Animal Waste, Water Quality and Human Health

38Animal Waste, Water Quality and Human Healthassociated with intestinal and extra-intestinal diseases in both humans and animals(Donnenberg & Whittam 2001). Most E. coli pathogroups that cause entericdisease in humans are host species-specific and are important agents ofwaterborne disease in children and adults in parts of the world where theinfrastructure and services necessary for adequate treatment of drinking-waterand sewage are rudimentary. In contrast to these human-restricted E. colipathogroups, members of the enterohemorrhagic E. coli (EHEC) pathogroup arezoonotic pathogens. Although E. coli O157:H7 has been isolated from a widevariety of other animals sources, including feral and domestic pigs, dogs, horses,raccoons, starlings, gulls, geese and flies (Renter & Sargeant 2002, Pedersen &Clark 2007), most outbreaks of infection have been linked to ruminants. Itremains unclear whether other animal species are significant pathogen sources orsimply act as passive carriers.EHEC produce one or more antigenic type of potent bipartite protein toxinstermed Shiga toxins (Stxs) (Gyles 2007; Karch et al. 2009; Karmali et al. 2010;Mohawk & O’Brien 2011; Tam et al. 2008b). The toxins are thought to bind tospecific glycolipid receptors on host cells and become internalised. Once in thecytoplasm, the toxin A subunit cleaves the host ribosomal RNA at a specificsite. As this ribosomal RNA is essential for protein synthesis this actioneventually leads to cell death. During EHEC infection, Stxs are absorbed intothe blood stream and damage endothelial cells lining the small vessels of organssuch as the intestine, kidney, pancreas and brain. Stxs not only cause cell deathbut also the release of mediators of inflammation from endothelial cells whichpromote clot formation in the small blood vessels. This formation ofmicrothrombi causes platelet depletion, haemolysis and prevents blood flow,resulting in ischemic damage to vital organs. Damage to the colon is manifest ashaemorrhagic colitis and to the kidney as the sometimes fatal haemolytic uremicsyndrome (HUS).In addition to Stx production, EHEC typically possess a large plasmid whichencodes a number of virulence-related genes including a special haemolysin (Limet al. 2010) and also specific chromosomal regions known as pathogenicityislands that are thought to contribute to bacterial virulence (Hayashi et al. 2001,Perna et al. 2001). One of the best characterized of these is the locus ofenterocyte attachment and effacement which encodes for a specific bacterialattachment factor called intimin and proteins which form a so-called type IIIsecretion system which acts like a molecular syringe and injects effectorproteins from a number of different pathogenicity islands into the eukaryotic cell(Coombes et al. 2008, Kaper et al. 1997, Ogura et al. 2007, Tobe et al. 2006).These effector proteins participate in bacterial binding to the microvilli ofentocytes and also change the cell’s morphology and physiology.