pathogenic E. coli) accounted for 44.8% of freshwater recreational water outbreaks (CDC, 2004). Likewise, zoonotic agents* contributed to drinking water outbreaks between 1991 and 2002: Giardia (16%), Cryptosporidium (7%), E. coli O157:H7 (5%), and Campylobacter (3%). In most cases, the specific source of the zoonotic agent was not identified.
*Zoonotic agents—pathogens that are naturally transmitted between animals and humans.
Additional Reading:
Centers for Disease Control and Prevention. 2004. Surveillance for waterborne-disease outbreaks associated with recreational water and drinking water—United States, 2001-2002. in Surveillance Summaries, October 22, 2004, MMWR: Morbidity and Mortality Weekly Report 53, No. SS-8. (http://www.cdc.gov/mmwr/indss_2004.html)
Hrudey, S.E., and Hrudey, E.L. 2004. Safe Drinking Water: Lessons Learned from Recent Outbreaks in Affluent Nations. IWA Publishing, London, UK. 486 pp.
