Animal Waste, Water Quality and Human Health

76Animal Waste, Water Quality and Human Healththat under the right environmental, hydrological, host and other conditions, humaningestion of water contaminated with low levels of livestock excreta can result inillness from these pathogens. For waterborne transmission to occur, sufficientamounts of human-infective Cryptosporidium parvum, G. duodenalis, E. coliO157:H7, Salmonella, or Campylobacter need to be shed in the excreta oflivestock, survive the numerous processes of the terrestrial or aquaticcomponents of a catchment that attenuate and/or inactivate this pathogen load(e.g., filtration, UV inactivation, desiccation, dilution, senescence), contaminaterecreational water and be ingested in infectious doses. As such, all else beingequal, the amounts of these pathogens generated by livestock populations isrelated to the risk levels of waterborne transmission to humans. Therefore, anaccurate calculation of the rate of pathogen production for the various livestockproduction systems will help assess livestock pathogen exposure risk and allowmore effective targeting of load, transport, and exposure interventions on highrisk livestock populations.3.1.4 Environmental loading rate of zoonotic pathogensin livestockThe environmental loading rate per animal is a metric that allows comparison of thepathogen production rate between livestock species, different age classes ordifferent animal production systems (Atwill et al. 2003, Dorner et al. 2004). It isdefined as the total amount of a specific pathogen excreted per animal per day.Given that all five priority waterborne zoonotic pathogens are shed primarily inthe faeces rather than the urine of infected animals, the environmental loadingrate excludes urine production in its calculation. For pathogens such aLeptospira, the measurement of urine production would be critical. Theenvironmental loading rate can be crudely estimated in various ways (Atwillet al. 2003, Dorner et al. 2004, Starkey et al. 2007, Ferguson et al. 2005), forexample, daily faecal production per animal ( f ) × the point prevalence of faecalshedding (P) × mean intensity or concentration of pathogens excreted byinfected animals (I p ), or fPI p . Examples of faecal production rates are presentedin Table 3.2. The amount an animal defecates per day is largely determined bydry matter intake (DMI) (Weiss 2004, Wilkerson et al. 1997), which, in turn, isinfluenced by the metabolic needs of the animal (e.g., body weight, lactationstatus), diet composition, and other factors. For example, an equation predictingwet excreta (faeces + urine) production per lactating dairy cow is: wet manure(kg/d) = 3.0 × DMI (kg/d) (Weiss 2004).This approach can be used for confined and extensive livestock populationswhere individual faecal samples are collected from a representative sample of