Animal Waste, Water Quality and Human Health

controlling E. coli-associated diarrhoeal diseases in calves, lambs, and piglets(Smith & Huggins 1983, Smith et al. 1987). While the results of these studiesusing mixtures of bacteriophages were promising, the work was not pursuedfurther until recently. Kudva et al. (1999) isolated bacteriophages that lyse E.coli O157 strains. However, the use of bacteriophages to control E. coli O157:H7 in cattle has met with mixed success (Sheng et al. 2006, Rozema et al. 2009).4.5.3 AntimicrobialsZoonotic waterborne pathogens in livestock 135Antimicrobials have been used at low levels for growth promotion in animalproduction since the 1950s. They act primarily to increase feed efficiency; themechanism of action is not known but is mediated largely through the effects ofantimicrobials on gut bacteria and inhibition of pathogenic bacteria is believedto be one of multiple pathways. A number of studies have shown the benefits ofincorporating antimicrobials into the diets of farm animals in reducing the levelsof enteric bacterial pathogens associated with foodborne and waterbornediseases (Rantala & Nurmi 1973, Goodnough & Johnson 1991, Johnson 1992).Antimicrobials such as penicillin and dihydrostreptomycin have also provenuseful in controlling leptospirosis infections and eliminating the carrier state indomestic animals (Alt et al. 2001). As mentioned above, transmission ofenteropathogens from the hen to the egg in elite, grandparent and parent breederflocks has been shown to be critical in decreasing levels of these zoonoticpathogens in commercial flocks. Several antibiotics, including gentamycin, havebeen effective at reducing bacterial cell numbers in poultry semen withoutnegatively influencing fertility. Antimicrobial dips to sanitize egg surfaces havealso been proved efficacious in reducing Salmonella-positive eggs (Berranget al. 2000, Cox et al. 2002). Treatment of cattle with neomycin sulphate hasbeen shown to decrease faecal shedding of E. coli O157:H7 (Elder et al. 2002,Woerner et al. 2006). However, approval of this antimicrobial for routine use incattle is unlikely because it and other closely related antibiotics are used in thetreatment of human infections.Despite their potential use in the control of zoonotic pathogens in animals, muchconcern has been expressed about the overuse of antibiotics in animal feeds for thepurposes of growth promotion. Concern is greatest where there is the possibility ofcross-resistance developing between antibiotics used in animal feeds and those usedin human clinical medicine. The development in Salmonella of resistance tofluoroquinolones and extended spectrum β-lactamases as well as the emergenceof multi-antibiotic resistance are thought to be a direct consequence of antibioticsupplementation of animal feeds (Threlfall 2002). These antibiotic resistancedeterminants are encoded by naturally occurring mobile genetic elements that