Animal Waste, Water Quality and Human Health

132Animal Waste, Water Quality and Human Healthyear in the USA and that each case on average costs $6,276 (2007 US$) (Withee et al.2009). If there is a linear relationship between the number of head of cattle colonizedwith the organism and human disease incidence, maximum vaccine (or othertreatment) costs could range from $2.29 to $9.14 per dose and yield an economicbenefit. However, the cost is dependent on the vaccine efficacy and the coveragerequired in order to achieve herd immunity. The market for such a vaccine is large,given that about 32 million cattle are slaughtered each year in the USA.In addition to vaccines, many other measures have been evaluated to determine ifthey can reduce the shedding of E. coli O157:H7 by cattle. These include the use ofprebiotics, probiotics, chlorate, specific antimicrobials, and bacteriophage therapy(Lejeune & Wetzel 2007, Sargeant et al. 2007). It is likely that the most efficacious ofthese treatments will eventually be used in tandem to reduce health risks associatedwith this important human pathogen.4.4.3 Management practices to increase resistance to diseaseNatural exposure to infection can lead to acquired resistance and is recommendedto aid in the control of some diseases. However, it appears that for the moreimportant waterborne pathogens (e.g. E. coli O157:H7 and Salmonella) naturalexposure does not confer protection to the host (Gyles 1998).Although it is often assumed that stress may lead to increased carriage orshedding of food-borne pathogens, little is actually known about the importanceor mechanism of stress in influencing the shedding of water- and foodbornepathogens (Rostagno 2009).4.5 CONTROL POINT 3: MANIPULATION OF THEMICROBIAL ECOLOGY OF THE HOST’SGASTROINTESTINAL TRACTColonization of animals by pathogenic micro-organisms is a natural process, butthe set of micro-organisms that colonize one animal species or one age-class ofanimal is by no means universal. The basis for this variability in hostsusceptibility is thought to be related to the specific environmental conditionsthat exist in the gastrointestinal tracts of different animals. The gastrointestinaltract is populated by competing micro-organisms which produce toxic(antimicrobial) substances, send out hormone signals that limit growth orstimulate the host’s immune system, and physically exclude others fromprotected or otherwise favourable spaces. In addition, viruses and bacterivorousuni-cellular and multi-cellular eukaryotes prey on other microbes, including